Y.F. Yan

Negative magnetoresistance observed in c-axis conduction in Bi2Sr2CaCu2O8+δ

Abstract The magnetoresistance for charge transport along the c -axis is negative in Bi 2 Sr 2 CaCu 2 O 8+ δ , unlike the in-plane transport. We show that Δϱ c 0 / ϱ c is thermally activated with a gap Δ that varies from 520 K to 997 K as the doping proceeds from under-to over-doped. We suggest that the applied field lowers the barrier impeding c -axis charge transport, by coupling to the Zeeman energy.

Violation of Kohler's rule in magnetoresistance of YBa2Cu3O7−δ

Abstract In both the 90K and 60K phases of YBa2Cu3O7−δ, the transverse magnetoresistance is positive and varies with temperature as T−4, instead of the T−2 behavior predicted by Kohlers rule.This unusual behavior in the MR is deeply related to the anomalous behavior of the Hall angle.

The low temperature microwave surface resistance of 90-K and 60-K YBa2Cu3O7−δ in magnetic fields

Abstract With a bolometric technique, we have measured, in intense fields, the low temperature microwave surface resistance of 90-K and 60-K YBCO single crystals. From the viscosity inferred from the field dependence of surface resistance, the quasiparticle scattering rate 1/τ in the vortex core of YBCO is found to be strongly suppressed. The long relaxation times of quasiparticle place YBCO in the “superclean regime” where the lifetime broadening of the core energy levels is much less than the energy spacing.

Sharp magneto-absorption resonances in the vortex state of Bi2Sr2CaCu2O8+δ

Abstract We have observed sharp resonances in the absorption of microwave radiation (30 to 50 GHz) in the vortex state of Bi 2 Sr 2 CaCu 2 O 8+δ . The unusual dispersion and temperature dependence suggest the excitation of an electronic mode strongly coupled to the vortex lattice.

Additive quasi-particle and vortex Hall currents in the cuprate superconductors

Abstract New data and analysis show that the Hall conductivity σ xy of cuprate superconductors consists of two additive contributions: a quasiparticle excitation term and a vortex motion term. Competition between the two leads to a sign reversal in YBCO but not in LSCO, where they both have the same sign.