Observation of a Field Induced Transition in the Vortex Solid of ${\mathrm{DyBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7/\left(Y_{1-\mathit{x}}{\mathrm{Pr}}_{\mathit{x}}\right){\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ Superlattices

We report on the observation of a change in the field dependence of the critical current density $J_c$ of ${\mathrm{DyBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7/\left({\mathrm{Y}}_{0.45}{\mathrm{Pr}}_{0.55}\right){\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ superlattices at a characteristic magnetic field $H_s^{*}$. This change shows up as an abrupt transition when analyzing the current-voltage ( $I-V$) characteristics in the framework of the vortex glass and collective flux creep theories. This behavior can be traced back to a sudden reduction in the collective pinning energy $U_c$ above $H_s^{*}$, and may reflect a softening transition of the vortex lines in the vortex solid. $H_s^{*}$ is very similar to the characteristic field $H_l^{*}$ in the vortex liquid phase, which we interpreted to be due to entanglement.