Thermal conductivity of REBa2Cu3O7-δ (RE = Er, Ho) below 1.2 K

The heat conductivity k of superconducting (SC) and non-superconducting (NSC) ErBa2Cu3O(7-δ) (ErBaCuO) and Sc HoBaCuO has been measured between 0.06 and ∼ 1 K and in magnetic fields up to 3 T. A proportionality k ∝ Tn with 2 < n < 3 has been observed in large temperature ranges. Heat is transported mainly by phonons. A change of n from 2.2 to 2.6 with increasing temperature is observed in SC ErBaCuO near TN = 0.60 K, the magnetic ordering temperature. This behavior is analogous to that of SC SmBaCuO. For T > TN, k decreases when external fields are applied. This applies for NSC ErBaCuO at all temperatures. For the latter material, this decrease can be described by an extra phonon scattering mechanism with a thermal resistivity Wpc ∝ T-2. Influence of spin waves i not probable, but can not be ruled out, either. In this paper the specific heat of HoBaCuO between 0.05 and 0.4 K is presented. The magnetic phase transition in this compound takes place at 0.17 K