Low Temperature Action inYBa2Cu3O7−δ

We have performed a high-resolution study of the temperature dependence of vortex dynamics for $5\times {10}^{-4\mathrm{}}<{T/T}_c<{10}^{-2\mathrm{}}$ in untwinned single crystals of the high- $T_c$ superconductor, ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{O}_{7-\delta }$. Through the introduction of a “bithermal” magnetization relaxation technique, we have uncovered a previously unresolved, linear temperature contribution to the semiclassical action for vortex tunneling. Finally, we have compared the weak and strong pinning regimes through heavy ion irradiation of a series of crystals.