Nonlinear effective barriers for flux diffusion and critical current density of ${\mathrm{HgBa}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ based upon ac susceptibility measurement

The complex ac susceptibility of ${\mathrm{HgBa}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{x}}$ was measured systematically as a function of temperature T, ac field amplitude, frequency, and dc field ${\mathit{H}}_{\mathit{d}}$ to study the flux dynamics in a time window of ${10}^{\mathrm{-}5}$–${10}^{\mathrm{-}2}$ s. The dependences of the effective barriers for flux diffusion U on current density J, U∝${\mathit{J}}^{\mathrm{-}\mathrm{\mu }}$, as well as ${\mathit{H}}_{\mathit{d}}$ and T were determined. The rate of current relaxation J(t), E vs J curves, and the so-called irreversibility line were deduced from U. The critical current density ${\mathit{J}}_{\mathit{c}}$ as a function of T and ${\mathit{H}}_{\mathit{d}}$ was estimated. μ=0.43 is close to 0.5, one of the glassy exponents predicted by the vortex glass and collective creep theories. The high and stable ${\mathit{J}}_{\mathit{c}}$(${\mathit{H}}_{\mathit{d}}$,T) and the highest ${\mathit{T}}_{\mathit{c}}$ indicate that the Hg-based superconductor is a promising material for technical applications. © 1996 The American Physical Society.