Unusual magnetic-field dependence of the electrothermal conductivity in the mixed state of cuprate superconductors

In the mixed state of a superconductor, the electrothermal conductivity, P, which measures the electrical current density produced by a thermal gradient, is supposed to be independent of both the magnetic field and the details of the vortex motion. Measurements of the thermopower and resistivity in YBa₂Cu₃O_7-δ, Bi₂Sr₂CaCu₂O_8+δ, and Tl₂Ba₂CaCu₂O_8+δ show an unusual, sharp structure in P at low field. In Bi₂Sr₂CaCu₂O_8+δ, the extra contribution is negative, causing a sign change of the thermopower versus applied magnetic field. The possible origins of the low-field anomaly are discussed. © 1996 The American Physical Society.