Effect of the bias field on the thermal conductivity of KH2PO4 near the Curie point

The thermal conductivity of KH2PO4 (Potassium Dihydrogen Phosphate, KDP) single crystal under dc bias fields up to 2 kV/cm was measured using a novel technique called the 3ω method. Without a bias field, a small but definite discontinuity was observed at the Curie temperature (Tc = 123 K), indicating the first-order nature of the transition. The value of the thermal conductivity was very small due to the protonic scattering of phonons. The dc bias field tended to increase the thermal conductivity of KDP only in the vicinity of Tc and round the discontinuity at the same time. These results clearly demonstrate the coupling between the heat carrying phonons and the soft mode associated with the ferroelectric transition.