Evidence for Field-Induced Excitations in Low-Temperature Thermal Conductivity ofBi2Sr2CaCu2O8

The in-plane thermal conductivity $\kappa$ of $\mathrm{Bi}{}_2\mathrm{Sr}{}_2\mathrm{CaCu}{}_2{\mathrm{O}}_8$ was studied as a function of magnetic field along the $c$ axis. Above 5 K, as recently reported, $\kappa \left(H\right)$ 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.