Hg NMR spin-lattice relaxation rates in an aligned powder sample of the anisotropic high-T c superconductor HgBa 2 CuO 41d (T c 96 K) show anisotropy in the temperature interval 10 -50 K, with an extra contribution present for the external magnetic field parallel to the c axis. The extra contribution shifts with magnetic field in a way related to the irreversibility temperature T irr , indicating that the thermal motion of flux lines (FL's) is the cause of the extra relaxation. A simple model is presented to analyze the relaxation data and to extract values for the correlation time t c and the effective activation energy U of the FL motion.

Single-phase Yl Ba2Cu309 -"has been synthesized and characterized. Although x-ray diffraction shows only the Yl Ba2Cu309 -"phase which has an orthorhombically distorted perovskite structure, resistance measurements reveal two transition temperatures, T, l =95.5 K and T,2 =93 K. Thermoelectric power measurements indicate that YlBa2Cu309is of n type.Strong enhancement and unusual temperature dependence for the "phonon-drag" effect which might be indicative of an unusual superconducting mechanism in this material have been observed.