Evidence for field-induced excitations in low-temperature thermal conductivity of Bi2Sr2CaCu2O8

The in-plane thermal conductivity kappa of Bi2Sr2CaCu2O8 was Studied as a function of magnetic field along the c axis. Above 5 K, as recently reported, kappa(H) decreases, then shows a kink followed by a plateau. By contrast, below 1 K, the thermal conductivity was found to increase with increasing field. This behavior is indicative of a finite density of states and is not compatible with the vanishing quasiparticle transport due to a field-induced phase transition as recently proposed to describe the plateau regime. Our low-temperature results are in agreement with recent works predicting a field-induced enhancement of thermal conductivity by Doppler shift of the quasiparticle spectrum. [S0031-9007(98)08250-7].