arXiv subjects

Carsten Timm

Publications and source records attributed to Carsten Timm.

94 records · Page 6Linked to original sources

arXiv1998

Two-Dimensional Quantum Ferromagnets

We present 1/N Schwinger boson and quantum Monte Carlo calculations of the magnetization and NMR relaxation rate for the two-dimensional ferromagnetic Heisenberg model representing a quantum Hall system at filling factor nu=1. Comparing the analytic and numerical calculations, we find that the SU(N) version of Schwinger boson theory gives accurate results for the magnetization at low temperatures, whereas the O(N) model works well at higher temperatures.

cond-mat.mes-hall↗

arXiv1996

Flux noise in high-temperature superconductors

Spontaneously created vortex-antivortex pairs are the predominant source of flux noise in high-temperature superconductors. In principle, flux noise measurements allow to check theoretical predictions for both the distribution of vortex-pair sizes and for the vortex diffusivity. In this paper the flux-noise power spectrum is calculated for the highly anisotropic high-temperature superconductor Bi-2212, both for bulk crystals and for ultra-thin films. The spectrum is basically given by the Fourier transform of the temporal magnetic-field correlation function. We start from a Berezinskii-Kosterlitz-Thouless type theory and incorporate vortex diffusion, intra-pair vortex interaction, and annihilation of pairs by means of a Fokker-Planck equation to determine the noise spectrum below and above the superconducting transition temperature. We find white noise at low frequencies omega and a spectrum proportional to 1/omega^(3/2) at high frequencies. The cross-over frequency between these regimes strongly depends on temperature. The results are compared with earlier results of computer simulations.

cond-mat.supr-con↗

arXiv1996

Generalization of the Berezinskii-Kosterlitz-Thouless theory to higher vortex densities

The Berezinskii-Kosterlitz-Thouless theory for superfluid films is generalized in a straightforward way that (a) corrects for overlapping vortex-antivortex pairs at high pair density and (b) utilizes a dielectric approximation for the polarization of the vortex system and a local field correction. Generalized Kosterlitz equations are derived, containing higher order terms, which are compared with earlier predictions. These terms cause the total pair density to remain finite for temperatures above the transition so that it is not necessary to introduce an ad hoc cut-off, as opposed to the original Berezinskii-Kosterlitz-Thouless theory. The low-temperature bound pair phase is destabilized for small vortex core energy. The behaviour of the stiffness constant and of the correlation length close to the transition is not affected by the higher order terms. A first-order transition as suggested by other authors is not found for any values of the parameters. The pair density is calculated for temperatures below and above the transition. Possible experiments and the applicability of the extended approach are discussed. The approach is found to be applicable even in a significant temperature range above the transition.

cond-mat↗

arXiv1995

Renormalization group approach to layered superconductors

A renormalization group theory for a system consisting of coupled superconducting layers as a model for typical high-temperature superconducters is developed. In a first step the electromagnetic interaction over infinitely many layers is taken into account, but the Josephson coupling is neglected. In this case the corrections to two-dimensional behavior due to the presence of the other layers are very small. Next, renormalization group equations for a layered system with very strong Josephson coupling are derived, taking into account only the smallest possible Josephson vortex loops. The applicability of these two limiting cases to typical high-temperature superconductors is discussed. Finally, it is argued that the original renormalization group approach by Kosterlitz is not applicable to a layered system with intermediate Josephson coupling.

supr-con↗