We study the effect of a single non-magnetic impurity and vortex core states in iron-selenide superconductors by solving the Bogoliubov-de Gennes equations self-consistently based on a three-orbital model. Various pairing symmetry are considered in the calculation. The impurity-induced in-gap bound states are found only for attractive impurity scattering potential, as in the cases of doping of Co or Ni, which is characterized by the strong particle-hole asymmetry, in the nodeless d_(x^2-y^2 ) wave pairing state. This property may be used to probe the pairing symmetry of iron-selenide 122-type superconductors. The orbital-resolved vortex core states of different pairing symmetries manifest themselves as distinguishable structures due to different behavior of the quasiparticle wavefunctions. The obtained vortices are classified by the invariant subgroups of the symmetry group of the meanfield Hamiltonian in the presence of magnetic field as isotropic s- and s±-wave vortices have G_5 symmetry for each orbital, whereas d_(x^2-y^2 ) wave vortices show G(* )¦6 symmetry for d_xzand d_yz orbitals and G(* )¦5 symmetry for d_xy orbital. In the case of d_(x^2-y^2 ) wave vortices, hybridized-pairing between d_xzand d_yz orbitals gives rise to a relative phase difference in terms of winding structures of vortices between these two orbitals and d_xy orbital, which is essentially caused by a transformation of co-representation of G(* )¦5 and G(* )¦6 subgroup. Calculation of particle densities show common charging feature of vortices in the cases of s± and d_(x^2-y^2 ) wave pairing states where the electron-like vortices are observed for d_xz and d_yz orbitals while hole-like vortices for d_xyorbital. The phase difference of orbital-resolved d_(x^2-y^2 ) wave vortices and their charging effects can be verified by further experimental observations. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Iron-based high temperature superconductors are a large family of materials that exhibit unconventional superconductivity and arise from antiferromagnetically-ordered parent compounds. One of the grand challenges in understanding the behavior of these materials is determining the physical mechanisms responsible for the transition into the superconducting state. This thesis describes two recent investigations to explore the magnetic and superconducting properties of NaFeAs in response to changes in temperature and nickel dopants. The peculiar interplay of magnetism and superconductivity in nickel-doped NaFeAs is elucidated using muon spin rotation. Our experimental findings on this novel system are supported with both computational and theoretical calculations. The second investigation describes an improvement to the analysis framework to the scanning tunneling microscopy technique that leverages recent advances in nonconvex optimization. This novel approach is applied directly to microscopy images of NaFeAs to provide unprecedented phase-sensitive access to the quasiparticle scattering spectrum in the material. These results place constraints on theoretical models that describe the local electronic structure and physics of NaFeAs.

Lai, Kwing To = 鐵基超導體的合成與分析 : 氟及釔合摻SmFeAsO / 黎烱韜. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references. / Abstracts in English and Chinese. / Lai, Kwing To = Tie ji chao dao ti de he cheng yu fen xi : fu ji yi he shan SmFeAsO / Li Jiongtao. / Abstract --- p.i / 摘要 --- p.iii / Acknowledgements --- p.v / Table of contents --- p.vii / List of table captions --- p.x / List of figure captions --- p.xi / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Superconductivity --- p.1 / Chapter 1.1.1 --- Physical properties --- p.3 / Chapter 1.1.1.1 --- Zero resistance --- p.3 / Chapter 1.1.1.2 --- Meissner effect --- p.4 / Chapter 1.1.1.3 --- Josephson effect --- p.5 / Chapter 1.1.2 --- Type I and II superconductivity --- p.7 / Chapter 1.2 --- Iron-based superconductors --- p.7 / Chapter 1.2.1 --- Classifications and structures --- p.8 / Chapter 1.2.2 --- Physical properties --- p.12 / Chapter 1.2.3 --- Iron-based superconductors versus Cuprate superconductors --- p.13 / Chapter 1.2.4 --- Correlation between high-Tc superconductivity and magnetism --- p.14 / Chapter 1.2.5 --- Quantum phase transition by doping --- p.16 / Chapter 1.2.6 --- Sample preparation of undoped and doped iron-based superconductors --- p.17 / Chapter 1.3 --- Objectives of this research project --- p.18 / References --- p.20 / Chapter Chapter 2 --- Literature Review --- p.22 / Chapter 2.1 --- Theories of superconductivity --- p.22 / Chapter 2.1.1 --- London equation --- p.22 / Chapter 2.1.2 --- Ginzburg-Landau theory --- p.24 / Chapter 2.1.2.1 --- Mean-field theory and Landau theory --- p.24 / Chapter 2.1.2.2 --- Spatial varying order parameter and Gauge symmetry --- p.27 / Chapter 2.1.2.3 --- Applications --- p.31 / Chapter 2.1.3 --- BCS theory --- p.35 / Chapter 2.2 --- Magnetism in condensed matters --- p.39 / Chapter 2.2.1 --- Ferromagnetism and Antiferromagnetism from local moments --- p.40 / Chapter 2.2.1.1 --- Mathematical explanation in the mean-field approach --- p.40 / Chapter 2.2.1.2 --- Exchange interaction --- p.43 / Chapter 2.2.2 --- Antiferromagnetism in magnetic metals: Spin density wave --- p.45 / References --- p.52 / Chapter Chapter 3 --- Methodology and Instrumentation --- p.53 / Chapter 3.1 --- Sample preparation --- p.53 / Chapter 3.2 --- Characterization --- p.55 / Chapter 3.2.1 --- Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDX) --- p.55 / Chapter 3.2.2 --- Transmission electron microscopy (TEM) and Electron diffraction --- p.55 / Chapter 3.2.3 --- X-ray diffraction (XRD) --- p.56 / Chapter 3.2.4 --- X-ray photoelectron spectroscopy (XPS) --- p.57 / Chapter 3.2.5 --- Physical properties measuring system (PPMS) --- p.58 / Chapter 3.2.5.1 --- Transport properties --- p.58 / Chapter 3.2.5.2 --- Magnetic properties --- p.60 / Chapter 3.2.6 --- Raman spectroscopy --- p.62 / Chapter 3.3 --- Precautions --- p.62 / References --- p.63 / Chapter Chapter 4 --- Results --- p.64 / Chapter 4.1 --- Fluorine-doped SmFeAsO --- p.64 / Chapter 4.1.1 --- Morphologies and microstructures --- p.67 / Chapter 4.1.2 --- Phase and composition --- p.69 / Chapter 4.1.3 --- Lattice constants --- p.72 / Chapter 4.1.4 --- Transport properties --- p.73 / Chapter 4.1.5 --- Magnetic properties --- p.74 / Chapter 4.2 --- Fluorine and Yttrium co-doped SmFeAsO --- p.78 / Chapter 4.2.1 --- Morphologies and microstructures --- p.79 / Chapter 4.2.2 --- Phase and composition --- p.82 / Chapter 4.2.3 --- Lattice constants --- p.84 / Chapter 4.2.4 --- Oxidation state --- p.85 / Chapter 4.2.5 --- Transport properties --- p.88 / Chapter 4.2.6 --- Magnetic properties --- p.89 / Chapter 4.3 --- Quality control of the superconducting products --- p.94 / Chapter 4.3.1 --- Intermediate product SmAs --- p.95 / Chapter 4.3.2 --- Intermediate product FeAs --- p.104 / Chapter 4.3.3 --- Effects of annealing temperature --- p.110 / Chapter 4.4 --- Summary --- p.112 / References --- p.112 / Chapter Chapter 5 --- Discussions --- p.113 / Chapter 5.1 --- Effects of F doping --- p.113 / Chapter 5.2 --- Effects of YF doping --- p.115 / Chapter 5.3 --- Improvements in the quality of the superconducting samples --- p.117 / Chapter 5.4 --- Summary --- p.124 / References --- p.125 / Chapter Chapter 6 --- Conclusions and Suggestions for Future work --- p.126 / Chapter 6.1 --- Conclusions --- p.'126 / Chapter 6.2 --- Suggestions for future work --- p.128 / References --- p.130 / Chapter Appendix A --- Sealing samples in small evacuated silica capsules for DTA measurements --- p.131 / Chapter Appendix B --- AC susceptibility measurement --- p.133 / Chapter Appendix C --- Suggested readings for beginners --- p.135 / Bibliography --- p.138

Thermal conductivity measurements are made on a variety of systems in order to probe low energy quasiparticle excitations. In particular, thermal conductivity measurements were made on the iron based superconducting material LaFePO at temperatures from 60 mK to 1 K and in fields from 0 T to 5 T in order to shed light on the symmetry of the superconducting order parameter. A substantial non-zero electronic contribution to the thermal conductivity is observed and interpreted as sub-gap electronic quasiparticles which is clear evidence for a nodal gap symmetry. A high scattering rate and non-T3 temperature dependence of the conductivity is evidence against the d-wave scenario. However, the field dependence does seem to suggest that the anisotropic s+- picture is a likely candidate for the order parameter, although more theoretical work is required to confirm this. Thermal conductivity measurements were also made on the spin-ice system Ho2Ti2O7 between 50 mK and 1.4 K in applied magnetic fields from 0 T to 8 T in an attempt to observe the much debated magnetic monopole-like quasiparticles. An applied magnetic field of 8 T was applied along to [111] direction as to fully polarize the magnetic moments in order to extract the phonon contribution of the thermal conductivity. The low field thermal conductivity reveals evidence for an additional heat transfer mechanism that also scatters phonons which is magnetic in nature. This is taken to be evidence for the existence of monopole-like excitations out of the spin-ice ground state and is described by existing Debye-Huckel theory. Thermal transport was used in conjunction with charge conductivity to study the unconventional quantum critical point (QCP) in the heavy-Fermion superconductor beta-YbAlB4 at temperatures down to 60 mK and in fields up to 2 T. The results show that the Wiedemann-Franz law (WFL) is obeyed down to the lowest measured temperatures indicating that the Landau quasiparticles remain intact near the QCP. A small suppression of the Wiedemann-Franz ratio (L/L0 = kappa / sigma T L0) is seen at finite temperatures (T < 1 K) with minimal dependence on magnetic field. Comparing with other similar quantum critical systems, it becomes apparent that inelastic scattering events have little effect on the transport and are mainly field independent in beta-YbAlB4. An overview of the design for a new thermal conductivity mount is also presented. The design hinges around the idea of building the experiment mount into a small copper box rather than on an open frame. Not only does this provide mechanical stability for safe transportation, it also reduces the noise caused by electromagnetic interference (EMI) in the sample thermometers by more than a factor of ten over the old wire frame design.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第23452号 / 理博第4746号 / 新制||理||1680(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 松田 祐司, 教授 石田 憲二, 教授 柳瀬 陽一 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

This thesis describes inelastic neutron scattering (INS) experiments on several iron-based materials. The experiments were primarily designed to investigate the link between magnetic dynamics and superconductivity. The work contributes to evidence that magnetic fluctuations influence or are influenced by superconductivity. It is demonstrated that the INS response of a material, in conjunction with theoretical models, can provide valuable information about both superconductivity and magnetism. I measured the magnetically ordered parent-compound SrFe2As2 to investigate the nature of magnetism in iron-based systems. Comparison of the data to models based on both itinerant and localised magnetism showed that an itinerant model offers the best description of the data. LiFeAs is a superconductor that shows no magnetic order, however I was able to distinguish a magnetic signal in its INS spectrum. The signal is consistent with the magnetic resonance observed in several other iron-based superconductors. This indicates that LiFeAs likely hosts an s± gap symmetry. I investigated two iron-phosphide systems, LaFePO and Sr2ScO3FeP, and in this case I was unable to identify any magnetic scattering. Comparison to LiFeAs showed that any signal in LaFePO is at least 7 times weaker. These results suggest that magnetic fluctuations are not as influential to the electronic properties of iron-phosphide systems as they are in other iron-based superconductors. In CsxFe2−ySe2 I found two independent signals that appear to be related to phase-separated magnetic and superconducting regions of the sample. I showed that fluctuations associated with the magnetically ordered phase are consistent with localised magnetism, and do not respond to superconductivity. The second signal, however, increases in intensity below the superconducting transition temperature Tc = 27K, consistent with a magnetic resonance. This could be indicative of a pairing symmetry in CsxFe2−ySe2 that is distinct from most other iron-based superconductors. Finally, the molecular intercalated FeSe compound Li0.6(ND2)0.2(ND3)0.8Fe2Se2 revealed strong magnetic fluctuations. Again the signal was consistent with a magnetic resonance responding to Tc = 43 K. The results suggest that Lix(ND2)y(ND3)1−yFe2Se2 is similar to the superconducting phase of CsxFe2−ySe2, placing constraints on theoretical models to describe the molecular intercalated FeSe compounds.

The newly discovered iron based superconductors have captivated the attention of the scientific community due to the unusual mechanism behind their superconductivity and their promise as the next generation high temperature superconductors. After a century of superconductor research, the physical mechanism behind high temperature superconductivity is still not understood. These new materials bring renewed hope in elucidating the pairing mechanism responsible with high temperature superconductors and achieving the ultimate goal of the field, room temperature superconductivity. Consequently, a deeper understanding of the intriguing properties of iron based materials is essential. A great deal about the pairing mechanism of Cooper electron pairs can be inferred from the symmetry of their pairing wave function or order parameter. One of the most involved probes for studying the pairing symmetry is the London penetration depth. The low temperature behavior of London penetration depth in superconductors is directly related to the density of states and provides a powerful tool for investigating low-lying quasiparticle energy and, for this very reason, can give valuable hints on superconducting gap symmetry. The work presented focuses on investigating the pairing symmetry in the Fe1+y(Te1−xSex) system using a radio-frequency tunnel diode oscillator (TDO) technique for precise measurements of the temperature dependence of their in-plane penetration depth. The TDO technique, based on an original concept involving the use of planar inductors in an novel configuration, was implemented on a dilution refrigerator to investigate a significant number of single crystal samples, with nominal Se concentrations of 36%, 40%, 43% and 45% respectively, down to temperatures as low as 50 mK. A systematic study together with a comprehensive analysis regarding the order parameter symmetry in the Fe1+y(Te1−xSex) system is presented. In many cases we found that London penetration depth shows an upturn below at low temperatures, indicative of a paramagnetic-type contribution. Also the low-temperature behavior of penetration depth is best described by a quadratic power law with no systematic dependence on the Se concentration. Most importantly, in the limit of T → 0, in some samples we observed a narrow region of linear temperature dependence, suggestive of nodes in the superconducting gap of Fe1+y(Te1−xSex).

Thesis advisor: Ziqiang Wang / Angle-resloved photoemission spectroscopy (ARPES) is a powerful technique to study the electronic structure in solids. Its unique ability of resolving the energy and momentum information of electrons inside a solid provides an essential tool in measuring the electronic structure of solids. ARPES has made great contributions in the understanding of correlated system such as high-T_c superconductors and ruthenates. The Metal-insulator transition is a fundamental problem in condensed matter physics. The calcium substituted strontium ruthenate, Ca_2-xSr_xRuO₄, provides a good platform to study the metal-insulator transition in multi-orbital systems. This system has a complex phase diagram that evolves from a <italic>p</italic>-wave superconductor to a Mott insulator. One of important projects of this thesis focuses on Ca_2-xSr_xRuO₄ The growing evidence for coexistence of itinerant electrons and local moments in transition metals with nearly degenerate d orbitals suggests that one or more electron orbitals undergo a Mott transition while the others remain itinerant. We have observed a novel orbital selective Mott transition (OSMT) in Ca_1.8Sr_0.2RuO₄ by ARPES. While we observed two sets of dispersing bands and Fermi surfaces (FSs) associated with the doubly-degenerate d_yz and d_zx orbitals, the Fermi surface associated with the d_xy orbital which has a wider bandwidth is missing as a consequence of selective Mott localization. Our theoretical calculations have demonstrated that this unusual OSMT is mainly driven by the combined effects of inter-orbital carrier transfer, superlattice potentials and orbital degeneracy, whereas the bandwidth difference plays a less important role. Another important project of this thesis focuses on the recently discovered iron-pnictides superconductors. The idea of inter-FS scattering associated with the near-nesting condition has been proposed to explain the superconductivity in the pnictides. The near-nesting condition varies upon the carrier doping which shifts the chemical potential. We have performed a systematic photoemission study of the chemical potential shift as a function of doping in a pnictide system based on BaFe₂As₂. The experimentally determined chemical potential shift is consistent with the prediction of a rigid band shift picture by the renormalized first-principle band calculations. This leads to an electron-hole asymmetry (EHA) due to different Fermi velocities for different FS sheets, which can be calculated from the Lindhard function of susceptibility. This built-in EHA from the band structure, which is fully consistent with the experimental phase diagram, strongly supports that inter-FS scattering over the near-nesting Fermi surfaces plays a vital role in the superconductivity of the iron pnictides. / Thesis (PhD) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Orientador: Eduardo Granado Monteiro da Silva / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-09-01T17:31:34Z (GMT). No. of bitstreams: 1 Kaneko_UlissesFerreira_D.pdf: 51229533 bytes, checksum: 3d39a6722771458fa1812d75c31661b4 (MD5) Previous issue date: 2017 / Resumo: Nesta tese, empregamos espectroscopia Raman para realizar um estudo detalhado dos fônons e das respostas eletrônicas em monocristais de LaFeAsO. Nós observamos as simetrias e frequências dos fônons nas fases tetragonal e ortorrômbica e também a dependência com a temperatura de um pico quase-elástico (do inglês: QEP) na simetria B1g com intensidade máxima em torno da transição magnética TN . A área e a altura do QEP B1g foram atribuídas às flutuações nemáticas de spin, enquanto que a largura do QEP B1g foi relacionada com taxa de relaxação dessas flutuações. Através da análise da largura do QEP B1g propomos que a transição estrutural esté relacionada a um congelamento gradual das flutuaçõees nemáticas de spin, o que deve ser um fenômeno geral presente nos demais supercondutores à base de Fe. Esse estudo foi complementado por medidas de espectroscopia Raman em BaFe2As2 e de difração de raios-X com luz síncrotron em LaFeAsO, BaFe2As2 e SrFe2As2 dopado com Co. Paralelamente, nós estudamos as estruturas cristalinas e magnéticas em compostos da série Ba1?xLaxTi1/2Mn1/2O3 através da técnica de difração de nêutrons / Abstract: In this thesis, we employed Raman spectroscopy to perform a detailed study of the phonons and electronic responses in single crystals of the LaFeAsO. We observed the symmetries and frequencies of the phonons in the tetragonal and orthorhombic phases and also the temperature dependence of the quasi-elastic peak (QEP) in the B 1g symmetry with maximum intensity around the magnetic transition T N . The B 1g QEP area and height were ascribed to spin nematic fluctuations, while the B 1g QEP width was related to the relaxation rate of these fluctuations. From the B 1g QEP width analysis we propose that the structural transition is related to a gradual freezing of the spin nematic fluctuations, which may be a general phenomenon present in other Fe-based superconductors. This study was complemented by measurements of Raman spectroscopy in BaFe2As2 and synchrotron X-ray diffraction in LaFeAsO, BaFe2As2 and Co-doped SrFe2As2 . In parallel, we studied the crystal and magnetic structures in compounds of the series Ba1?xLaxTi1/2Mn1/2O3 through the neutron diffraction technique / Doutorado / Física / Doutor em Ciências / 140978/2012-7 / CNPQ

The origin of the nematic phase in the iron-based superconductors is still unknown, and an understanding of its microscopic mechanism could have important implications on the unconventional superconductivity in these materials. This thesis describes a series of experiments using scanning tunneling microscopy (STM) and spectroscopy (STS) to visualize the nematic electronic structure in NaFe1-xCoxAs as a function of energy, temperature, strain, and doping. We first start with background material on the iron-based superconductors and the iron pnictides in particular. We then extensively explore the physical details of NaFe1-xCoxAs which is the main material of study in this thesis. Additional attention is paid to the electronic structure due to its relation to quasiparticle interference (QPI) measurements made with STS. The theoretical underpinnings of STM and STS are then derived as well as further details of QPI. Many of the experiments described in this thesis were performed on a custom-built, low temperature STM which the author helped build. We describe the design of this system and report on benchmarking tests that were used to characterize the system's performance. Both pristine, undoped NaFeAs and LiFeAs were measured by STM, and we compare and contrast these two materials which come from the same structural family. The electronic local density of states (LDOS) of NaFeAs was measured at various temperatures in all three phases of the material (tetragonal paramagnetic, orthorhombic paramagnetic, and orthorhombic spin density wave (SDW)). The electronic structure in the SDW phase is highly anisotropic. QPI signals in this phase are found to be well-explained by comparison to a joint density of states (JDOS) model using the reconstructed bandstructure fit to angle-resolved photoemission spectroscopy data. The electronic anisotropy is found to persist into the nominally tetragonal phase. This persistence arises from built-in crystallographic strain coupling to high amplitude, unidirectional, antiferroic fluctuations. These fluctuations renormalize the bare Green's function which gives rise to anisotropic scattering. We then describe the construction of a novel device created for variable-strain STS. Antiphase domains in NaFeAs are visualized and found to change in size as a function of unidirectional strain. These domains are tracked as a function of temperature and found to disappear at exactly the nematic transition temperature proving that this is the temperature at which long-range order is lost. By measuring Co-doped samples, we find that the domains disappear before optimal doping in an underdoped sample with superconducting transition temperature of 18 K. However, the electronic structure remains anisotropic implying that nematic fluctuations persist. These fluctuations are found even in overdoped samples and disappear with superconductivity at heavy doping.