Magnetic and electronic properties of iron-based superconducting systems

<p>This thesis is motivated by the large variety of high-temperature superconductors that contain iron in the superconducting layer. This number has grown rapidly since the discovery in 2008 of the iron-pnictides (and chalcogenides), where iron and arsenic form the superconducting layer. Also of interest are the iron-cuprate hybrid materials, where one out of three copper atoms is replaced by iron. The aim is to understand the superconducting, magnetic and electronic properties of these materials in respect to their iron content. This thesis describes some of these properties for the iron-pnictide compounds of CeFeAsO₁₋xFx and AFe₂As₂ (A=Ba, Sr), and for the ironcuprate hybrids of FeSr₂YCu₂O₆₊y and FeSr₂Y₂₋xCexCu₂O₁₀₋y. Here it has been found that CeFeAsO₁₋xFx follows a 3D fluctuation conductivity above the superconducting transition and the thermal activation energy is correlated to the critical current density within a two fluid-flux creep model below the superconducting transition. NMR measurements show that there is considerable charge disorder within the superconducting doping region. The AFe₂As₂ show a positive magnetoresistance, which could be interpreted through three-carrier transport. Superconducting samples of SrFe₂As₂ display a large enhancement in the magnetoresistance below the superconducting transition up to 1600 %, which is due to three-carrier transport through metallic and superconducting regions in an inhomogeneous state. The superconducting properties of the iron-cuprate FeSr₂YCu₂O₆₊y in respect to the location of iron was studied under the influence of electron and hole doping and with additional magnetic impurities. FeSr₂Y₂₋xCexCu₂O₁₀₋y shows a disorder induced spin-glass state and strong localization depending on the doping.</p>

Magnetic and electronic properties of iron-based superconducting systems

<p>This thesis is motivated by the large variety of high-temperature superconductors that contain iron in the superconducting layer. This number has grown rapidly since the discovery in 2008 of the iron-pnictides (and chalcogenides), where iron and arsenic form the superconducting layer. Also of interest are the iron-cuprate hybrid materials, where one out of three copper atoms is replaced by iron. The aim is to understand the superconducting, magnetic and electronic properties of these materials in respect to their iron content. This thesis describes some of these properties for the iron-pnictide compounds of CeFeAsO₁₋xFx and AFe₂As₂ (A=Ba, Sr), and for the ironcuprate hybrids of FeSr₂YCu₂O₆₊y and FeSr₂Y₂₋xCexCu₂O₁₀₋y. Here it has been found that CeFeAsO₁₋xFx follows a 3D fluctuation conductivity above the superconducting transition and the thermal activation energy is correlated to the critical current density within a two fluid-flux creep model below the superconducting transition. NMR measurements show that there is considerable charge disorder within the superconducting doping region. The AFe₂As₂ show a positive magnetoresistance, which could be interpreted through three-carrier transport. Superconducting samples of SrFe₂As₂ display a large enhancement in the magnetoresistance below the superconducting transition up to 1600 %, which is due to three-carrier transport through metallic and superconducting regions in an inhomogeneous state. The superconducting properties of the iron-cuprate FeSr₂YCu₂O₆₊y in respect to the location of iron was studied under the influence of electron and hole doping and with additional magnetic impurities. FeSr₂Y₂₋xCexCu₂O₁₀₋y shows a disorder induced spin-glass state and strong localization depending on the doping.</p>

Electron Conduction Processes in Tantalum-Germanium Multilayers

<p>An explosion of both theoretical and experimental research into structurally disordered materials in the late 1970s has greatly increased our understanding of these complex systems. A number of facets of the conduction processes remain unexplained, however, particularly in the area of non-simple metals. Multilayers of disordered tantalum and amorphous germanium with individual layer thicknesses of between 4 & 120A [Angstrom] and 13 & 220A [Angstrom]respectively have been prepared by vapour deposition and the in-plane resistance measured from 1.5 to 300K. Results for samples with germanium layers of sufficient thickness to prevent tunnelling between the conducting tantalum layers can be interpreted in terms of conduction in the tantalum layers alone. In these samples the behaviour of the resistance as a function of temperature and the tantalum layer thickness can be explained in terms of the interplay between quantum interference effects and disorder enhanced electron-electron interaction effects. At high temperatures the negative temperature coefficient of resistance arises from the destruction of coherent interference in the backscattered direction by phonons. From the data, the electron-phonon scattering rate is found to be comparable in magnitude to that expected for scattering in either the "clean" or "dirty" limits while the temperature dependence of the scattering rate lies between that expected for each of these limits. At lower temperatures a turn over to a positive temperature coefficient of resistance is seen as spin-orbit scattering and superconducting fluctuations become important. At still lower temperatures the resistance is dominated by electron-electron interaction effects and we have observed a transition from three-dimensional to two-dimensional behaviour as the tantalum layer thickness is reduced. Evidence for the onset of superconductivity is seen for samples with a low temperature sheet resistance of less than 3000 Omega/whitesquare. We have also investigated samples with thin germanium layers (<40A [Angstrom]) in which coupling between the layers causes an increase in the superconducting transition temperature. We present some preliminary measurements which suggest that the transition from isolated to coupled tantalum layers, as the germanium layer thickness is reduced, can be followed in the form of the fluctuation conductivity.</p>

Electron Conduction Processes in Tantalum-Germanium Multilayers

<p>An explosion of both theoretical and experimental research into structurally disordered materials in the late 1970s has greatly increased our understanding of these complex systems. A number of facets of the conduction processes remain unexplained, however, particularly in the area of non-simple metals. Multilayers of disordered tantalum and amorphous germanium with individual layer thicknesses of between 4 & 120A [Angstrom] and 13 & 220A [Angstrom]respectively have been prepared by vapour deposition and the in-plane resistance measured from 1.5 to 300K. Results for samples with germanium layers of sufficient thickness to prevent tunnelling between the conducting tantalum layers can be interpreted in terms of conduction in the tantalum layers alone. In these samples the behaviour of the resistance as a function of temperature and the tantalum layer thickness can be explained in terms of the interplay between quantum interference effects and disorder enhanced electron-electron interaction effects. At high temperatures the negative temperature coefficient of resistance arises from the destruction of coherent interference in the backscattered direction by phonons. From the data, the electron-phonon scattering rate is found to be comparable in magnitude to that expected for scattering in either the "clean" or "dirty" limits while the temperature dependence of the scattering rate lies between that expected for each of these limits. At lower temperatures a turn over to a positive temperature coefficient of resistance is seen as spin-orbit scattering and superconducting fluctuations become important. At still lower temperatures the resistance is dominated by electron-electron interaction effects and we have observed a transition from three-dimensional to two-dimensional behaviour as the tantalum layer thickness is reduced. Evidence for the onset of superconductivity is seen for samples with a low temperature sheet resistance of less than 3000 Omega/whitesquare. We have also investigated samples with thin germanium layers (<40A [Angstrom]) in which coupling between the layers causes an increase in the superconducting transition temperature. We present some preliminary measurements which suggest that the transition from isolated to coupled tantalum layers, as the germanium layer thickness is reduced, can be followed in the form of the fluctuation conductivity.</p>

Vortex Dynamics and Instabilities in Tax Ge1-x/Ge Multilayers

<p>In this thesis the magnetic response of a layered type-II superconducting system is explored across the entire range of fields, temperatures and currents where superconductivity exists, with the results providing valuable insight into the role of reduced dimensionality in determining the behaviour of type-II materials such as the new high temperature superconductors. The system in question consists of alternating layers of amorphous Ta or TaxGe1-x (x approximation 0.3) with amorphous Ge where the individual layer thicknesses vary between 17A [angstrom] and 210A [angstrom]. These multilayers were fabricated by vapour deposition in a high vacuum chamber which allowed the creation of samples with uniform layers of high purity. The resistive transport properties have been measured from Tc (approximation 1-3K) to temperatures as low as 50mK in some cases, and in fields of up to 15T. The upper critical fields have been determined from the fluctuation conductivity both with the field parallel and perpendicular to the layer plane of the samples. The results show clearly the dependence of the dimensionality on the superconducting layer thickness and the degree of coupling across the Ge layers. For the samples with the most two-dimensional properties the zero field resistive transition is governed by the unbinding of thermally created vortex-antivortex pairs as described by the Berezinskii-Kosterlitz-Thouless theory. A detailed investigation of the perpendicular field vortex states and dynamics has been performed, including measurement of the activation energies needed for thermally activated vortex motion. Qualitative difference are observed between the activation energies in two- and three-dimensional samples, with the barriers being generally higher in 3D. The non-linear current-voltage characteristics of the samples provide evidence for the existence of a vortex glass state which melts into a liquid below Hc2, although the divergence of the activation barriers in the glass can be restricted by the finite sample thickness. A brief investigation of the corresponding parallel field regime showed considerably less dissipation, due largely to the transparent nature of the Ge layers to the magnetic field. At the highest currents an instability is observed in the vortices which can drive the samples discontinuously back into the normal state. This instability is shown to be of the type predicted by Larkin and Ovchinnikov (LO), including quantitative agreement between the measured and predicted values of the critical vortex velocity. Several features of the instability are noted which are not specifically predicted by the LO theory, and comparisons are drawn between these and the prevailing vortex state at lower currents.</p>

Vortex Dynamics and Instabilities in Tax Ge1-x/Ge Multilayers

<p>In this thesis the magnetic response of a layered type-II superconducting system is explored across the entire range of fields, temperatures and currents where superconductivity exists, with the results providing valuable insight into the role of reduced dimensionality in determining the behaviour of type-II materials such as the new high temperature superconductors. The system in question consists of alternating layers of amorphous Ta or TaxGe1-x (x approximation 0.3) with amorphous Ge where the individual layer thicknesses vary between 17A [angstrom] and 210A [angstrom]. These multilayers were fabricated by vapour deposition in a high vacuum chamber which allowed the creation of samples with uniform layers of high purity. The resistive transport properties have been measured from Tc (approximation 1-3K) to temperatures as low as 50mK in some cases, and in fields of up to 15T. The upper critical fields have been determined from the fluctuation conductivity both with the field parallel and perpendicular to the layer plane of the samples. The results show clearly the dependence of the dimensionality on the superconducting layer thickness and the degree of coupling across the Ge layers. For the samples with the most two-dimensional properties the zero field resistive transition is governed by the unbinding of thermally created vortex-antivortex pairs as described by the Berezinskii-Kosterlitz-Thouless theory. A detailed investigation of the perpendicular field vortex states and dynamics has been performed, including measurement of the activation energies needed for thermally activated vortex motion. Qualitative difference are observed between the activation energies in two- and three-dimensional samples, with the barriers being generally higher in 3D. The non-linear current-voltage characteristics of the samples provide evidence for the existence of a vortex glass state which melts into a liquid below Hc2, although the divergence of the activation barriers in the glass can be restricted by the finite sample thickness. A brief investigation of the corresponding parallel field regime showed considerably less dissipation, due largely to the transparent nature of the Ge layers to the magnetic field. At the highest currents an instability is observed in the vortices which can drive the samples discontinuously back into the normal state. This instability is shown to be of the type predicted by Larkin and Ovchinnikov (LO), including quantitative agreement between the measured and predicted values of the critical vortex velocity. Several features of the instability are noted which are not specifically predicted by the LO theory, and comparisons are drawn between these and the prevailing vortex state at lower currents.</p>

Composition variation and electron irradiation effects on the fluctuation conductivity in Y1–zPrzBa2Cu3O7−δ single crystals

A comparative analysis of the changes in the fluctuation conductivity and characteristics of the superconducting state of YBa2Cu3O7–δ single crystals caused by various types of defects is carried out. These defects appeared due to irradiation with high-energy electrons (radiation doses from 1.4 to 8.8 1018 cm–2), changes in oxygen deficiency (0.08 ≤ δ ≤ 0.23) due to annealing at different temperatures, or doping with praseodymium (Y1–zPrzBa2Cu3O7−δ, 0 ≤ z ≤ 0.5 at optimal oxygen concentration). It is shown that the introduction of such defects leads to a significant expansion of the temperature range of the existence of excess conductivity, and upon doping with praseodymium, it also leads to the appearance of a thermally activated deflection on the temperature dependence of the electrical resistance. The effect of such defects on the superconducting transition temperature, Tc, and the coherence length along the c axis, ξc(0), is studied. In particular, ξc(0) more than quadruples (at z = 0.43), while the 2D-3D crossover point shifts towards higher temperatures. Possible reasons for the suppression of superconductivity in YBa2Cu3O7–δ upon irradiation with fast electrons and the qualitatively different temperature dependences of its resistivity in the basal plane, ρab (T), are discussed.