Quantum tunneling of vortices in single crystal Tl2 CaBa2 Cu2O8 superconductors

Data are presented on the temperature-dependent time-logarithmic magnetic relaxation rate S(T) = |dM/d ln t| of the high-Tc superconductor Tl2CaBa2 Cu2O8. It is found that at low temperatures the relaxation rate has the form S(T) = A(H) exp[(T/T*)2], which does not extrapolate to zero at T = 0, thus excluding conventional thermally activated flux creep and providing evidence of quantum vortex tunneling. From a quantum flux tunneling theory, it is shown that S(T) ∝ 1/ηerp2. The measurements of the relaxation rate thus provide information about the effective viscosity ηe of fluxons