Thermal fluctuations and disorder strongly influence the behaviour of hightemperature superconductors. In particular the vortices play a key role in determining their properties. In this thesis the main focus lies on phase transitions, both in ultra-thin films and in three-dimensional systems, which are driven by vortex fluctuations. The last paper concerns the influence of antiferromagnetism on superconductivity in a simple model. A brief review of these topics is given in the introductory part. The main results are: The phase transition in ultra-thin superconducting/superfluid films is studied within the two-dimensional Coulomb gas model, which is known to have a Berezinskii-Kosterlitz-Thouless transition at low vortex densities. We construct the phase diagram from grand canonical Monte Carlo simulations on a continuum, without any restrictions on the vortex density. The dynamical universality classes for vortices in superconductors in zero magnetic field are studied by means of Monte Carlo simulations, with particular attention to the role of screening of the vortex interaction. We construct a formula for the k = 0 helicity modulus directly in terms of the vortex line fluctuations, which can serve as a useful way to detect superconducting coherence in model calculations. A method for simulating vortex lines on a continuum is developed, and used to study the melting of the Abrikosov vortex lattice. We study the critical dynamics for vortices in the presence of columnar defects. The linear resistivity and current-voltage characteristics are calculated in Monte Carlo simulations, and the critical behaviour extracted using finite size scaling. We reconsider the scaling properties as the magnetic field is tilted away from the direction of the columns. The influence of antiferromagnetic correlations on the superconducting properties is studied in a simplified lattice fermion model for superconductivity in the presence of an antiferromagnetic background. We find that the superconducting critical temperature is enhanced by antiferromagnetic order, and that a gap with dx2-y2-wave symmetry is the most stable. / QC 20100512

Over the past 20 years a great deal of progress has been made towards understanding the physics of the high-temperature (high-Tc) cuprate superconductors. Much of the low- energy physics of these materials appears to be captured by two-dimensional Hubbard or t-J models which have provided significant insight into a number of properties such as the pseudogap, antiferromagnetism and superconductivity itself. However, intrinsically planar models are unable to account for the large variations in Tc observed across materials nor do they capture the electron-phonon (el-ph) interaction, the importance of which a number of experimental probes now indicate. This thesis examines the el-ph interaction in cuprates using a combination of analytical and numerical techniques. Starting from the microscopic mechanism for coupling to in-plane and c-axis polarized oxygen phonons, the theory of el-ph coupling is presented. The el-ph self-energy is derived in the context of Migdal-Eliashberg theory and then applied to understanding the detailed temperature and doping dependence of the renormalizations observed by Angle-resolved photoemission spectroscopy. The qualitative signatures of el- boson coupling in the density of states of a d-wave superconductor are also examined on general grounds and a model calculation is presented for el-ph coupling signatures in the density of states. Following this, the theory is extended to include the effects of screening and the consequences of this theory are explored. Due to the quasi-2D nature of the cuprates, screening is found to anomalously enhance the el-ph contribution to d-wave pairing. This result is then considered in light of the material and doping dependence of Tc and a framework for understanding the materials variations in Tc is presented. From these studies, a detailed picture of the role of the el-ph interaction in the doped cuprates emerges where the interaction, working in conjunction with a dominant pairing interaction, provides much of the materials variations in Tc observed across the cuprate families. Turning towards numerical techniques, small cluster calculations are presented which examine the effects of a local oxygen dopant in an otherwise ideal Bi2Sr2CaCu2O8+δ crystal. Here, it is demonstrated that the dopant locally enhances electronic properties such as the antiferromagnetic exchange energy J via local el-ph coupling to planar local oxygen vibrations. Finally, in an effort to extend the scope of this work to the underdoped region of the phase diagram, an examination of the properties of the single-band Hubbard and Hubbard-Holstein model is carried out using Determinant Quantum Monte Carlo. Here focus is placed on the spectral properties of the model as well as the competition between the the antiferromagnetic and charge-density-wave orders. As with the small cluster calculations, a strong interplay between the magnetic and lattice properties is observed.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 87-88). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 87-88). Also available on the Internet.

Over the past 20 years a great deal of progress has been made towards understanding the physics of the high-temperature (high-Tc) cuprate superconductors. Much of the low- energy physics of these materials appears to be captured by two-dimensional Hubbard or t-J models which have provided significant insight into a number of properties such as the pseudogap, antiferromagnetism and superconductivity itself. However, intrinsically planar models are unable to account for the large variations in Tc observed across materials nor do they capture the electron-phonon (el-ph) interaction, the importance of which a number of experimental probes now indicate. This thesis examines the el-ph interaction in cuprates using a combination of analytical and numerical techniques. Starting from the microscopic mechanism for coupling to in-plane and c-axis polarized oxygen phonons, the theory of el-ph coupling is presented. The el-ph self-energy is derived in the context of Migdal-Eliashberg theory and then applied to understanding the detailed temperature and doping dependence of the renormalizations observed by Angle-resolved photoemission spectroscopy. The qualitative signatures of el- boson coupling in the density of states of a d-wave superconductor are also examined on general grounds and a model calculation is presented for el-ph coupling signatures in the density of states. Following this, the theory is extended to include the effects of screening and the consequences of this theory are explored. Due to the quasi-2D nature of the cuprates, screening is found to anomalously enhance the el-ph contribution to d-wave pairing. This result is then considered in light of the material and doping dependence of Tc and a framework for understanding the materials variations in Tc is presented. From these studies, a detailed picture of the role of the el-ph interaction in the doped cuprates emerges where the interaction, working in conjunction with a dominant pairing interaction, provides much of the materials variations in Tc observed across the cuprate families. Turning towards numerical techniques, small cluster calculations are presented which examine the effects of a local oxygen dopant in an otherwise ideal Bi2Sr2CaCu2O8+δ crystal. Here, it is demonstrated that the dopant locally enhances electronic properties such as the antiferromagnetic exchange energy J via local el-ph coupling to planar local oxygen vibrations. Finally, in an effort to extend the scope of this work to the underdoped region of the phase diagram, an examination of the properties of the single-band Hubbard and Hubbard-Holstein model is carried out using Determinant Quantum Monte Carlo. Here focus is placed on the spectral properties of the model as well as the competition between the the antiferromagnetic and charge-density-wave orders. As with the small cluster calculations, a strong interplay between the magnetic and lattice properties is observed.

In this report the analysis of microstrip electrodynamic structures on basis of high-temperature superconductors is carried out and a mathematical model of microstrip devices with lumped and distributed nonlinear properties is created. For this purpose nonlinear integral equations method and method of moments are used. In the issue of the work a SHF filter based on the equivalent circuit of elements with lumped parameters is also studied. It is ascertained that the received mathematical model allows to achieve more proper results of modeling on compensation of variation of current-density distribution nearby the edges of conductor break.

The class of high Tc superconductors share one common structural aspect, the existence of planes of copper and oxygen ions. These planes are thought to be the source of the superconducting behaviour. They can be represented as a two-dimensional lattice of ions, which facilitates their study using numerical models. One such model is the t − J model. In most studies utilising numerical models, the planes have been considered isotropic. However, recent analysis of cuprate structure has illustrated that this may not be representative of the copper oxide planes. A number of cuprate structures exhibit different phases in which the planes are not isotropic, such as the low temperature orthorhombic and low temperature tetragonal phases. This work will examine the effects of introducing anisotropy into the t − J model in order to understand how these phases affect the results gained from numerical studies. / x, 89 leaves : ill. ; 29 cm.

Abstract A stripline structure is closed and therefore protected against surrounding EMI and it is easy to bury in multiplayer structures, which offer higher circuit density. This thesis focuses on the evaluation (and verification via actual structures) of the correct simulation of striplines and, as a new aspect, the advantages of using a dual-stripline. Multiple design methods and electromagnetic simulation systems were tested and properties of these are compared. For a reliable design it is still necessary to use at least two tools, at first a very fast tool having excellent circuit parameter optimization methods and then some electromagnetic simulator, which can be used to the complete the realizable layout. That is, because all the electromagnetic simulators suffer from the same limiting factors, the memory capacity of the computer and the unacceptable calculation time. It has been discovered through modelling that the "cat-eye" shape having many more and larger local inaccuracies at the thinned edge areas of the sintered conductor in LTCC structures increases the conductor losses. Therefore it is important to develop new manufacturing methods capable of producing better-shaped conductors. A combination of broadside coupled parallel connected striplines has been tested both in High Temperature Superconducting ( HTS ) and LTCC materials. A two-conductor stripline, a dual stripline, raises the power handling capability of a microwave bandpass HTS filter. In addition, it offers the possibility to use a normal metal protection layer at the surface of the superconductor without degradation of electrical properties, thus increasing the power handling capability even more. The dual stripline solution in LTCC would offer some preferable properties in high power filters only. The shape of the ground plane used for trimming the coupling between resonators was also found to have a remarkable influence on the quality factor of the resonator. A quite narrow ground strip can offer a much better quality factor with the same coupling level than a meshed or continuous ground plane, but it requires accurate design and manufacturing methods. It would help to design filters with lower loss in the passband without compromises in the attenuation outside the passband.

The Bi2Sr2CaCu2O7+d, i.e. Bi2212 phase of bismuth derived high temperature superconductors (HTS), powder precursor were synthesized via sol – gel technique using ethylendiamintetraacetic acid (Chelaton II) as a chelating agent. Metal nitrate’s solutions were mixed with EDTA suspension. The pH value was adjusted to 9 by NH4OH by reason forming of stable metal’s complexes. The mixture was heated to 80 °C for gelation. Solution taken before solid gel was form is used for preparation of Bi2212 layer on to a-Al2O3 surface via spin coating deposition’s technique. Reactive powder used for bulk sample preparation was made by calcination (800 °C) of pyrolyzed xerogel (500 °C). Sintering in oxygen atmosphere was proceeding at temperature from 850 to 880 °C. Bulk sample properties were compared with sample prepared by common ceramic method. Simultaneous TG-DTA, IR spectroscopy and heating microscopy were used for sample characterization. Furthermore, construction of furnace for sintering in O2 atmosphere was described.

In this thesis, the results of analyzing the electronic structure of two iron-based superconductors: FeSe and LiFeAs are presented. To access the electronic structure, angle-resolved photoemission spectroscopy was used. In our analysis, we focus on the structure of the superconducting gap and the influence of nematicity on the electronic structure. We have revealed changes in the electronic structure of FeSe caused by nematicity in all parts of the Brillouin zone. A scale of these changes is smaller than it was believed earlier. Also, we have observed an anomalous shift of the dispersions in opposite directions with temperature in this material. We have observed anisotropic superconducting gap on all sheets of the Fermi surfaces of both: FeSe and LiFeAs. We have shown that in LiFeAs, rotational symmetry is broken in the superconducting state, which manifests not only in the gap symmetry but also in the shapes of the Fermi surfaces sheets. This result indicates a realization of a novel phenomenon of superconductivity-induced nematicity:1 Iron-based superconductors 1.1 Introduction to iron-based superconductors 1.2 LiFeAs - special iron-based superconductor 1.3 FeSe - structurally simplest iron-based superconductor 2 Angle-Resolved Photoemission 3 Temperature evolution of the electronic structure of FeSe 3.1 Effects of nematicity from low-temperature measurements 3.2 Temperature dependent shift of the dispersions 3.3 Discussion and conclusions 4 Three-dimensional superconducting gap in FeSe 4.1 Superconducting gap on the electron-like pockets 4.2 Superconducting gap on the hole-like pocket 4.3 Discussion and conclusions 5 Superconductivity-induced nematicity in LiFeAs 5.1 Superconducting gap 5.2 Nematicity 5.3 Discussion and conclusions Summary