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22 K Superconductivity Induced in AeFe2As2 Post-Growth via Reactive Gases ( Subscripts on the numbers in AeFe2As2)

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22 K Superconductivity Induced in AeFe2As2 Post-Growth via Reactive Gases ( Subscripts on the numbers in AeFe2As2)
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Tam, Gordon N
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[Gainesville, Fla.]
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Doctorate ( Ph.D.)
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University of Florida
Degree Disciplines:
Physics
Committee Chair:
Stewart,Gregory R
Committee Co-Chair:
Biswas,Amlan
Committee Members:
Hirschfeld,Peter J
Tanner,David B
Nino,Juan C

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fluorine -- iron -- superconductivity
Physics -- Dissertations, Academic -- UF
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Abstract:
Iron based superconductors are among the least understood superconductors. One aspect that has puzzled researchers for nearly as long as their existence since discovery is the tendency for undoped AeFe2As2 (Ae = alkaline earth metal) to be superconducting either as-grown or by chemical reaction near ambient conditions. Various works on undoped AeFe2As2 report this superconductivity manifesting in resistivity drops and some degree of Meissner effect in the magnetic susceptibility. In our study, single crystals of BaFe2As2 were grown using the self-flux method to avoid contamination of the lattice. Brief exposure of about 20 minutes to 5% fluorine gas post-growth resulted in a full resistive drop around 22 K and magnetic canning in an overwhelming majority of our samples (> 90%), with an occasional sample showing an incomplete resistive drop or no change at all. We also show similar effects of fluorine in SrFe2As2 and 3% chlorine in BaFe2As2. An increase in free electron concentration is indicated by Hall effect measurements. XPS results show the appearance of a fluorine 2s peak with the simultaneous suppression of the arsenic 3d peak. These results provide strong evidence for electron doping via atomic substitution of F for As as the source of the observed superconductivity. Our results represent a fast and reliable method for inducing a 5-10 micron thick superconducting layer in AeFe2As2 at room temperature. We compare our findings with the other reports of superconductivity in AeFe2As2 (and one case of FeTe1-xSx) in an attempt to solve the decade long mystery of superconductivity in undoped AeFe2As2. ( en )
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Thesis (Ph.D.)--University of Florida, 2019.
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Adviser: Stewart,Gregory R.
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Co-adviser: Biswas,Amlan.
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by Gordon N Tam.

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22 K SUPERCONDUCTIVITY INDUCED IN AeFe 2 As 2 POST GROWTH VIA REACTIVE GASES By GORDON N. TAM A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2019

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2019 Gordon N. Tam

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3 ACKNOWLEDGMENTS I thank my advisor Dr. Greg ory Stewart my supervisory committee (Dr. Juan Nino, Dr. Peter Hirschfeld, Dr. David Tanner, and Dr. Amlan Biswas) and Dr. Jungsoo Kim for their guidance, motivation, and continued patience through the years. I thank all members of the electronics, machine, and pump shops for their troubles in building and fixing of all the equipment I have managed to get my hands on. I thank several people that I have worked extensively with at the University of Florida in regards to specific experimentation : Dr. Derrick VanGennep of the Hamlin Group in the Physics Department for XRD meas urements and discussions Hiraku Maruyama of the Nino Research Group in the Materials Science and Engineering Department for SEM and EPMA measurements imaging of the doped crystal structure, Brendan Faeth (former undergradua te in the Stewart Group) for LabView programming and crystal growth and harvesting and Dr. Eric Lambers of the Major Analytical Instrumentation Center for XPS measurements, interpretations, and analysis.

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4 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ ............... 3 LIST OF FIGURES ................................ ................................ ................................ ......................... 6 ABSTRACT ................................ ................................ ................................ ................................ ..... 8 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .................. 10 History ................................ ................................ ................................ ................................ .... 10 Motivation ................................ ................................ ................................ ............................... 17 2 MATERIALS AND METHODS ................................ ................................ ........................... 26 Electron Transport Equipment ................................ ................................ ................................ 26 Resistivity Probe ................................ ................................ ................................ .............. 26 Electronics ................................ ................................ ................................ ....................... 27 Setup ................................ ................................ ................................ ................................ 28 Sample Preparation ................................ ................................ ................................ ................. 30 Growth ................................ ................................ ................................ ............................. 30 Harvesting ................................ ................................ ................................ ........................ 32 Treatment ................................ ................................ ................................ ......................... 33 Thinning ................................ ................................ ................................ .......................... 35 Annealing ................................ ................................ ................................ ........................ 36 3 RESULTS AND DISCUSSION ................................ ................................ ............................. 45 Effects of Flu orine ................................ ................................ ................................ .................. 45 Electron Transport ................................ ................................ ................................ ........... 45 Magnetic Susceptibility ................................ ................................ ................................ ... 46 Surface Morphology ................................ ................................ ................................ ........ 47 Location of Fluorine and Extent of Penetration ................................ ................................ ..... 48 Predictions ................................ ................................ ................................ ....................... 48 X Ray Diffraction ................................ ................................ ................................ ............ 48 X Ray Photoelectron Spectroscopy ................................ ................................ ................. 50 Fluori ne Depth ................................ ................................ ................................ ................. 51 Analysis ................................ ................................ ................................ ................................ .. 52 Additional Work ................................ ................................ ................................ ..................... 53 Interpretation of Literature ................................ ................................ ................................ ..... 55 4 CONCLUSIONS ................................ ................................ ................................ .................... 68 APPENDIX DELTA METH OD CALCULATION ................................ ................................ 70

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5 LIST OF REFERENCES ................................ ................................ ................................ ............... 72 BIOGRAPHICAL SKETCH ................................ ................................ ................................ ......... 80

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6 LIST OF FIGURES Figure page 1 1 First reported discovery of superconductivity. ................................ ................................ .. 20 1 2 Meissner effect for a superconductor. ................................ ................................ ................ 21 1 3 Phonon mediated pairing of electrons. ................................ ................................ .............. 21 1 4 The differences between a type I and type II superconductor. ................................ .......... 22 1 5 Magnetic flux vortices in a type II superconductor. ................................ .......................... 23 1 6 LaFeAsO structure. ................................ ................................ ................................ ............ 23 1 7 Unit cell of the 122 structure ................................ ................................ ............................. 24 1 8 Phase diagram of BaFe 2 As 2 with Co doping. ................................ ................................ .... 24 1 9 The two largest interstitial sites. ................................ ................................ ........................ 25 2 1 The copper block of the probe ................................ ................................ ........................... 39 2 2 Diagram of the 5 probe Hall Effect setup. ................................ ................................ ......... 40 2 3 BaFe 2 As 2 crystal ................................ ................................ ................................ ................ 40 2 4 Incomplete tra nsition ................................ ................................ ................................ ......... 41 2 5 The lapping device. ................................ ................................ ................................ ............ 42 2 6 Comparison between unannealed and annealed ................................ ................................ 43 2 7 Disappearance of superconductivity from annealing ................................ ......................... 43 2 8 Extended annealing at 100 C ................................ ................................ ............................ 44 3 1 Resistivity transition on a thinned BaFe 2 As 2 crystal ................................ ......................... 58 3 2 Overlapping magnetic and structural transitions of a thinned crystal. ............................... 58 3 3 Molar magnetic susceptibility as a function of field ................................ .......................... 59 3 4 Surface morphology of a reacted crystal. ................................ ................................ .......... 60 3 5 Fluorine atomic percentage from EPMA ................................ ................................ ........... 61 3 6 Unit cell of Ba122 with fluorine substitution ................................ ................................ .... 61

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7 3 7 Single crystal XRD spectrum of the (00 l ) lines ................................ ................................ 62 3 8 High angle single crystal XRD on a chlorinated crystal ................................ .................... 62 3 9 Virgin BaFe 2 As 2 survey scan. ................................ ................................ ............................ 63 3 10 Fluorinated BaFe 2 As 2 survey scan ................................ ................................ ..................... 64 3 11 Low binding energy XPS data ................................ ................................ ........................... 65 3 12 Fluorine depth penetration ................................ ................................ ................................ 65 3 13 Timeline of successive chlorine treatments ................................ ................................ ....... 66 3 14 Resistance of SrFe 2 As 2 reacted with fluorine ................................ ................................ .... 67 A 1 An example of the delta method. ................................ ................................ ....................... 71

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8 Abstract of Dissertation Pr esented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy 22 K SUPERCONDUCTIVITY INDUCED IN AeFe 2 As 2 POST GROWTH VIA REACTIVE GASES By Gordon N. Tam August 2019 Chair: Greg ory Stewart Major: Physics Iron based superconductors are among the least understood superconductors. One aspect that has puzzle d researchers for nearly as long as their existence since discovery is the tendency for undoped AeFe 2 As 2 (Ae = alkaline earth metal) to be superconducting either as grown or by chemical reaction near ambient conditions. Various works on undoped AeFe 2 As 2 rep ort this superconductivity manifesting in resistivity drops and some degree of Meissner effect in the magnetic susceptibility. In our study, single crystals of BaFe 2 As 2 were grown using the self flux method to avoid contamination of the lattice. Brief expo sure of about 20 minutes to 5% fluorine gas post growth resulted in a full resistive drop around 22 K and magnetic canning in an overwhelming majority of our samples (> 90%), with an occasional sample showing an incomplete resistive drop or no change at al l We also show similar effects of fluorine in SrFe 2 As 2 and 3% chlorine in BaFe 2 As 2 An increase in free electron concentration is indicated by Hall effect measurements. XPS results show the appearance of a fluorine 2s peak with the simultaneous suppression of the arsenic 3d peak. These results provide strong evidence for electron doping via atomic substitution of F for As as the source of the observed superconductivity.

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9 Our results represent a fast and reliable method for inducing a 5 10 m thick superconducting layer in AeFe 2 As 2 at room temperature We compare our findings with the other reports of superconductivity in AeFe 2 As 2 (and one case of FeTe 1 x S x ) in an attempt to solve the decade long mystery of superconductivity in undoped AeFe 2 As 2

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10 CHAPTER 1 INTRODUCTION History Superconductivity remains one of the most interesting, yet least understood phenomena in all of physics despite being discovered over a century ago. Superconductivity was ori ginally discovered in 1911 by Heike Kamerlingh Onnes in the form of zero electrical resistance in elemental mercury at 4.2 K [1] ( F igure 1 1 ). Thus began the extensive search for other materials that would also serve as lossless transport media. Superconductors have a critical temperature, abbreviated T C below which we observe its superconducting properties. Above T C the material is said to be in the normal st ate. Properties of a superconductor are most commonly characterized in our laboratory using 3 measurements: electron transport, magnetic susceptibility, and heat capacity. The electrical resistivity of a superconductor drops abruptly to zero at T C This is the least stringent of the 3 when measuring bulk superconductivity, as a single superconducting filament (i.e. not bulk behavior) will provide a short, leading to zero electrical resistance. The second property of superconductivity is the nearly complete expulsion of magnetic flux from the bulk of the material at and below T C ( F igure 1 2 ) Magnetic flux at the surface decays exponentially, with a penetration depth roughly given by the London penetration depth [2] This phenomenon is termed the Meissner e ffect The flux expulsion is due to induced supercurrents in the material. This is drastically different from a perfect conductor, where the expulsion would only persist while the applied magnetic field is still changing. True Meissner e ffect is indicative of a bulk superconductor. The volume of a sample that is superconducting can b e determined by comparing zero field cooled (ZFC) and field cooled (FC) data [3,4] A perfect bulk superconductor will expel all magnetic field regardless of if it is cooled in field or not. More

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11 often what occurs is partial flux expulsion due to pinning forces which will result in flux getting trapped in the interior of the sample if field cooled. A more extreme case of this is canning (or shielding) where only the surface of the sample (tens to hundreds of nanometers thick) or a second phase at the grain boundaries is superconducting, giving the maximum disp arity between ZFC and FC. The third property can be seen as a jump in the specific heat at T C corresponding to the opening of the band gap. The band gap is an energy gap created in the band structure that in the case of electron phonon coupled supercond uctors is explained by BCS Theory [5] Developed by Bardeen, Cooper, and Schrieffer in 1957, BCS Theory explains the phonon mediated pairing of electrons in conventional superconductors. An electron passing through a posi tive lattice will cause a temporary deformation in the lattice, producing a region of higher positive charge density ( F igure 1 3 ). Because of the larger mass of the positive ions, they will take longer to return to their unperturbed positions (i.e. longer relaxation time). This local region of positive charge will cause an effective attractive interaction with a nother electron, of opposite spin and momentum, as it passes through. The two electrons are thus paired and effectively act as a boson. As temperatu re is further and further decreased below T C a great deal of electrons become paired producing a Bose Einstein condensate of electron pairs. The pairing of the electrons results in a spontaneous lowering of the energy of the system, producing the aforemen tioned energy gap. Superconductors are classified in various ways such as type I vs type II and conventional vs unconventional. Conventional superconductors are defined by their 2 main characteristics: phonon mediated pairing and s wave gap symmetry. There is no prescription for unconventional superconductors, since they are everything other than conventional. The pairing interaction for a

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12 good deal of unconventional superconductors are thought to be due to magnetic spin fluctuations (e.g. in cuprates and i ron based superconductors). The symmetry of the order parameter for unconventional superconductors varies greatly, from number of bands to sign changes across the different electron and hole pockets. In regards to IBS (iron based superconductors), there ar e a multitude of potential symmetries [6] with s+ being the prevailing theory [6 9] In this specific case, the electron pockets carry the positive sign while the hole pocket is negative and there is no sign change in the gap function through a 90 degree rotation [6,10,11] Type I and II superconductors refers to an entirely different property. A type I superconductor has a strict cutoff for the critical field known as H C Below H C there is no flux penetra tion into the bulk (there is always flux within a penetration depth of the surface) and above T C there is full penetration. Type II superconductors possess two critical fields, with the lower and upper critical fields known as H C1 and H C2 respectively. B elow H C1 the material behaves as a type I. However, b etween H C1 and H C2 there exists a vortex state that allows field penetration only through the vort ices that have formed within the material ( F igures 1 4 and 1 5 ). The general rule of thumb is that type I superconductors are typically pure elements (Nb is one of several exceptions) with lower T C s as opposed to the compounds which are usually type II superconductors. Not only can type II superconductors have much higher T C s, they also bring with them some interesting properties such as flux pinning in maglev technology and current dissipation in Kosterlitz Thouless theory. Kosterlitz Thouless (KT) theory [12] postulates a phase transition in two dimensional systems that arises from a dissociation of vortex antivortex pairs. It was o riginally developed for superfluids but was found to be applicable to highly anisotropic superconductors too [13,14] The Kosterlitz Thouless transition temperature, hereafter referred t o as T KT occurs at a lower

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13 temperature than the characteristic superconducting critical temperature T C The resistivity of a material obeying this theory would follow (1 1) where (1 2) for X 1 << 1 [15 17] Minnhagen explicitly states that given the lack of features of the curve formed by equation (1 1) and the large number of adjustable parameters, a fit to the curve is A secon d and more conclusive test is to measure the relationship between applied current and longitudinal voltage. Above T C we obtain normal Ohmic behavior, but below T C the expectation is that there will be a power law dependence given by (1 3) wh KT Below T KT the vortex antivortex pairs are bound in the absence of an applied current. The addition of a nonzero current produces a Magnus force on the pairs. This leads to a dissociation of the pairs d ue to the different spin directions producing the dissipation relation given by equation (1 3) [16] This is the foundation on which we test for two dimensional superconductivity. As will be seen, our observed superconductivity is not two dime nsional, and as such our data do not obey V ~ I 3 in our V I measurements (not shown). However at higher temperatures V ~ I behavior (i.e. Ohmic) is observed as expected. Iron based superconductors are grouped into five main structures but also contain a few defect structures [18] The five main structures ( with examples ) are 1111 (LaFeOAs), 122

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14 (BaFe 2 As 2 ), 111 (LiFeAs), 11 (FeSe), and 21311 (Sr 2 ScO 3 FeP); the structures being aptly named based off of the stoichiometry. All structures follow the same prescription: a plane of Fe atoms with nonequivalent As (or P, Se, Te) atoms above and below. The result is Fe centered As tetrahedra forming the backbone of this family of superconductors. These FeAs layers can exist on their own (e.g. FeSe) or are separated by intermediate layers such as LaO in LaFeAsO ( F ig ure 1 6) strongly resembling the layered structure in th e cuprate superconductors and are therefore classified as layered superconductors. The crystal symmetries of all 5 of the main structures are primitive and tetragonal at room temperature with the exception of the body centered 122 group ( F igure 1 7 ). Upon cooling they (except for the Li 111 structure [18,19] ) undergo a structur al phase transition ( at a temperature denoted T S ) to the lower symmetry orthorhombic structure. In the parent ( i.e. undoped) compounds, this structural pha se transition also coincides with a magnetic phase transition (except for FeSe [18,20] ) from paramagnetic to antiferromagnetic. The magnetic transition is most often referred to as T SDW for spin density wave, a more generalized term to account for the Fermi surface nesting that is frequently seen in IBS [9,21,22] Chemical doping and pressure application [23] separates and suppresses the two transitions (in many, but not all of these materials) [24] with the magnetic transition suppressed more rapidly than the structural one [25] IBS are some of the most versatile and robust superconductors. Superconductivity can be induced in IBS by applying pressure or doping. IBS have seen success in electron doping, hole doping and even isovalent doping (where the dopant is from the same column in the periodic table) [26] Doping can also occur in either the insulating or the conducting layers, unlike in the cuprates where any doping at the copper site leads to a rapid destruction of superconductivit y

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15 [27] Th e general rule in the IBS is: if the magnetism can be suppressed no matter the method, superconductivity can be induced. Unlike popular perception the first IBS discovered was LaFePO by Kamihara et al. with a T C of 5 K in 2006 [18,28] superconductivity in F doped LaFeAsO in 2008 [29] that drew the attention of many researchers and reinvigorated the field. This marked the discovery of a new class of high temperature superconductors. Scientists found T C up to 56 K in Gd 0.8 Th 0.2 FeAsO soon after [30] The current record for IBS is 100 K in FeSe thin films on SrTiO 3 substrates [31] (although this T C has proven hard to reproduce). Since this thesis deals with inducing superconductivity in the 122s (namely BaFe 2 A s 2 ), it is worth narrowing our focus to the 122s. Superconductivity in BaFe 2 As 2 by Co doping for Fe was first discovered by Sefat et al. [32] a comparison we will make throughout this paper. Cobalt acts as an electron donor and can be seen from basic chemistry arguments (Co lies to the right of Fe and therefor e has more valence electrons) and experimentally by Hall effect data [32 34] The phase diagram of Ba(Fe 1 x Co x ) 2 As 2 is shown in F igure 1 8 The parent compound has overlapping structural (T S ) and magnetic (T N in the F igure 1 8 ) phase transitions (as mentioned previously), typically reported around 140 K [35 37] Upon doping, both these transitions are supp ressed to lower temperature with the magnetic transition being suppressed more rapidly. The structure changes from the higher symmetry tetragonal structure to the slightly less symmetric orthorhombic structure at T S The magnetic moments from the Fe transi tion from the paramagnetic state to the ordered antiferromagnetic state at T SDW Only after sufficient suppression of the magnetism does the superconducting state emerge. Both magnetism and

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16 superconductivity compete for the density of states despite the re being a region (green in F igure 1 8 ) where the two coexist. The superconducting region in the phase diagram that is formed by the T C boundary is known as the superconducting dome [25,38] The peak in the dome corresponds to the optimal doping amount (Co in this example) To the left and the right of the peak are referred to a s underdoped and overdoped respectively [39] The region beneath the structural transition and above the magnetic transition in the phase diagram exists the electroni c nematic phase. It is called such, because there is a broken 4 fold rotational symmetry while preserving the translational symmetry. Consequences of this nematic phase are seen in electronic and orbital anisotropies. The nematic phase has also been seen t o extend into the tetragonal phase however The origin of the nematic phase and its influence on the superconducting phase are still debated [6,39 42] For hole doped Ba122 (e.g. Ba 1 x K x Fe 2 As 2 ) there is no splitting between T S and T SDW [6,43,44] Magnetic and nematic phases still experience suppression prior to the superconducting phase as in electron doped Ba122. Since our data supports electron doping, we will be drawing comparisons to the phase diagram for Co doped Ba122 throughout. Though t he values for T C in IBS are still dwarfed by those of their cuprate predecessors e.g. mercury based cuprate s possess T C s exceeding 150 K [45,46] IBS expanded upon our knowledge of superconductivity, both in the similarities and differences with the cuprates As Mazin points out in [10] Bernd Matt hias (one of the fathers of superconductivity and the most prolific discoverer of superconductors) established a handful of golden rules for searching for a new superconductor. As time went on, not only were most of them proven to be wrong, but in fact the exact opposite turned out to be more promising. It appears that the more we learn about superconductors, the less we understand in terms of what the ingredients are to make a

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17 superconductor. The moral is that superconductors vary vastly from magnetism to order parameter to pairing interaction etc. ( refer to [10] for a more detailed table) and cover a greater spectrum of materials than anyone could have imagined so the more we know, the better Motivation As one can see from the phase diagram of BaFe 2 As 2 ( F igure 1 8 ), the parent compound (i.e. x = 0) is inherently non superconducting as grown. This is true for all members of the AeFe 2 As 2 family (Ae = alkaline earth metals) such as SrFe 2 As 2 It is well known that superconductivity can be induced in BaFe 2 A s 2 by application of pressure [47 50] and chemical doping on any of the three lattice site s ( e.g. hole doping of K for Ba, electron doping of Co for Fe, isovalent doping of P for As) [26] Numerous researchers report cases of superconducting 122s in the absence of doping and pressure with attempt s to explain this u nu sual occurrence of superconductivity. Saha et al. reported 21 K superconductivity in their self flux as grown SrFe 2 As 2 crystals [51] but early on eliminated the possibility of the superconductivity being a surface effect. Contrary to the benefits just 5 minutes caused nearly all traces of superconductivity to vanish. Interestingly enough, they were able to re introduce superconductivity back into the sample with pressure. Because of the sensitivity of their samples to heat treatment and pressure, they attribute d the superconductivity to be strain induced during the growth process, resembling post growth pressure induced superconductivity. On the other hand, Kim et al. observed 22 K in In flux grown BaFe 2 As 2 [52] Their results appeared to differ from those of Saha et al. in every way. They find evidence for low er dimensional superconductivity (i.e. not bulk), in the form of surface sensitivity and low

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18 superconducting critical current density. They also observe improvement with annealing and therefore rule out strain as the source. Another case of superconductin g AeFe 2 As 2 was reported in thin films of SrFe 2 As 2 by Hiramatsu et al. in [53] They noticed that exposure to water vapor in the ambient atmosphere over several hours was enough to produce a T C of 25 K. They explore the possibilities of the constituent ions settling into interstitial sites, namely the I6 and I9 interstitials depicted in F igure 1 9. They observe a shrinking of the c axis length in their x ray diffraction data. However inserting an ion on either site lead s to an expansion of c axis length, with I9 being the preferred of the two as it causes significantly less expansion than the I6. Though they have no answer for the origin of the superconductivity, then conclude it must be induced by ch emical pressure on the basis of the change in c axis length. The discovery of Hiramatsu et al. prompted the study of water on BaFe 2 As 2 by Kim et al. [16] They provide strong evidence for 2 D superconductivity with T C at 22.4 K. In addition to showing agreement with KT theory through equations (1 1 ) and ( 1 3) they also analyzed the magnetoresistance and observed the behavior expected from a 2 D superconduct or [15] The appearance of unusual superconductivity in IBS is not limited to 122s. Deguchi et al. [54] ex tended the mystery to the 11s. They found that by immersing non superconducting FeTe 0.8 S 0.2 induce a 9 K superconducting transition. It should be mentioned FeTe 1 x S x is hig hly sensitive to growth conditions and is generally superconducting as grown i.e. requires no doping, with a similar T C of 8.5 10 K for 0.1 < x < 0.3 [55,56] In all cases mentioned, the serendipitous superconductivity occurred contrary to expected norms. Regardless of how superconductivity was attained in those samples, it is intriguing to

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19 note that all these insta C very similar to that of their electron doped counterparts, 22 K for Ba ( Fe 1 x Co x ) 2 As 2 and 9 K for FeTe 1 x S x Though pressure and doping may have been involved, they were not intentional. This manuscript encom passes an extensive study of 250+ BaFe 2 As 2 samples subjected to various treatments in an attempt to not only explain the origin of the superconductivity in our samples but also in the aforementioned reports.

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20 Figure 1 1 : First reported disc research notes.

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21 Figure 1 2 : Meissner effect for a superconductor. The normal state of a material ignoring slight intrinsic paramagnetic and diamagnetic effects (left) compared to the perfect d iamagnetic superconducting state (right). Figure 1 3 : P honon mediated pairing of electrons. An electron passing t hrough from right to left will exert an attractive Coulomb force on the neighboring positive ions temporarily increasing the charge density. This higher positive charge region will cause an attractive interaction with a second electron of opposite spin and momentum (i.e. traveling left to right) thereby pairing the two electrons Figure adapted from [57] Note: The timescale is not depicted; the second electron arrives at a much later time as to avoid the Coul omb repulsion of the first electron

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22 Figure 1 4 : The differences between a type I and type II superconductor : A) Magnetization induced as a response to applied magnetic field with a distinct cutoff for a type I superconductor and a monotonic drop off be tween H C1 and H C2 B) Applied field as a function of temperature with 2 boundaries for type II Figure from [58]

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23 Figure 1 5 : Magnetic flux vortices in a type II superconductor between B C1 and B C2 (H C1 and H C2 in the text) Figure from [59] Figure 1 6 : LaFeAsO structure with the As tetrahedra highlighted Figure from [18,29]

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24 Figure 1 7 : Unit cell of the I10 (body centered, 10 atoms per unit cell) 122 structure Figure from [60] Figure 1 8 : Phase diagram of BaFe 2 As 2 with Co doping showing the various transition boundaries Figure from [25]

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25 Figure 1 9 : The two largest interstitial sites a ) in relation to the unit cell with expand ed views of b ) I9 and c ) I6 Figure from [53]

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26 CHAPTER 2 MATERIALS AND METHODS In the following chapter, I will present details and specifications on the methods we primarily used to conduct our experiments and the apparatus to carry them out. So me sections may contain brief results for emphasis or to illustrate a point. Electron Transport Equipment Resistivity Probe Electron transport measurements were primarily carried out in a specially designed resistivity probe built in the Physics Dept. mac hine shop At the top of the probe there are two 18 pin hermetically sealed connectors and a valve for evacuating the chamber of the probe. 18 pairs of 40 AWG wires are soldered to the 36 pins on the connectors. Of the 18 pairs, 10 are copper and 8 are man walled aluminum neck of the probe down to the copper block. Between the block and the neck is perpendicular plate used to shield the block from radiation traveling from the top of the probe. At the bottom, the wires are wrapped around a copper post that serves as the primary thermal connection of the contents of the probe with the cryogen reservoir. The tight wrapping of the wires around the copper post, anchored with GE 7031 varnish, helps remove heat that is brought in by the wires from the connector end of the probe. The copper block houses the samples, thermometer, and heater. 36 copper pins are mounted on the block correspondi ng to the 36 wires ( F igure 2 1 ) The CERNOX thermometer is situated in a cavity in the block and connected to 4 of the wires. The heater is an 830 Ohm, W rated resistor that is firmly and thermally secured to the block with STYCAST 2850 FT. The ex tra pin s allow for measurements of several samples at a time. Our electronic equipment however limits us to 3 simultaneous samples.

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27 The sample chamber of the probe is closed off with a brass can that is machined to a 4 degree taper angle matching that of the tap er joint on the probe. The vacuum seal is created by applying Dow Corning high vacuum grease on the taper joint and evacuating the probe via the nipple valve at the top. The copper block is weakly, thermally linked to the cryogenic reservoir by a stainless steel arm with the addition of a copper wire shunt held down with screws at each end. Varying the gauge of the copper wire allows adjusting the amount of the thermal connectivity between the copper block and the cryogen bath. This allows for strong tempe rature control with the heater. Temperature variations in the probe are on the order of 50 milliKelvin. Electronics The measurements are remotely controlled by a LabView program linked to the electronics by a GPIB controller. Up to 3 samples are connected in series via relays. The current to the thermometer is supplied by a Keithley 2400 with the voltage measured by a Keithley 2700. The heater is attached to a voltage limited Keithley 220 programmable current source. The samples themselves have much small er resistances and therefore require more precise instrumentation. The current is provided by a Keithley 6220 precision current source, and the voltage is measured by a Keithley 2182a nanovoltmeter. The delta method is employed here and preferred over the current reversal method. The delta method compensates for a thermoelectric voltage created from heating of the sample. This is done by using three points (instead of two as seen in the current reversal method) measured in rapid succession and approximating the thermal voltage drift as linear ( refer to Appendix for a more detailed explanation). The triggering between the KE 6220 and KE 2182a for the delta method mode is linked by a n RS 232 cable.

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28 Setup Resistivity and V I measurements are performed in the a b plane of the crystal using a 4 wire setup. The copper wires have lower resistance and are thus used for the current leads, while manganin wires are used for the voltage leads. Copper wires are also preferred when running currents through the sample to mi nimize resistive heating. by Epotek H20E LV epoxy (a two component, low viscosity silver epoxy) The platinum wires are annealed by briefly heating with a cigarette lig hter. The annealing drastically reduces the stiffness of the wires and allows them to be bent with ease. Curing for the epoxy is performed at For Hall effect measurements, the samples were glued down to an aluminum pin using stycast and mounted with the ab plane perpendicular to the field direction. (In a resistivity sample, the orientation of the sample in relation to the probe does not matter, because the direction of the current is established by the platinum contact s.) The aluminum pin laid on its side firmly secured to the empty space on the left side of the copper block (Figure 2 1) with the crystal mounted surface facing right. A fifth platinum wire is attached to the sample to measure the Hall voltage. The Hall v oltage allows us to obtain the carrier sign and concentration by ( 2 1 ) w here I is the current, B is the magnetic field, d is the lateral distance between the voltage leads, e is the elementary charge, V H is the Hall voltage, and A is the cross sectional area. side voltage leads ( F i gure 2 2 ). In practice it is very hard to line up the transverse voltage leads. Even if they are aligned visually, they migh t not be aligned conductively due to chemical and surface inhomogeneities and lack of

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29 sample symmetry. Since Hall voltages are generally very small (on the order of nanovolts for our samples), even a small offset can mask our true signal. The potentiometer allows us to null any offset voltage created by misalignment of the transverse voltage leads. The above described technique is frequently called the Hall bar method for the linear, bar like geometry of the sample. An alternative way to measuring resistivi ty or Hall effect is the v an der Pauw method. The van der Pauw method only requires 4 contacts for Hall effect as opposed to 5 for our setup. In addition, sample width and distance between contacts do not need to be measured. The real power of the van der Pauw technique lies in the ability to measure samples of arbitrary shape, granted the following conditions are met: 1) the contacts are on the perimeter of the sample; 2) the size of the contacts are much smaller than the size of the sample; 3) the sample is homogeneous and of uniform thickness; and 4) the sample does not contain isolated holes [61 64] Aside from meeting the listed criteria, van der Pauw involves measuring two voltages per polarity thereby requiring taking twice as long to measure the resistivity or Hall voltage [61] The main reasons for using the Hall bar method instead of the van der Pauw method are sample homogeneity and uniform thick ness. The shape of the sample is generally not an issue, as the crystals can be cleaved to be bar shaped. As will be seen later in magnetic susceptibility, EPMA, and XPS, there is a considerable degree of chemical inhomogeneity. It will also be mentioned l ater that the thinning process produces samples that can have a considerable thickness nonuniformity relative to the absolute thickness. After the samples have been mounted and the brass can attached, the probe is evacuated and placed in a dewar. Samples with compromised structural integrity, such as those that have been reacted with fluorine, are pumped on slowly to prevent pressure gradients from stripping

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30 off the platinum wires. Upon cooling the brass can to 4.2 K, the boiling point of liquid helium the vapor pressure of any residual gases in the probe drops to essentially zero due to freezing of the gases. The brass can taper joint seal is leak tight to even superfluid helium 4. An initia l cooling is done in liquid nitrogen to achieve a temperature of 77 K. This is first done because of the lower cost and greater abundance of nitrogen. The liquid nitrogen is then removed from the dewar and replaced with liquid helium to reach 4.3 K. By pum ping on the dewar, we can reach a final temperature of 1.2 K. The initial LN 2 cooling from 300 K to 77 K takes 1 1.5 hours, and the LHe cooling to 4.3 K takes an additional 30 minutes. Because the cooling is very rapid, measurements are only performed whil e slowly warming with the heater to ensure the sample and thermometer are in thermal equilibrium. Sample Preparation Growth All of our samples are grown using the flux growth method in house at the University of Florida. Flux growth is a solution growth m ethod. The solvent (also known as the flux) is heated to increase solubility of the constituents, producing a supersaturated solution. Several considerations should be taken into account: melting point, solubility, phase diagrams, and reactivity. By analyz ing binary phase diagrams, one can fine tune the ratio of components to flux and temperature for a specific compound. There are several advantages to using the flux growth method [65] It is preferred over melt growth because the vapor pressures at the melting point of some elements are unreaso nably high. With flux growth we can pick a solvent with a sufficiently low melting point, provided it has decent inter solubility with the crystal constituents There is also an added benefit of having a low melting point solvent after formation of the cry stals. By reheating the flux past its melting point, the solid crystals can be easily extracted. Compared to some of the other methods, solution

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31 growth can be done with fairly simple equipment. Solution growth gives us a great deal of control on what is be ing synthesized, whether one is trying to get congruent or incongruent melting. The two biggest issues encountered with flux growth are the lack of ternary phase diagrams and the possible incorporation of the flux into the crystal lattice [18,66,67] Ternary (and higher order) phase diagrams are almost nonexistent and most compounds of interest to this work consist of 3+ e lements. Using binary phase diagrams for the solubility of each constituent with the preferred flux choice, we can generally make educated guesses, with a little trial and error. Contamination of the lattice with foreign impurities can skew the physical p roperties of the crystal [67] To avoid this pitfall, our samples are grown using self flux [27,68] meaning the flux is part of the desired product Therefore the flux chosen for our BaFe 2 As 2 compounds was [65] This is much higher than the melting poi of FeAs is still relatively low compared to those for melt growth. Our laboratory equipment can safely go up to 1 5 issue. There is no universal agreement on the best method to synthesize BaFe 2 As 2 compounds via self flux [51,69,70] Considerations include temperature, duration, ratio, and rate of cooling We start our samples by combining high purity elements from Alfa Aesar: 99.9% Ba chunks, 99.998% Fe powder, and 99.999% As powder. They are added to an alumina cruci ble in an inert atmosphere glove box in a 1:5:5 ratio to yield BaFe 2 As 2 with an additional 3 parts FeAs for the flux. The alumina crucible is then placed in an evacuated niobium container which is arc melted shut. Some researchers prefer sealing in a quart z tube. Niobium is more durable and can

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32 withstand pressures of 100 bar but care must still be taken because of the high vapor pressure of some elements at elevated temperatures The niobium container is transferred to a Thermo Scientific Lindberg tube fu rnace to solubility in the FeAs flux. The container is held at ultra high purity argon gas is fed through the mullite tube to minimize oxidation of the niobium container. Harvesting After the crystals have formed, they need to be extracted from the flux. Some researchers prefer use of a catch crucible with wool and centrifugation of the tube at high temperature to separate the molten flux from the solid crystals. This is a benefit of the quartz tube and box furnace setup but also requires knowledge of the temperature at which the crystals form. Harvesting is where having a low melting point flux shines. It is as simple as reheating the crucible on a hot plate past the melting point and fishing the crystals out with a pair of and reseal it in a pyrex/quartz tube with some pyrex wool, reheat, and then centrifuge. The molten flux gets trapped in the wool while the c rystals rest on top. This method is not ideal for our self flux grown samples for 2 reasons: 1) the high melting point of FeAs could damage the centrifuge and 2) the melting point of BaFe 2 As 2 is not known but would likely be close to that of FeAs. Our h ar vesting self flux grown BaFe 2 As 2 involves brute force. A scalpel, sturdy pair of forceps, and a microscope are used to mechanically separate the crystals. The crystals possess shiny and smooth surfaces and can be distinguished from the dull and gritty flux This is not

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33 always the case, as tin flux can be shinier than the crystals themselves but does not possess the flat, plate like geometry associated with the tetragonal BaFe 2 As 2 crystals. Crystal sizes are typically on the order of 3 mm x 2 mm x 0.25 mm. T here are a lot of factors affecting crystal size s and shapes however, such as ratio of flux, heating and cooling rates, size and shape of vessel, and some degree of randomness The shape of the raw crystals out of the pot are almost never tetragonal. Twinning creates a large degree of unparallel edges (in addition to the stress of the harvesting procedure). Curved edges can also be seen when crystals grow up to the edge of the cylindrical crucible. More often than not, at least one straight edge is preserved (Figure 2 3) allowing us to orient our samples based off that edge. In crystals with no straight edges, boundaries can usually be seen on the surface under a microscope, and the crystal can then be cleaved accordingly. Treatment For th e purposes of reacting the samples, we had two reaction vessels made to contain the hazardous fluorine and chlorine gases. The primary focus of this thesis is on fluorination but will include sections containing chlorination along with the advantages and d isadvantages over fluorination. It was discovered early on that using the same reaction vessel for both showed fluorine contamination in our chlorinated samples, necessitating the need for separate reaction vessels. The cylindrical reaction vessel is comp lid for viewing. There are two ball valves on opposite sides of the vessel, one for evacuating the vessel and one for injecting the gas. A viton o ring is used for the seal between the lid and the vessel beca use of its strong chemical resistance. An analog pressure gauge is attached to one of the valves for monitoring the pressure Aluminum was chosen for the reaction vessel material because of the passivation that occurs from the AlF 3 layer that is created [71]

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34 The two gas mixtures used are 5% fluorine with a 95% helium balance and 3% chlorine with a 97% nitrogen balance. The vessel is first evacuated in a fume hood, then pressurized to atmosphere of pressure with a gas mixture. After the specif ied reaction time, the gasses are vented into the fume hood and the sample is carefully extracted. Fluorine provides the best results between 20 30 minutes. In this context, best results refer to the quality of the superconducting transition and the appearance of the reacted crystal. The quality of a transition is determined by the onset temperature T C the width of the transition T C and in our case whether the resistive transition goes fully to zero. Shorter exposure times, 5 10 minutes, often resu lt in samples with low T C s and incomplete transitions ( F igure 2 4 ). The longer the crystal is exposed to the fluorine the more likely the sample becomes compromised. Despite the low concentration, the aggressiveness of the fluorine causes the crystals to r apidly decay. The crystals first begin to lose luster and show discoloration even with a few minutes of exposure. The crystals then start to flake and require delicate handling. In excess of 40 minutes, the integrity of the crystals is greatly compromised, and crystals are likely to crumble to pieces. In our experiences, in rare cases, virgin BaFe 2 As 2 can be superconducting as grown, but this is usually a few crystals in an entire batch or about 3 4 % of all measured crystals Because of a false positive w e came across early on, we measure the resistivity prior to fluorination to ensure any changes observed are due to the fluorination. The fluorine does not discriminate and will also attack the silver epoxy. The platinum wires are highly resistant, but the epoxy readily degrades. To circumvent this, for sample s that are to be fluorinated we will apply a larger volume of epoxy to buffer the damage caused by the fluorine to where the sample contacts are made. Even then, sometimes the leads do not survive and r equire reattachment.

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35 Chlorine is significantly less reactive than fluorine for presumably 2 reasons: lower concentration and less electronegative. Unlike fluorination which is carried out at room temperature, chlorination requires heating the sample on a hot plate for the duration of the treatment. We found success in inducing superconductivity with the chlorine gas mixture heated Thinning The finite depth of penetration of fluorine (discussed later to be 5 10 m) causes issues in certain measurements because this is a very small fraction of the full sample thickness that can be as large as a few hundred microns Therefore, in order to focus on the properties of the fluorinated layer rather than the bulk, we oft en need to thin the sample down to the point where the two are comparable (e.g. < 25 m) Polishing the thickness of a 3 mm x 2 mm crystal down to 5 10 m is difficult and relies heavily on luck. First and foremost, v irgin BaFe 2 As 2 crystals are brittle an d micaceous. Fluorination further affects the compound and makes it even more fragile. Therefore, the first step is to glue the crystal down to a flat pin to provide structural support. Secondly, we are at the mercy of how the crystal forms. Despite the te ndency of crystals to form according to their structure, in this case tetragonal, in practice it is difficult to get crystals with parallel surfaces, some reasons being during growth (e.g. twinning, dislocations, etc.) and others imparted by the mechanical stress of the harvesting procedure (e.g. cracking, bending, etc.). We can remedy this to some extent by finding a single flat surface and glueing it down to the pin while the exposed, non parallel surface receives a pre polishing to be parallel with the b ottom surface. The choice of glue also plays a large part As glue dries and solidifies, it has a tendency to shift, which in turn causes shifting of the sample. This can be mostly mitigated by firmly pressing the sample to the pin until the stycast fully hardens. While curing the epoxy for the

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36 resistivity contacts, we did notice the stycast can still slightly shift from the heat. Among the various glues we tested ( DUCO cement, a Gorilla glue two component non conductive epoxy stycast 1266, stycast 2850 F T ), the stycast s appeared to shift the least with drying and heating. DUCO cement also produced another concern: as it solidified, bubbles would form, leading to the concern of how porous the glue was and whether or not the fluorine would find access and m ake contact with the bottom surface and edges of the sample The pin is then inserted into the lapping tool ( F igure 2 5 ) that is specifically designed to reduce tilting by fixing the orientation of the sample but on such small scales tilting cannot be co mpletely eliminated. The weighted lapping tool is inverted onto a sheet of 2000 grit sandpaper. The wide, flat base forces the piston and pin to remain parallel (as much as possible) with the sandpaper. This method preserves a flat and uniform surface over the more conventional polishing wheel method. The stycast will unavoidably form a layer of non negligible thickness between the sample and the pin, so the crystal thickness is measured by a Mitutoyo Absolute digital height gauge with 1 m resolution and 3 m accuracy prior to being glued down. The thickness is then determined by subtraction of the amount removed, shining light on the urgency for the starting crystal to initially possess two flat, parallel surfaces Any pre polishing thus ge ts included as another source of error. Annealing Annealing is a technique commonly employed by researchers to improve certain properties of a material. It is a process involving the heating of a material and subsequent cooling. It was previously mentione d that the Pt wire contact leads are annealed to alleviate stress in the wires making it easier to solder to the pins on the probe. This is especially important

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37 in the context of fluorinated samples, where the stress in stiff, unannealed wires can easily b reak the contact between the fluorinated epoxy and the fluorinated crystal surface. Annealing has shown success in improving the quality of superconductors. Specifically regarding 122s, researchers have found increased residual resistivity ratio [72] (indicative of better lattice ordering) increased T C onset, and decreased T C [38,52,73] In those cases, annealing is done at 700 We have made attempts to improve the tran sitions (namely sharpening the transition widths) by extending the annealing process to our fluorinated samples. Several graphs are presented to elucidate the varying degrees to which annealing can improve a sample. Taking the silver epoxy into considerati on, we opted to initially annea l 800 The first example chosen is shown in F igure 2 6 Annealing was done for 8 hours at 100 C onset by 3 K while reducing T C by a f actor of 2. The second example refers back to F igure 2 4 The first four hours of annealing shows considerable improvement but almost no change beyond eight hours. In one sample we tested annealing at a higher temperature of 200 F igure 2 7 It is clear that 30 minutes of fluorina tion produced a superconductivity remain aside f ro m a very slight downturn near 17 K. An additional test was lowed by another 24 hours ( F igure 2 8 ). This allowed us to conclude that disappearance of the superconductivity in the sample in F igure 2 7 was the correct choice.

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38 Ultimate ly, we decided to focus on results from unannealed samples since we believe annealing affects the surface morphology and depth of fluorine penetration, both questions we attempt to answer. It is important to note that the curing temperature for the epoxy i s comparable to that of the annealing temperature ideal to these samples. In cases where the epoxy for the contacts does not survive the fluorination and leads must be reattached, we must keep in mind that any changes can be due to this brief (15 minute)

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39 Figure 2 1: The copper block of the probe. Stycast is used for thermal conduction, electrical insulation, and permanent attachment between the block, pins, and heater. The thermometer is slotted into a bore on the left an d thermally anchored with Wakefield grease. Dental floss is used to strap down the excess length of the copper and manganin wires. The flat space between the thermometer end and the pins is used for mounting the aluminum pin for Hall effect measurements.

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40 Figure 2 2 : Diagram of the 5 probe Hall E ffect setup The two positive voltage terminals are strategically placed as far apart from each other and as close to the current leads as possible to ensure the negative voltage terminal is enclosed. Figure 2 3: BaFe 2 As 2 crystal shown on 0.5 cm x 0.5 cm grid paper. Originally an XRD sample, the underlying polycarbonate diffractometer slide can be seen, along with a small jut on the bottom edge from the stycast. Two edges (left and bottom) are preserved from the crystal symmetry but the top edge was used for more spacing between resistance leads.

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41 Figure 2 4 : Incomplete transition from insufficient (10 minutes) fluorine exposure. The t ransition is very broad and does not go all the way to zero. Suspiciously enough, T C onset remains 22 K.

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42 Figure 2 5 : The lapping device: removable pin, piston, and base. The top hole allows for a set screw while the bottom one allows pressurized air to be used to eject the pin. The sample is glued to the top of the pin, and the system is inverted for polishing.

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43 Figure 2 6 : Comparison of the drastically improved transition between unannealed (black squares) and annealed (red circles) Figure 2 7 : Disappearance of superconductivity from annealing thermal energy is believed to drive out the fluorine impurities.

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44 Figure 2 8 : Extended annealing at 100 C Two 24 hour annealing treatments were performed on a fluorinated crystal. Improvement is saturated after the first 24 hours (red circles) with no further improvement after an additional 24 hours (blue triangles).

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45 CHAPTER 3 RESULTS AND DISCUSSION Effects of Fluorine Electron Transport The resistivity of undoped BaFe 2 As 2 monoto nically decreases with decreasing temperature and is differentiable everywhere except at the overlapping T S and T SDW transitions at 138 K BaFe 2 As 2 is a metal, so at higher temperatures, there is an enhanced density of thermal excitations (i.e. increased e lectron speed and density of lattice vibrations) leading to a larger scattering rate and therefore a higher resistivity. Below T SDW the system undergoes a long range stripe magnetic ordering, and simultaneous tetragonal orthorhombic structural transition. It is believed that the large, sudden drop of resistivity is due to the magnetic ordering that suppresses electronic scattering, overcompensating the effects of decreased carrier density The SrFe 2 As 2 system follows the same prescription except the conjoined transitions appear much higher at 205 K [74,75] The brief post growth reaction of the crystals with 5% F gas causes a 22 K 2 K onset transition temperature into the superconducting state ( F ig ure 3 1 ). The quality of the transition (i.e. T C C and lowest resistance value achieved below T C ) was most affected by duration of fluorination but also varied from sample to sample. The resist ivity curves shown in Figure 3 1 were measured on a thin n ed sample (i.e. thickness is on the order of the depth of fl uorine penetration ). The foreign fluorine atoms act as an impurity introduced to the lattice, producing scattering sites which in turn raise the normal state resist ivity Therein lies the importance of a thin sample: current will always find the least resi stive path so in a thick crystal the current will mostly bypass the fluorinated layer and return nearly the same resist ivity as an unfluorinated sample. A closer examination of the

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46 structural/magnetic transition (on a different sample) shows an increase of nearly 2 K with fluorination ( F i gure 3 2 ). This is in stark contrast with the typical magnetic suppression observed prior to the emergence of superconductivity, and there is certainly no splitting of the transitions. The persistence of the magnetic state in the fluorinated layer can also be observed by analyzing the ratio of the resistivity just above T SDW to that just above T C The resistivity ratios are the same before and after fluorination implying no suppression of magnetism (i.e. if magnetism is supp ressed, we would expect to see an effect in the magnetic region of the resistivity). Hall effect was then performed using the 5 probe setup mentioned previously to determine the effects on carrier concentration. The same sample was measured before and afte r fluorination to ensure the results were not skewed by uncertainties in the sample dimensions. The probe was held at 149 K to prevent influence from the various transitions that occur in this material. A thin sample was used to avoid results being dominat ed by the unfluorinated bulk. A 10 T magnetic field was applied perpendicular to the plane of the sample (out of the page in F igure 2 2 ). We observe an increase in Hall coefficient from 1.8 x10 9 m 3 /C (in agreement with literature [33,34] ) to 4.1 x 10 10 m 3 /C corresponding to an increase in the electron carrier concentration (in a single band model) by a factor 4.4. Magnetic Susceptibility The presence of a superconducting transition in the resistivity could be due to filaments. To test the extent and dimensionality of the superconductivity one must resort to a more powerful test. Zero field cooled magnetic sus ceptibility was performed on a fluorinated sample in a SQUID (superconducting quantum interference device) magnetometer. A weak canning effect appears near 7 K (much lower that the T C from resist ance data) with 20 Oe of applied field and is rapidly suppres sed with increasing field ( F igure 3 3 ). With as little as 200 Oe (extremely low for 122 compounds [52,76,77] ) the transition is fully suppressed. We therefore conclude

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47 that H C2 in our samples is around 200 Oe. The fact that canning is present conclusively means that the superconductivity extends beyond filaments. The lack of bulk superconductivity that could be measured by field cooling implies that there is limited penetration of fluorine into the crystals. The extent of the two dimensionality hinges on the thickness of the fluorinate layer, which will be determined by other means in a later section Surface Morphology The surface morphology of fluorinated crystals was examined by EPMA (electron probe microanalysis) and SEM (scanning electron microscopy) using an Electron Probe Microanalyzer at the Nanoscale Research Facility at the University of Florida. The voltage and current were 15 kV and 21 nA respectively. The surfaces post fluorination are presented in F ig ure 3 4 The cracks present in the SEM images (top panel) likely formed during the growth process as they are much smaller than those that would be induced from harvesting. A stacking fault can also be clearly seen near the right edge of the images. The elemental distribution is relatively homogenous for all constituents including fluorine. A second fluorinated sample measured under the same conditi ons but with half the duration of fluorine exposure, showed a drastically different fluorine distribution ( Figure 3 5 ). The second sample possessed islands with higher fluorine concentration that the surrounding regions. The fluorine inhomogeneity is like ly tied to the observation of a low critical current density (when compared to Co doped Ba122 for reasons that will become clear later) [32,7 6,78,79] low critical field (200 Oe) [52,76,77] and low diamagnetic transition temperature ( 7 K) for our samples. We define the critical current density at 4.3 K to be the current density required to restore the sample back to the normal state value of resistance. By this definition, our measured value of the critical current density is 800 A/cm 2 an order of magnitude smaller than Co doped Ba122 [32]

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48 Location of Fluorine and Extent of Penetration Predictions We produced a crystallographic rendition of a unit cell with As substitution in F igure 3 6 [80] are used for the elements since the differences in electronegativity between them are all less than 1.7 (other ionic radii might also be appropriate) In terms of size considerations, fluorine is very similar to that of iron and arsenic, with a better match to the latter. The valency of fluorine (1 ) is also of the same sign as that of arsenic (3 ) so from an electronic standpoint, substitution of fluorine for arsenic makes more sense. X R ay Diffraction Single crystal x ray diffraction was performed on multiple fluorinated crystals using 1.54 Cu k alpha radiation. The crystals are oriented with the c axis perpendicular to the x ray slide, returning only the (00 l ) reflections Since the crystal structure is body centered tet ragonal, the center barium atoms form planes with each other, effectively cutting the distance between reflection planes in half. The Miller indices are a function of reciprocal space, so half the distance in real space corresponds to a doubling in recipro cal space. Therefore only the even integer peaks undergo constructive interference and are present in the spectrum Both the fluorine and x rays have a penetration depth. To get any meaningful diffraction patterns, we must confirm that x ray depth is com parable to the fluorine depth. To estimate the penetration depth of the x rays in BaFe 2 As 2 we use the x ray attenuation equation ( 3 1 ) with ( 3 2 ) where is the density of BaFe 2 As 2 = 6.638 g/cm 3 [81] x is the distance, X n is the mass fraction, and m n is the mass attenuation coefficient for each element (values obtained

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49 from the NIST database). The mass attenuation coefficie nt is dependent on x ray wavelength and atomic number. Using equations 3 1 and 3 2 the x ray depth for a 1/e intensity attenuation is 6.6 m, very similar to the fluorine depth. The (00 l ) lines for one sample are shown in F igure 3 7 before and after fluo rination. The ( 3 3 ) and the relationship between lattice constants and d spacing for tetragonal crystal symmetry is ( 3 4 ). When this study was begun, a ny change in T C was expected to be accompanied by a change in c axis length (a historically important parameter in the discussion of 122s ) [18,82] However, i n all flu orinated scans, no c axis change is observed. This was an independent clue (besides our XPS data) that the F was not going into the lattice interstitially, where it would be expected to expand the lattice. Instead, a possible explanation for this null resu lt is due to the similarity in the sizes of F and As (compare the values in F igure 3 6 ) assuming that the F substitutes for As This marked the juncture whe n we decided to characterize our chlorinated samples further Chlorine has a larger ionic radius (1 .80 [80] ) by nature of possessing an extra electron shell. In line with this expectation, a definitive peak shift to lower angles was observed in the chlorinated single crystal BaFe 2 As 2 x ray spectrum (beam along the c axis to give only (00 l ) reflections) but only the (0014) peak (with the largest shift) is shown in F igure 3 8 By mathematical manipulation of equation 3 3 we can see that a change in d spacing will be more prominent at higher angles and is th e reasoning for our choosing of the (0014) peak. Using equations 3 3 and 3 4 the observed peak shift of after chlorination is an increase in the

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50 c axis lattice parameter of 0.027 , consistent with an expansion of the lattice we would expect by ch lorine substitution. X Ray P hotoelectron S pectroscopy X ray photoelectron spectroscopy is a surface sensitive experimental technique. It involves irradiating a material with x rays and measuring the kinetic energy of the electrons that escape. It is a mea surement that probes only the top few nanometers of a sample. The greater the kinetic energy of the electron picked up by the detector the lower the binding energy. For this reason, XPS spectra are commonly presented as a function of decreasing binding ene rgy. XPS was performed on both virgin and fluorinated crystals at the Major Analytical Instrumentation Center at the University of Florida using monochromatic aluminum radiation. The spectra were taken at various take off angles (30 4 5 and 90 ) on a 200 m diameter spot size. The power setting was 50.0 W with an electron pass energy of 93.90 eV. The XPS survey scans are presented in Figure 3 9 (virgin) and Figure 3 10 (fluorinated) along with atomic percentages The data are dominated by oxygen and carbon (moreso in the virgin sample). This carbon contamination is what spectroscopists call adventitious carbon, inevitable in nearly every air exposed sample. Adventitious carbon (c omprised of C C, C O, and C=O ), in c onjunction with oxidation explains the large presence of oxygen in the scans. The benefit of this ubiquitous contaminant is that it can be, and is, used to calibrate energy lines between different samples and compounds. For comparison of the effects of f luorine, we look to the low binding energy regime of the spectra (Figure 3 11 ), where the peaks a re produced by electrons in the valence bands of the respective elements. Therefore, the shift between the two spectra would be expected since fluorine would h ave a significant effect on the binding environment. Upon 20 minutes of fluorination, the As3d double peak is barely visible in the background. This also comes with the

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51 sudden appearance of the F2s peak. This is indicative of fluorine substituting for arse nic. We can even further infer th at in the top few atomic layers (c axis length of 1.30 nm for perspective) almost all of the arsenic has been displaced based off of the nearly nonexistent As3d peaks in the spectrum. Fluorine Depth Fluorine depth was dete rmined by successive electron transport measurements combined with careful thickness measurements after successive polishing The crystal is glued down to a flat pin using stycast with the edges also covered. This forces the reaction with fluorine to be confined to the top surface. We employ techniques outlined in the Thinning section without the restriction of starting with a thin crystal. Since the only surface being fluorinated is the top and we are only concerning ourselves with how much material must be removed for the transition to completely vanish, the crystal does not need to be thin. In order to measure the depth of the fluorine, the surface being polished must also be the surface being measured. Consequently, the electrical contacts must be removed and reattached with each successive polishing. Figure 3 12 shows the resistive transition with depth. The original crystal was fluorinated for 30 minutes, and though it shows the characteristic drop at 20 K, it does not go fully to zero. Following the removal of 5 m from the fluorinated surface, the sample exhibits a very slight drop in T C onset. The more distinguishing features are the sharper transition and a lower minimum a chieved resistance. These are likely caused by the unavoidable annealing that occurs with curing of the lead contacts at 150 C for 15 minutes Removing an additional 6 m from the surface of the sample completely eliminates any trace of superconductivity. A second sample subjected to this procedure (not shown) produced nearly identical results: almost no change with 5 m removed and no transition remaining at 10 m. From these results, we can infer that the fluorine reaches a maximum depth between 5 and 10 m.

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52 Our lack of a more precise answer is due to a number of factors. A few are already mentioned previously, such as sample inhomogeneity tilting, and shifting from the stycast and repeated reattachment of the platinum wires Th ereattachement not only a nneals the sample and presumably the distribution of the fluorine but also enables the stycast to undergo additional shifting. One of the largest sources of uncertainty comes from the 3 m accuracy of the height gauge. For these reasons our depth determina tion is confident to 3 m accuracy Analysis Equipped w ith the knowledge of where the fluorine impurities go in the lattice from XPS data we therefore shall interpret the results from that standpoint. From only the consideration of oxidations states, ars enic substitution would make the most sense, since they would both act as anions while barium and iron are cations in this compound. The superconducting transition in the resistance appears near 22 K, much like we would expect from electron doped Ba122 (i .e. optimally Co doped Ba122). This analogy still has a lot of holes though (pun unintended). Cobalt doping changes the normal state behavior and separates the structural and magnetic transitions. Fluorine doping, as we will call it, almost perfectly prese rves the normal state temperature dependence, save the slight increase in T SDW We should not expect the two situations to be identical however, since the reactions and products are not the same. Hall effect showed a significant increase in electron concentration. The direction of the change (electron doping) is consistent with fluorine substitution for arsenic, but the magnitude seems egregiously large. Fang et al. [33] and Rullier Albenque et al. [34] analyzed Co doped Ba122 from a two band model starting point. In that system, electron conduction dominates the transport and allows the analysis to be simplified down to a one band model. Unfortunately, since there is no prior knowledge and the lack of ARPES data on our system such

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53 approximations and interpretations cannot be made. All we can do is naively interpret the data from a simple one band m odel suggesting that the electron density increases. However, the increase in the magnitude by a factor of 4.4 is likely incorrect. To more accurately interpret this result, we would need more empirical data, such as ARPES measurements to see the effects o f fluorination on the Fermi suface The low critical current density, critical field, and diamagnetic susceptibility transition temperature as compared with Co doped Ba122 are reasonable in the context of the observation of fluorine congregating in islands [83] This behavior was not observed in all s amples. However, the superconducting transition in the resistance data is present among both groups (island and homogeneous), therefore the congregation is not a requirement for T C so we do not consider it a defining property of fluorine induced supercond uctivity. The location of the displaced arsenic atoms is still under investigation. No mass gain or loss has been observed in reaction of a single crystal to fluorine. The small degree to which fluorine penetrates the surface makes such minute mass differences difficult to measure. Additional Work Throughout the course of our study, we experimented with several other strongly electronegative elements and polar molecules to try to produce superconductivity in BaFe 2 As 2 An earlier student [16 ] found semi reliable success heavily reacting Ba122 with water vapor by holding crystals over a vat of boiling water leading to an unrecognizable deeply pitted and corrugated crystal surface. We tried more mild, control led conditions by plac ing a crystal and a beaker of water in a sealed reaction vessel (before we had designated fluorine and chlorine vessels). The reaction vessel was placed on a hotplate and set to a fixed temperature. Aside from rendering the surface

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54 unmeasurable with very l arge contact resistance (on the order of kilohms), we were unable to obtain any successful results. Other unsuccessful chemicals we experimented with were bromine, iodine, phosphorus, sulfur, antimony, pyridine (C 5 H 5 N), ethanol, glacial acetic acid, and nitric acid. A large number of these reactants dissolved the Ba122 crystals with bromine being the most aggressive, disintegrating the crystals within a few minutes. Most of the reactions were carried out in a fume hood. Of special consideration are the liquids: the acids for obvious reasons, bromine for the tendency to readily evaporate at room temperature, and ethanol and pyridine because of the high flammability. The solids were sealed in a quartz/pyrex tube with crystals and hea ted to various temperatures (as high as 900 C) depending on the vapor pressure of the substance, in a box furnace The most notable reactant was phosphorus. One sample reacted with phosphorus produced a superconducting transition. Roughly 30 samples late r without being able to produce the same effect, we concluded that this was a false positive due to a randomly superconducting crystal sans reaction, as mentioned in the treatment section. y successful results with was chlorine. Chlorinated crystals were not subjected to the extensive study we performed on fluorinated crystals. The main reason for this was because of the long reaction times needed. To induce a superconducting transition comp arable to a 20 minute, room temperature fluorination, a 74 hour, 100 C reaction was required (Figure 3 13). The success rate of a crystal surviving the chlorination was about 75 80%, lower than with fluorination but also based on a much smaller number of samples For time considerations, we elected to use fluorine except in cases where fluorine did not reveal anything, such as in XRD.

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55 Lastly, since the original unusual superconductivity discovery was on SrFe 2 As 2 the picture would be incomplete without sho wing that our reaction method induces a superconducting transition in Sr122 also. Figure 3 14 shows a superconducting transition in resistance vs temperature on a Sr122 single crystal. The T C onset in this sample is 23.5 K, comparable to the 20 K T C found in optimally Co doped Sr122 (i.e. Sr(Fe .9 Co .1 ) 2 As 2 ) [84] Interpretation of Literature Our 22 K superconducting BaFe 2 As 2 mirrors that of all the other cases of unusual superconductivity in 122s. Despite the f act that they are all from the same family of superconductors, the methods for causing superconductivity are very different (i.e. water vapor, In flux) Regardless of what the method is though, one feature remains consistent across our samples and those by which our study was motivated by : T C is always 22 K. Looking beyond the possibility of this being just coincidental, it is reasonable that in fact the mechanism is indeed the same. Armed with our result of electron doping, we will go back and test th e p lausibility of electron doping in their studies. 2 As 2 [51] appears to conclusively be due to strain. Their ability to reversibly transition between the normal state and superconducting state by he ating and applying pressure needs no further discussion. The sensitivity of samples to growth conditions is well known (but not well understood). Therefore, the most likely explanation is, as they claim, strain induced during the ir growth process. Hiramats u et al. induced 25 K superconductivity in SrFe 2 As 2 thin films [53] by exposure to water vapor. They had two predictions on where the components (i.e. H 2 O, O 2 OH etc.) could have gone in conside ration to the sizes of the interstitials but neither of which agreed with the decreased c axis length they observed. The polar nature of H 2 O lends itself well to an electron donor theory. There is still however the inexplicable c axis shrinkage. We were a ble to

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56 eliminate chemical pressure in our samples because of the similarity in size between As and F as evidenced by our XRD data. We do observe an expansion of the lattice (opposite of chemical pressure) in our chlorine substitution, but the root of the s uperconductivity remains electron samples, but that does not rule out our electron donor theory. Building off the work of Hiramatsu et al., Kim et al. [16] further tested the extent of the phenomenon of water vapor but instead on BaFe 2 As 2 single crystals. They found T C onset to be 21 2 K in their samples with 50% success rate. They extend their results to fit Kosterlitz Thouless theory, but that is beyond the scope of this paper. As an aside, we considered KT theory for our samples also until we discovered the superconducting layer in our samples to span several microns. All ar guments in the previous paragraph hold true here also since both reports use water vapor as the reactant. Deviating from water vapor, we turn our attention to In flux grown BaFe 2 As 2 by Kim et al. [52] with a T C onset varying from 19 23 K. Their ability to improve T C by annealing rules out strain. It is known that fluxes can get incorporated into the crystal lattice during growth. If this were the case, the indium atoms could give up electrons to achieve its preferred oxidation state of 3 + which would result in electron doping. Since this occurs during g rowth, they should have observed more of a bulk effect rather than the surface sensitivity and small current density that were seen much like in our samples Another IBS we would like to draw comparisons with is FeTe 0.8 S 0.2 Deguchi et al. showed that by immersing non superconducting as grown FeTe 0.8 S 0.2 (recall from the introduction that this compound can be superconducting based off growth conditions) in alcoholic beverages that they could induce 9 K superconductivity. At first glance

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57 fo llow the mold of producing a 22 K superconductor We must do an in group comparison to notice the similarities. In 122s, no matter how superconductivity is induced, a 22 3 K superconductor is produced, where the T C is comparable to that which would b e expected from Ba(Sr)Fe 2 As 2 when doped with the optimal amount of cobalt Similarly, the 9 K 10 K found in the superconducting as grown samples with compositions ranging from 10 30% s ulfur [55,56] They also suspect the cause to be charge donation, except by intercalation rather than doping.

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58 Figure 3 1 : Resisti vity transition pre (black squares) and post (red circles) fluorination on a thinned BaFe 2 As 2 crystal The fluorine impurities cause an increase in normal state resistivity and induce a superconducting transition at 22 K. Figure 3 2 : Overlapping m agnetic and structural transitions of a thinned crystal. T SDW increases by 2 K after fluorination and no splitting occurs.

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59 Figure 3 3 : Molar magnetic susceptibility as a function of field. A strong diamagnetic signal is present below 7 K for fields up to 150 Oe. A larger crystal was used with a longer exposure time to allow for adequate fluorination. ( Note: emu is a cgs unit whose definition changes depending on the quantity of interest. For magnetic susceptibility it is equivale nt to cm 3 [85] .)

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60 Figure 3 4 : Surface morphology of a reacted crystal. SEM images taken from secondary electrons (top left panel) and backscattered electrons (top right panel). EPMA reveals no large elemental inhomogeneities in the sample surfaces (bottom four panels).

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61 Figure 3 5 : Fluorine atomic percentage from E PMA on a sample exposed for 20 minutes. Distinct isolated regions possessing a higher fluorine concentration can be seen. Figure 3 6 : Unit cell of Ba122 with fluorine substitution on the arsenic site. The ionic radii listed in the legend are from Shan non [80]

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62 Figure 3 7 : Single crystal XRD spectrum of the (00 l ) lines No discernable shift is observed between the the two sets of data. Figure 3 8 : High angle single crystal XRD on a chlorinated crystal. The larger of the two (0014) peaks is shown. The re is a visible shift to lower angle represent ing an increase in lattice size.

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63 Figure 3 9 : Virgin BaFe 2 As 2 s urvey scan at a 90 degree take off angle Several impurities are present such as oxygen, carbon, and silicon.

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64 Figure 3 10 : Fluorinated BaFe 2 As 2 survey scan at a 90 degree take off angle A large drop occurs in the amount of carbon present and can be seen in both the relative atomic percentage and a diminished background to the immediate left of the C1s peak that would be caused by C1s electrons deeper in the surface.

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65 Figure 3 11 : Low binding energy XPS data of virgin (blue) and fluorinated (red) showing almost no trace of the As3d peaks and the sudden appearance of the F2s peak. All peaks are shifted to the left by the affects of fluorine on the binding environment. Figure 3 12 : F luorine depth penetration determined from R vs T Removal of 5 m (red dots ) shows a slight improvement in the transition (see text for discussion) and 11 m (green triangles ) restores the normal state behavior.

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66 Figure 3 13: Timeline of successive chlorine treatments to induce a complete superconducting transition. 22 K. There is almost no change in the normal state resistance between the virgin and all exposures up to 50 hours, followed by a drop at 74 hours. The increase in normal state resistance brought about by the introduction of chlorine impurities with each If chlorine replacement saturates after 50 hours only the latter effect would remain, leading to the lowering of the normal state resistance at 74 hours.

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67 Figure 3 14: Resistance of SrFe 2 As 2 reacted with fluorine. The structura l and magnetic transitions occur near 200 K with T C

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68 CHAPTER 4 CONCLUSIONS We have attempted to solve a long standing mystery which has plagued researchers in the field of IBS: what causes some 122 structure samples to superconduct without app arent doping? 2 K? This question was first raised by Hiramatsu et al. [53] in 2009, when they reported th is approximate same T C in thin film samples of SrFe 2 As 2 simply exposed to air (and the concomitant water vapor) for 2, 4, and 6 hours. Hiramatsu et al. at that time offered no microscopic explanation of the effect of the exposure to air, other than some speculations that the In later work, Saha et al. [51] and Kim et al. [52] occurrence of T C in SrFe 2 As 2 and BaFe 2 As 2 at T C = 21 and 22.5 K respectively, again with no conclusive microscopic model. Saha et al. argued for strain, which contradicted the results of Kim et al. Based on the somewhat aggressive reaction of water vapor (Hiramatsu et al. [53] and Kim, et al. [16] ) with the 122 materials, we decided to investigate exposure of the 122 parent compounds (particularly BaFe 2 As 2 but also SrFe 2 As 2 ) to the halide elements, F, Cl, Br, and I, reproducibly induce superconductivity in the 122 BaFe 2 As 2 with only 20 minute exposure at room temperature to a gas mixture containing 5% F. Similar results, although not so thoroughly studied in this work, came with exposure to 3% Cl for several days at 100 C Further, our studies rather definitively indicate that the microscopic effect of exposure to F gas is to replace As with F, with no change in the c axis lattice parameter (arguing against the chemical pressure argument of Hiramatsu et al. [53] ) X ray photoemission data made clear that F replaces As,

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69 while Hall e ffect measurements were consistent with electron doping caused by the extra F electron compared to As. The actual cause of the superconductivity in terms of the generation of a microscopic mechanism still however remains open. Trying to fit the analogy of electron doping of Co in BaFe 2 As 2 to our present work runs into the dilemma that Co doping gradually decreases the spin density wave transition to T = 0 at about 8% replacement of Fe by Co (i. e. Ba(Fe 0.92 Co 0.08 ) 2 As 2 ) creating a superconducting dome with a maximum T C of ~ 22 K at 8% Co [25] However, T C begins to rise above 0 for much lower (~ 4%) C o doping, whereas F apparently regardless of concentration induces only one value of T C the aforementioned T C = 22 K. Further, Co suppresses T SDW and F either has no effect or slightly (~ 2 K) enhances it. Thus, this thesis presents only a partial so puzzle in the 122 structure parent compounds BaFe 2 As 2 and SrFe 2 As 2 Why the T C 22 K awaits ARPES data and further microscopic calculation of the band structure. Interestingly, our F doping results tend to argue against the idea common in the IBS for the cause of superconductivity, namely that doping suppresses T SDW to T = 0 and causes a quantum critical point, which generates spin fluctuations and unconventionally pairing superconductivity. In our F results, T SDW apparently (in thinned samples where the resistivity is mainly sampling F affected sample) stays far away from T=0. Further testing is currently being conducted. If both fluorine and chlorine are acting as electron donors, we should be ab le to induce superconductivity using the prototypical electron donor, hydrogen. We are attempting to crack hydrogen to separate the diatomic molecule to pass into BaFe 2 As 2 crystals. Due to the small size of hydrogen, we expect it to settle into an interstitial site and donate its electron, thereby producing the same effect.

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70 APPENDIX DELTA METHOD CALCULATION Disclaimer: The following calculation is carried out in [86] albeit with a few sign errors. We reproduce the calculations below with the proper correctio ns. The basis of this method involves current reversal to negate the thermoelectric offset voltage in conjunction with a short sweep time so the thermoelectric drift can be linearly approximated (Figure A 1) where V Mn are measured voltages, V n are the true voltages, V EMF is the constant thermoelectric voltage offset taken at the time of the first measurement, V is the amount of drift in thermoelectric voltage between measureme nts. We define two intermediate variables: Averaging the two gives us r esulting in a cancelation of both the offset and drift voltages.

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71 Figure A 1. An example of the delta method. Three measurements are used to produce one true voltage data point. The figure has been adapted and modified for clarity from [86]

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80 BIOGRAPHICAL SKETCH Gordon received a Bachelor of Science degree in physics with a minor in mathematics from the University of Nevada Las Vegas in 2009. In 2012 he received a Master of Science degree in physics from the University of Florida followed by a Doctor of Philosophy degree in 2019.