THERMAL-CONDUCTIVITY OF AN UNTWINNED YBA2CU3O7-DELTA SINGLE-CRYSTAL - AN ALTERNATIVE INTERPRETATION IN TERMS OF CARRIER CONDENSATION

Data on the electronic contribution to the thermal conductivity of the untwinned YBaCuO single crystal are examined. Rather than focussing on the relaxation time, we explicitly take into account the effect of carrier condensation below the critical temperature in the spirit of a two-fluid model. Both the cases of full and partial condensation (at zero temperature) are examined. The electron-electron relaxation time is approximated to be a superposition of a Fermi-liquid contribution and of a fluctuation-like term. In the case of the more realistic partial condensation it is necessary to assume a leveling off of the relaxation time at low temperature. The bump in the thermal conductivity below T(c) at about T(c)/2 is very well reproduced by this phenomenological theory.