Effects of columnar disorder on flux-lattice melting in high-temperature superconductors

TL;DR

This study uses Monte Carlo simulations to examine how columnar disorder influences the flux-lattice melting transition in high-temperature superconductors, revealing material-dependent effects on transition temperature and phase behavior.

Contribution

It provides a comparative analysis of the effects of columnar defects on flux-lattice melting in YBCO and BSCCO, highlighting differences in transition thresholds and phase transitions.

Findings

Transition temperature remains unchanged below a defect strength threshold.

Higher defect strengths shift the transition to higher temperatures and change its order.

Presence of columnar defects leads to a pinned Bose glass phase with decreased order.

Abstract

The effect of columnar pins on the flux-lines melting transition in high-temperature superconductors is studied using Path Integral Monte Carlo simulations. We highlight the similarities and differences in the effects of columnar disorder on the melting transition in YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) and the highly anisotropic Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (BSCCO) at magnetic fields such that the mean separation between flux-lines is smaller than the penetration length. For pure systems, a first order transition from a flux-line solid to a liquid phase is seen as the temperature is increased. When adding columnar defects to the system, the transition temperature is not affected in both materials as long as the strength of an individual columnar defect (expressed as a flux-line defect interaction) is less than a certain threshold for a given density of randomly distributed columnar pins. This threshold strength is lower for YBCO than for BSCCO. For higher strengths the transition line is shifted for both materials towards higher temperatures, and the sharp jump in energy, characteristic of a first order transition, gives way to a smoother and gradual rise of the energy, characteristic of a second order transition. Also, when columnar defects are present, the vortex solid phase is replaced by a pinned Bose glass phase and this is manifested by a marked decrease in translational order and orientational order as measured by the appropriate structure factors. For BSCCO, we report an unusual rise of the translational order and the hexatic order just before the melting transition. No such rise is observed in YBCO.