Low Temperature Action inYBa2Cu3O7−δ

We have performed a high-resolution study of the temperature dependence of vortex dynamics for 5[times]10[sup [minus]4][lt][ital T]/[ital T][sub [ital c]][lt]10[sup [minus]2] in untwinned single crystals of the high-[ital T][sub [ital c]] superconductor, YBa[sub 2]Cu[sub 3]O[sub 7[minus][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.