Theoretical Description of Nearly Discontinuous Transition in Superconductors with Paramagnetic Depairing

TL;DR

This paper demonstrates through theoretical analysis and Monte Carlo simulations that in certain type-II superconductors with both paramagnetic and orbital depairing effects, the expected first-order transition at Hc2 becomes a crossover due to fluctuations, explaining experimental observations.

Contribution

It provides a theoretical and simulation-based explanation for the absence of a true first-order transition in real superconductors with paramagnetic depairing.

Findings

First-order transition at Hc2 is replaced by a crossover due to fluctuations.

The crossover occurs in clean superconductors with singlet pairing and high condensation energy.

Experimental observations in CeCoIn5 are consistent with the theoretical prediction.

Abstract

Based on a theoretical argument and Monte Carlo simulations of a Ginzburg-Landau model derived microscopically, it is argued that, in type-II superconductors where {\it both} the paramagnetic {\it and} orbital depairings are important, a strong first-order transition (FOT) at $H_{c2}$ expected in the mean field (MF) approximation never occurs in real systems and changes due to the fluctuation into a crossover. The present result explains why a {\it nearly} discontinuous crossover at $H_{c2}$ with {\it no} intrinsic hysteresis is observed only in a clean superconducting material with a singlet pairing and a high condensation energy such as CeCoIn$_5$.