THERMAL-CONDUCTIVITY OF POLYCRYSTALLINE YBA2CU4O8 AND THE PHONON TRANSPORT MODEL

We have measured the thermal conductivity kappa of bulk YBa2Cu4O8 (1-2 -4) between 30 and 310 K and at pressures p up to 1 GPa. At p = 0, kap pa = 10 Wm(-1)K(-1) near 100 K, twice that of sintered YBa2Cu3O7-delta (1-2-3). kappa also decreases with increasing T more rapidly than in 1-2-3, to 7.4 Wm(-1)K(-1) at 300 K. Under pressure, kappa increases sl owly with p above 150 K, but d kappa/dp decreases at lower T. Our resu lts are in excellent agreement with a semi-classical model for phonon thermal conductivity, provided we assume significant charge transfer w ith pressure. The calculated phonon mean free path is much larger than the lattice spacing at all T.