Magnetic field - temperature phase diagram of quasi-two-dimensional organic superconductor lambda-(BETS)_2 GaCl_4 studied via thermal conductivity

TL;DR

This study maps the magnetic field-temperature phase diagram of the organic superconductor lambda-(BETS)_2 GaCl_4 using thermal conductivity, revealing how sample quality influences the nature of the superconducting transition and potential spatial modulation of the order parameter.

Contribution

It provides the first detailed phase diagram of this material via thermal conductivity, highlighting the effects of sample purity on the superconducting state and transition characteristics.

Findings

Dirty samples show Pauli-limited H_{c2} with sharp kappa(H) changes.

Clean samples exhibit no H_{c2} saturation and display second-order transition features.

Behavior suggests formation of a spatially modulated superconducting state in clean samples.

Abstract

The thermal conductivity kappa of the quasi-two-dimensional (Q2D) organic superconductor lambda-(BETS)_2 GaCl_4 was studied in the magnetic field H applied parallel to the Q2D plane. The phase diagram determined from this bulk measurement shows notable dependence on the sample quality. In dirty samples the upper critical field H_{c2} is consistent with the Pauli paramagnetic limiting, and a sharp change is observed in kappa(H) at H_{c2 parallel}. In contrast in clean samples H_{c2}(T) shows no saturation towards low temperatures and the feature in kappa(H) is replaced by two slope changes reminiscent of second-order transitions. The peculiarity was observed below ~ 0.33T_c and disappeared on field inclination to the plane when the orbital suppression of superconductivity became dominant. This behavior is consistent with the formation of a superconducting state with spatially modulated order parameter in clean samples.