The mixed state in '1212' superconductors

Abstract

Since the discovery of high-temperature superconductors (HTSs), many attempts have been made to explain the vortex state in these materials, but to date no consensus has yet been reached. One particular vortex-state phenomenon that remains controversial is the sign reversal of the Hall resistivity, which is seen in practically all HTSs. A thorough understanding of the mixed state would allow for the understanding of flux pinning in these materials. Aside from solving an old mystery, this would also make it possible to optimize the pinning capability of HTSs. Such optimization would make it possible to greatly improve the current-carrying ability of these materials, thus making them even more viable for industrial applications, especially in power transmission.
This study offers to shed light on these issues by examining and comparing the mixed state oftwo isomorphic superconductors, TlBa₂CaCu₂O_7-δ (Tl-1212, T_c ~ 90 K) and HgBa₂CaCu₂O_6+δ (Hg-1212, T_c ~ 120 K). Since the structural anisotropy of these two are practically identical, any similar properties in their mixed state could be ascribed mainly to the physical structure of an HTS and any dissimilar properties could be ascribed mainly to the electronic structure of an HTS. The 30 K difference in the T_c of these species has been ascribed to differences in electronic structure, primarily the redistribution of oxygen in the unit cell as a result of exchanging Tl³⁺ cations with Hg²⁺ cations. This study is also expected to shed more light on this possibility. It is also possible for such oxygen redistribution to alter mixed-state behavior.
The following phenomena were studied in particular: (1) transitions in the H-T phase diagram, (2) the behavior of the mixed-state component of the Hall angle, tan Θ_H, and (3) the sign reversal of Hall resistivity.