Comment on ``Loss of Superconducting Phase Coherence in YBa_2Cu_3O_7 Films: Vortex-Loop Unbinding and Kosterlitz-Thouless Phenomena''

Recently, Kotzler et al. measured the frequency-dependent conductance for YBa_2Cu_3O_7 and interpreted their results as evidences that the decay of the superfluid density is caused by a 3D vortex loop proliferation mechanism and a dimensional crossover when the correlation length $\xi_c$ along the c axis becomes comparable to the sample thickness $d$ [PRL 87, 127005(2001)]. In this Comment, we show that the complex conductance data presented by Kotzler et al. have characteristic key features not compatible with their analysis, which are instead described by the existing phenomenology of 2D vortex fluctuation associated with a partial decoupling of CuO_2-planes.Comment: 2 pages, 1 figure, accepted in PR

Splitting of the superconducting transition in the two weakly coupled 2D XY models

The frequency $\omega$ and temperature T dependent complex conductivity $\sigma$ of two weakly coupled 2D XY models subject to the RSJ dynamics is studied through computer simulations. A double dissipation-peak structure in $Re[\omega\sigma]$ is found as a function of T for a fixed frequency. The characteristics of this double-peak structure, as well as its frequency dependence, is investigated with respect to the difference in the critical temperatures of the two XY models, originating from their different coupling strengths. The similarity with the experimental data in Festin {\it et al.} [Physica C 369, 295 (2002)] for a thin YBCO film is pointed out and some possible implications are suggested.Comment: 4 pages, 4 figure

Ubiquitous finite-size scaling features in IV characteristics of various dynamic XY models in two dimensions

Two-dimensional (2D) XY model subject to three different types of dynamics, namely Monte Carlo, resistivity shunted junction (RSJ), and relaxational dynamics, is numerically simulated. From the comparisons of the current-voltage (I-V) characteristics, it is found that up to some constants I-V curves at a given temperature are identical to each other in a broad range of external currents. Simulations of the Villain model and the modified 2D XY model allowing stronger thermal vortex fluctuations are also performed with RSJ type of dynamics. The finite-size scaling suggested in Medvedyeva et al. [Phys. Rev. B 62, 14531(2000)] is confirmed for all dynamic models used, implying that this finite-size scaling behaviors in the vicinity of the Kosterlitz-Thouless transition are quite robust.Comment: 7 pages, 4 pictures, accepted in Physica

Finite-Size Effects and Dynamical Scaling in Two-Dimensional Josephson Junction Arrays

In recent years many groups have used Fisher, Fisher, and Huse (FFH) dynamical scaling to investigate and demonstrate details of the superconducting phase transition. Some attention has been focused on two dimensions where the phase transition is of the Kosterlitz-Thouless-Berezinskii (KTB) type. Pierson et al. used FFH dynamical scaling almost exclusively to suggest that the dynamics of the two-dimensional superconducting phase transition may be other than KTB-like. In this work we investigate the ability of scaling behavior by itself to yield useful information on the nature of the transition. We simulate current-voltage (IV) curves for two-dimensional Josephson junction arrays with and without finite-size-induced resistive tails. We find that, for the finite-size effect data, the values of the scaling parameters, specifically the transition temperature and the dynamical scaling exponent z, depend critically on the magnitude of the contribution that the resistive tails make to the IV curves. In effect, the values of the scaling parameters depend on the noise floor of the measuring system.Comment: 24 pages, 8 figures; submitted to Physical Review

Characteristic properties of two-dimensional superconductors close to the phase transition in zero magnetic field

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