<p>The main focus of this thesis lies on the critical properties of twodimensional (2D) superconductors in zero magnetic field. Simulations based on variants of the 2D XY model are shown to give characteristic features close to the phase transition which agree qualitatively with experimental data. Thus, it is concluded that these common characteristic features are caused by two-dimensional vortices.</p><p>The thesis consists of an introductory part and five separate publications. In the introductory part of the thesis the basic results of the Ginzburg-Landau model, which gives a phenomenological description of superconductors, are described. In 2D systems, the superconductive phase transition in the absence of a magnetic field is governed by the unbinding of thermally created vortices and is called the Kosterlitz-Thouless (KT) phase transition. An introduction to this kind of transition is given. The important features of the current-voltage (IV) characteristics and the nonlinear conductivity, which can be used to study the KT transition, are discussed. The scaling analysis procedure, a powerful tool for the analysis of the properties of a system in the vicinity of phase transition, is reviewed. A scaling form for the nonlinear dc conductivity, which takes into account finite-size e ects, is discussed.</p><p>The static 2D XY model, which is usually used to describe superfluids, superconducting films as well as the high-Tc superconductors with high anisotropy, is introduced. Three different types of dynamic models, namely resistively shunted junction, relaxational, and Monte Carlo dynamics are superimposed on the 2D XY model for the evaluation of the dynamic properties. TheVillain model and a modifiedXY model using a p-type interaction potential exhibit different densities of the thermally created vortices. Since the dominant characteristic physical features close to the KT transition are associated with vortex pair fluctuations these two models are investigated.</p><p>The introductory part closes with a short introduction to each of the five published articles.</p>

The main focus of this thesis lies on the critical properties of twodimensional (2D) superconductors in zero magnetic field. Simulations based on variants of the 2D XY model are shown to give characteristic features close to the phase transition which agree qualitatively with experimental data. Thus, it is concluded that these common characteristic features are caused by two-dimensional vortices. The thesis consists of an introductory part and five separate publications. In the introductory part of the thesis the basic results of the Ginzburg-Landau model, which gives a phenomenological description of superconductors, are described. In 2D systems, the superconductive phase transition in the absence of a magnetic field is governed by the unbinding of thermally created vortices and is called the Kosterlitz-Thouless (KT) phase transition. An introduction to this kind of transition is given. The important features of the current-voltage (IV) characteristics and the nonlinear conductivity, which can be used to study the KT transition, are discussed. The scaling analysis procedure, a powerful tool for the analysis of the properties of a system in the vicinity of phase transition, is reviewed. A scaling form for the nonlinear dc conductivity, which takes into account finite-size e ects, is discussed. The static 2D XY model, which is usually used to describe superfluids, superconducting films as well as the high-Tc superconductors with high anisotropy, is introduced. Three different types of dynamic models, namely resistively shunted junction, relaxational, and Monte Carlo dynamics are superimposed on the 2D XY model for the evaluation of the dynamic properties. TheVillain model and a modifiedXY model using a p-type interaction potential exhibit different densities of the thermally created vortices. Since the dominant characteristic physical features close to the KT transition are associated with vortex pair fluctuations these two models are investigated. The introductory part closes with a short introduction to each of the five published articles.

<p>Dynamic properties of type-II superconductors have been experimentally studied in zero and low magnetic fields using SQUID magnetometry and <i>I–V</i> measurements.</p><p>In zero magnetic field close to the critical temperature, the physical properties of type-II superconductors are dominated by spontaneously created vortices. In three dimensions (3D) such vortices take the form of vortex loops and in two dimensions (2D) as vortex-antivortex pairs.</p><p>The 2D vortex dynamics has been probed using mutual inductance and flux noise measurements on YBa₂Cu₃O₇ (YBCO) and MgB₂ thin films in zero and low magnetic fields. In such measurements, information about vortex correlations is obtained through a temperature dependent characteristic frequency, below (above) which the vortex movements are uncorrelated (correlated). The results obtained in zero magnetic field indicate that sample heterogeneities influence the vortex physics and hinder the divergence of the vortex-antivortex correlation length.</p><p>In low magnetic fields the vortex dynamics is strongly dependent on the applied magnetic field and a power law dependence of the characteristic frequency with respect to the magnetic field is observed. The results indicate that there is a co-existence of thermally and field generated vortices.</p><p>The <i>I–V</i> characteristics of untwinned YBCO single crystals show that only a small broadening of the transition region influences the length scale over which the vortex movements are correlated. The dynamic and static critical exponents therefore exhibit values being larger in magnitude as compared to values predicted by relevant theoretical models. The results also suggest that the copper oxide planes in YBCO decouple slightly below the mean field critical temperature and hence, the system has a crossover from 3D to 2D behaviour as the temperature is increased. </p><p>From temperature dependent DC-magnetisation measurements performed on untwinned YBCO single crystals in weak applied fields, detailed information about the critical current density and the irreversibility line is obtained.</p>

Dynamic properties of type-II superconductors have been experimentally studied in zero and low magnetic fields using SQUID magnetometry and I–V measurements. In zero magnetic field close to the critical temperature, the physical properties of type-II superconductors are dominated by spontaneously created vortices. In three dimensions (3D) such vortices take the form of vortex loops and in two dimensions (2D) as vortex-antivortex pairs. The 2D vortex dynamics has been probed using mutual inductance and flux noise measurements on YBa2Cu3O7 (YBCO) and MgB2 thin films in zero and low magnetic fields. In such measurements, information about vortex correlations is obtained through a temperature dependent characteristic frequency, below (above) which the vortex movements are uncorrelated (correlated). The results obtained in zero magnetic field indicate that sample heterogeneities influence the vortex physics and hinder the divergence of the vortex-antivortex correlation length. In low magnetic fields the vortex dynamics is strongly dependent on the applied magnetic field and a power law dependence of the characteristic frequency with respect to the magnetic field is observed. The results indicate that there is a co-existence of thermally and field generated vortices. The I–V characteristics of untwinned YBCO single crystals show that only a small broadening of the transition region influences the length scale over which the vortex movements are correlated. The dynamic and static critical exponents therefore exhibit values being larger in magnitude as compared to values predicted by relevant theoretical models. The results also suggest that the copper oxide planes in YBCO decouple slightly below the mean field critical temperature and hence, the system has a crossover from 3D to 2D behaviour as the temperature is increased. From temperature dependent DC-magnetisation measurements performed on untwinned YBCO single crystals in weak applied fields, detailed information about the critical current density and the irreversibility line is obtained.

In this thesis, superconducting nanostructures for quantum detection of electromagnetic radiation are studied. In this regard, electrodynamics of topological excitations in 1D superconducting nanowires and 2D superconducting nanostrips is investigated. Topological excitations in superconducting nanowires and nanostrips lead to crucial deviation from the bulk properties. In 1D superconductors, topological excitations are phase slippages of the order parameter in which the magnitude of the order parameter locally drops to zero and the phase jumps by integer multiple of 2\pi. We investigate the effect of high-frequency field on 1D superconducting nanowires and derive the complex conductivity. Our study reveals that the rate of the quantum phase slips (QPSs) is exponentially enhanced under high-frequency irradiation. Based on this finding, we propose an energy-resolving terahertz radiation detector using superconducting nanowires. In superconducting nanostrips, topological fluctuations are the magnetic vortices. The motion of magnetic vortices result in dissipative processes that limit the efficiency of devices using superconducting nanostrips. It will be shown that in a multi-layer structure, the potential barrier for vortices to penetrate inside the structure is elevated. This results in significant reduction in dissipative process. In superconducting nanowire single photon detectors (SNSPDs), vortex motion results in dark counts and reduction of the critical current which results in low efficiency in these detectors. Based on this finding, we show that a multi-layer SNSPD is capable of approaching characteristics of an ideal single photon detector in terms of the dark count and quantum efficiency. It is shown that in a multi-layer SNSPD the photon coupling efficiency is dramatically enhanced due to the increase in the optical path of the incident photon.