AC Losses in High-T C Superconductors with Finite Thickness
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C8.13.1
Final Report on The Funded Project No: 07-02-2-11/03
AC Losses in High-TC Superconductors with Finite Thickness
Maamar Benkraouda and M. Mustafa Physics Department, U.A.E. University P.O. Box: 17551 Al-Ain, United Arab Emirates
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ABSTRACT The structure and dynamics of magnetic vortices in high-Tc superconductors are studied. The model we use is applicable to highly anisotropic high-Tc superconductors, such as Bi and Th compounds, where the Josephson coupling is neglected between the superconducting layers. In this paper, we studied the ac losses occurring in a finite number of superconducting layers due to an applied ac current. It is found that the ac losses increase with the frequency and the amplitude of the applied current. By introducing random pinning centers in the layers, the effect of pinning on the reduction of ac losses is also studied.
C8.13.3
INTRODUCTION High-Temperature superconductors (HTS) are type-II superconductors. This means that magnetic field penetrates these superconductors in the form of vortices without destroying superconductivity. Because superconductivity occurs only in the CopperOxide layers in HTS, this introduces anisotropy in the structure of vortex lattice; they become pancake-like vortices instead of cylindrical tubes [1]. These vortices, under the effect of a dc or an ac applied current, could easily move because of the Lorentz force. This motion causes dissipation and hence undermines the essence of superconductivity. For this, the study of ac losses becomes significant in HTS. To overcome this problem and maintain a non-dissipative transport current, it is necessary to pin these vortices in order to eliminate the resistance due to vortex motion. Pinning is mostly associated with substituting impurities [2] and introduced defects [3] by methods such as radiation damage [4]. However, various promising techniques like periodic pinning [4], stress field pinning [5], and intrinsic pinning mechanism [6] as well as twin boundaries [7] or grain boundary [8] are considered to be effective. The main goal of the study here is to simulate the dynamics of pancake vortices in a finite stack of Josephson-decoupled layers in the case of an applied ac transport current to the top layer. We will consider the two cases: (1) pinning free and (2) interlayer random but intralayer uniofrmly distributed pinning force. By applying different current amplitudes at different frequencies, the flux flow and the flux difference between the top and bottom layers are calculated as function of time. Finally, the average power dissipation versus frequency is calculated for different pinning force distribution. All these quantities can be measured experimentally and compared to our calculations.
1. Description of the model We consider a finite stack of N Josphson-decoupled superconducting layers and assume that the pancake vortices have perfect hexagonal shape that won’t change through the experiment. Thermal fluctuations and Josephson coupling effects are neglected. At equilibrium, with ap
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