Langevin Dynamics of the vortex matter two-stage melting transition in Bi_2Sr_2CaCu_2O$_{8+\delta}$ in the presence of straight and of tilted columnar defects

TL;DR

This study uses Langevin molecular dynamics simulations to explore the two-stage melting transition of vortex matter in Bi_2Sr_2CaCu_2O_{8+eta} with columnar defects, revealing details about phase transitions and effects of defect tilt.

Contribution

It provides detailed Langevin dynamics analysis of vortex melting, confirming two-stage transition and examining the impact of tilted columnar defects on phase behavior.

Findings

Vortex matter melts in two stages: nanocrystalline to nanoliquid, then to homogeneous liquid.

Tilted columnar defects do not affect melting and delocalization transition locations.

Irreversibility line and dynamical properties are influenced by defect tilt.

Abstract

In this paper we use London Langevin molecular dynamics simulations to investigate the vortex matter melting transition in the highly anisotropic high-temperature superconductor material Bi_2Sr_2CaCu_2O$_{8+\delta}$ in the presence of low concentration of columnar defects (CDs). We reproduce with further details our previous results obtained by using Multilevel Monte Carlo simulations that showed that the melting of the nanocrystalline vortex matter occurs in two stages: a first stage melting into nanoliquid vortex matter and a second stage delocalization transition into a homogeneous liquid. Furthermore, we report on new dynamical measurements in the presence of a current that identifies clearly the irreversibility line and the second stage delocalization transition. In addition to CDs aligned along the c-axis we also simulate the case of tilted CDs which are aligned at an angle with respect to the applied magnetic field. Results for CDs tilted by $45^{\circ}$ with respect to c-axis show that the locations of the melting and delocalization transitions are not affected by the tilt when the ratio of flux lines to CDs remains constant. On the other hand we argue that some dynamical properties and in particular the position of the irreversibility line should be affected.