Unusual normal and superconducting state properties observed in hydrothermal Fe1-xSe flakes

TL;DR

This study reveals that hydrothermally grown Fe1-xSe flakes exhibit distinct electronic and superconducting properties, including higher Tc and isotropic upper critical fields, contrasting with CVT-grown FeSe that shows nematicity.

Contribution

It demonstrates that hydrothermal growth produces FeSe with no nematicity and enhanced superconducting properties, differing from chemically vapor-transport grown samples.

Findings

No electronic nematicity detected in hydrothermal FeSe.

Higher Tc of 13.2 K compared to CVT FeSe.

Isotropic upper critical field of ~42 T exceeding the Pauli limit.

Abstract

The electronic and superconducting properties of Fe1-xSe single-crystal flakes grown hydrothermally are studied by the transport measurements under zero and high magnetic fields up to 38.5 T. The results contrast sharply with those previously reported for nematically ordered FeSe by chemical-vapor-transport (CVT) growth. No signature of the electronic nematicity, but an evident metal-to-nonmetal crossover with increasing temperature, is detected in the normal state of the present hydrothermal samples. Interestingly, a higher superconducting critical temperature Tc of 13.2 K is observed compared to a suppressed Tc of 9 K in the presence of the nematicity in the CVT FeSe. Moreover, the upper critical field in the zero-temperature limit is found to be isotropic with respect to the field direction and to reach a higher value of ~42 T, which breaks the Pauli limit by a factor of 1.8.