A theory of thermal-fluctuation-induced depinning line in weakly pinning high-T_c superconductors

The expression for the thermal-fluctuation-induced depinning line, T = T_P (B_e), is derived theoretically for weakly pinning high-T_c superconductors. In the derivation, it is shown that the thermal displacement of pinned fluxoids, u_th reduces the critical current density as J_c = J_c0 ( - 〈u²_th〉/d²_p) ^{3 2}, where J_c0 is the critical current density in the absence of u_th and d_p is the effective half width of the summed-up pinning potential. Then T_p (itB_e) is determined as the temperature at which J_c becomes zero due to the increase of 〈u²_th〉 with the increase of T in the external flux density of B_e. The relation of T_p(B_e) with the irreversibility line of T_i(B_e) and the flux melting line of T_m(B_e) is also discussed.