• PDF / 248,929 Bytes
• 4 Pages / 420.48 x 639 pts Page_size
• 11 Downloads / 249 Views

DOWNLOAD

REPORT

---

AC SUSCEPTIBILITY STUDIES OF THE HIGH TC SUPERCONDUCTORS: DISSIPATIVE EFFECTS IN La2.xSrxCuO4-y AND YBa 2 Cu 3 0 7 .Y SYSTEMS

XINSHENG LING, M. E. FILIPKOWSKI, E. K. HELLER, J. I. BUDNICK Department of Physics & Institute of Materials Science, University of Connecticut, Storrs, CT 06269

ABSTRACT We have studied the variation of dissipative effects with doping in the high Tc superconductors La2.xSrxCuO4.y and YBa2Cu307-y by studying the lossy component of ac susceptibility as a function of temperature. We find relatively strong excess losses at low temperature in both systems, which indicate stronger flux creep at grain boundaries, in lower level doped ceramic samples produced by a similar process. This effect is more significant in the La2.xSrxCuO4.y (214) system than in the YBa2Cu3O7.y(123) system. We suggest that the microscopic mechanisms of weakening of the order parameter in the grain boundaries may be different in the two systems. In the 214 system, as the charge carrier density in the CuO2 planes is reduced , the spatial fluctuation of the order parameter in the CuO2 planes is enhanced. In the 123 system, the initial removal of 0 from the sample, which weakens the coupling between planes and increases the spatial fluctuation of the order parameter between CuO2 planes. The grain boundaries are degraded in both systems. The degradation in the 214 samples is more significant than in the 123 samples. INTRODUCTION AC susceptibility measurements on high Tc superconducting ceramics YBa 2 Cu 307.y have been used to study the ac losses at the grain boundaries[1]. The hysteretic ac losses in type-Il superconductors at low frequencies are caused by viscous motion of magnetic flux lines. Three kinds of mechanisms are involved[2]. Firstly, flux lines creeping between pinning sites or flowing driven by a supercurrent induced by the ac field is responsible for bulk pinning losses. A loss peak would occur at the temperature the pinned flux lines start to flow. Secondly, when flux lines move in and out of the sample during each cycle, the surface barriers cause surface losses. Thirdly, viscous losses, analogous to eddy current losses in normal conductors, would occur even in the absence of flux pinning, caused by annihilation of vortices when moving vortices with flux lines in a direction opposite to the magnetic field. In a uniform conventional type-f superconductor, Nb 3 Al for example, at temperatures above Tc, a small ac loss from normal electron eddy current losses shows up. During the cooling through Tc, the transition from flowing to pinned of flux lines gives a loss peak while the real component shows a steep increase of shielding ( change from 0 to almost -1/41t ). At low temperatures, the ac loss is smaller than that of the normal state, because the surface of the sample can sustain a stable supercurrent to shield off all the interior losses. The bulk pinning losses and the surface losses are both important in high Tc superconducting ceramics, while the viscous loss play a smaller role[3]. The flux creep and flow