Superconducting phase diagram and FFLO signature in $\lambda$-(BETS)$_2$GaCl$_4$ from rf penetration depth measurements

TL;DR

This study maps the superconducting phase diagram of $mbda$-(BETS)$_2$GaCl$_4$ using rf penetration depth measurements, revealing evidence of FFLO state signatures and complex surface impurity effects.

Contribution

First detailed phase diagram of $mbda$-(BETS)$_2$GaCl$_4$ showing FFLO signatures and angle-dependent Pauli limiting effects.

Findings

Identification of a tricritical point at 1.6 K and 10 T.

Evidence of FFLO state in high field low temperature region.

Observation of surface impurity effects on penetration depth data.

Abstract

We report the phase diagram of $\lambda$-(BETS)$_2$GaCl$_4$ from rf penetration depth measurements with a tunnel diode oscillator in a pulsed magnetic field. We examined four samples with 1100 field sweeps in a range of angles with the magnetic field parallel and perpendicular to the conducting planes. In the parallel direction, $H_{c2}$ appears to include a tricritical point at 1.6 K and 10 T with a phase line that increases to 11 T as the temperature is decreased to} 500 mK. The second phase line forms a clearly defined high field low temperature region satisfying several of the conditions of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. We show remarkably good fits of $H_{c2}$ to WHH in the reentrant $\alpha>1$, $\lambda_{so}=0$ regime. We also note a sharp angle dependence of the phase diagram about the field parallel orientation that characterizes Pauli paramagnetic limiting and further supports the possibility of FFLO behavior. Unrelated to the FFLO study, at fields and temperatures below $H_{c2}$ and $T_c$, we find rich structure in the penetration depth data that we attribute to impurities at the surface altering the superconducting properties while maintaining the same crystallographic axes as $H_{c2}$.