Thermal Depinning of Flux Lines inHgBa2CuO4+δfrom199Hg Spin-Lattice Relaxation

{sup 199}Hg NMR spin-lattice relaxation rates in an aligned powder sample of the anisotropic high-{ital T}{sub {ital c}} superconductor HgBa{sub 2}CuO{sub 4+{delta}} ({ital T}{sub {ital c}}=96 K) show anisotropy in the temperature interval 10{endash}50 K, with an extra contribution present for the external magnetic field parallel to the {ital c} axis. The extra contribution shifts with magnetic field in a way related to the irreversibility temperature {ital T}{sub irr}, indicating that the thermal motion of flux lines (FL{close_quote}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 {tau}{sub {ital c}} and the effective activation energy {ital U} of the FL motion. {copyright} {ital 1996 The American Physical Society.}