DISSIPATIVE QUANTUM-CREEP STUDIES IN SINGLE-CRYSTAL LA2CUO4+DELTA

Temperature-dependent magnetic relaxation was studied in a La2CuO4+del ta crystal in the low-temperature range 2-10 K. The experimental data exhibited a non-vanishing magnetic relaxation in the low-temperature l imit. These data could be well interpreted by a quantum-creep theory, suggesting the non-vanishing relaxation at low temperatures due to qua ntum tunneling of vortices. The effective Euclidean action determined as HBAR/\d ln M/d ln t\ showed a quadratic temperature dependence. The quantitative analysis of the effective Euclidean action yields a clas sical activation energy, crossover temperature and zero-temperature qu antum-tunneling rate.