Thermodynamics of the vortex liquid in heavy ion-irradiated superconductors

TL;DR

This paper investigates how heavy ion irradiation affects the thermodynamic and vortex properties of anisotropic superconductors, revealing differences in pinning behavior and fluctuation effects between YBa2Cu3O7−δ and KxBa1−xBiO3.

Contribution

It demonstrates the extended influence of irradiation-induced defects on thermodynamics and vortex pinning, challenging existing views on the Bose glass transition in these materials.

Findings

Irradiation shifts the specific heat maximum in YBa2Cu3O7−δ.

Pinning appears at different temperatures relative to the specific heat maximum in the two compounds.

The pinning energy at the Bose glass transition is large compared to the total free energy gain.

Abstract

It is shown that the large effect of heavy ion-irradiation on the thermodynamical properties of the anisotropic superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ extends well into the superconducting fluctuation regime. The presence of the induced amorphous columnar defects shifts the specific heat maximum at the normal-to-superconducting transition. This effect is similar to that recently put into evidence in cubic K$_{x}$Ba$_{1-x}$BiO$_{3}$ ($x \simeq 0.35$). In both compounds, vortex pinning manifests itself as a sharp angular dependence of the \em equilibrium \rm torque. In YBa$_{2}$Cu$_{3}$O$_{7-\delta}$, pinning by the defects appears at the temperature $T_{C_{p}}^{max}$ of the specific heat maximum, well above the magnetic irreversibility line $T_{irr}(H)$. In isotropic K$_{x}$Ba$_{1-x}$BiO$_{3}$, the onset of the pinning-related torque anomaly tracks the onset of the specific heat anomaly and the irreversibility line. In YBa$_{2}$Cu$_{3}$O$_{7-\delta}$, fluctuations of the amplitude of the order parameter (and not vortex line wandering) are ultimately responsible for the vanishing of pinning. In K$_{x}$Ba$_{1-x}$BiO$_{3}$, vortex pinning disappears only at the superconducting-to-normal transition. The results indicate that in both compounds, the pinning energy at the ``Bose glass'' transition is large with respect to the total free energy gain in the superconducting state. By implication, the mechanism of this latter transition should be reconsidered.