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 (Formula presented)(Formula presented)(Formula presented), (Formula presented)(Formula presented)(Formula presented)(Formula presented), and (Formula presented)(Formula presented)(Formula presented)(Formula presented) show an unusual, sharp structure in P at low field. In (Formula presented)(Formula presented)(Formula presented)(Formula presented), 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.