Evidence for Field-Induced Excitations in Low-Temperature Thermal Conductivity of Bi₂Sr₂CaCu₂O₈

The in-plane thermal conductivity κ of Bi₂Sr₂CaCu₂O₈ was studied as a function of magnetic field along the c axis. Above 5 K, as recently reported, κ\(H\) 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.