New superconducting phases in field-induced organic superconductor lambda-(BETS)2FeCl4

TL;DR

This paper models the upper critical field in a quasi-2D organic superconductor considering the LOFF state, predicting phase transitions and effects of ion substitution on the superconducting transition order.

Contribution

It introduces a theoretical framework for the Hc2(T) behavior in lambda-(BETS)2FeCl4, including the impact of the LOFF state and ion substitution effects.

Findings

Predicted cascade of first-order vortex phase transitions.

Identified change from second to first order transition with ion substitution.

Proposed experimental study of Hc2(T) in tilted fields to verify the model.

Abstract

We derive the parallel upper critical field, Hc2, as a function of the temperature T in quasi-2D organic compound lambda-(BETS)2FeCl4, accounting for the formation of the nonuniform LOFF state. To further check the 2D LOFF model we propose to study the Hc2(T) curve at low T in tilted fields, where the vortex state is described by the high Landau level functions characterized by the index n. We predict a cascade of first order transitions between vortex phases with different n, between phases with different types of the symmetry at given n and the change of the superconducting transition from the second order to the first order as FeCl4 ions are replaced partly by GaCl4 ions.