Relaxation and anomalous T- and H-dependence of the μ coefficient in (K, Ba)BiO₃ superconductors

Ac shielding and classical dc relaxation experiments have been used to study the flux creep phenomena in the cubic (K, Ba)BiO₃ superconductor (T_c ∼ 30 K). The relaxation rate is found to be constant (S ∼ 1.5%) at low temperature and magnetic field and increases sharply as the vortex-glass transition line is approached. This behavior can be attributed to an anomalous decrease of the μ exponent (U(J) = U₀(J₀/J)^μ) close to T_g(H). In this regime, the temperature dependence of the apparent critical current J is then directly related to μ(T) as J(T) - J₀/[kT/U₀ · ln(1/ωτ)]^μ(T). A similar analysis can be made on the J(B) data recently published by Abulafia et al. (Phys. Rev. Lett., 77 (1996) 1597) on YBaCuO single crystals.