Experimental discovery of Sarma state in atomically thick superconducting FeSe films under high magnetic fields

TL;DR

This paper reports the first experimental observation of the Sarma state, a gapless spin-polarized superfluid, in atomically thin FeSe films under high magnetic fields, revealing new physical properties and potential for superconducting spintronics.

Contribution

It provides unambiguous experimental evidence of the Sarma state in FeSe films, demonstrating its emergence under high magnetic fields and anisotropic behavior.

Findings

Observation of Zeeman splitting coherence peaks crossing the Fermi level.

Two-fold symmetry in critical magnetic field due to anisotropic g-factor.

Induction of Sarma state in monolayer FeSe with out-of-plane magnetic fields.

Abstract

Many-body ground states of imbalanced Fermi gas have been studied both theoretically and experimentally for several decades because of their fundamental significance in condensed matter physics, cold atom physics and nuclear physics. The Sarma state, a gapless spin-polarized superfluid, is one of those long sought-after exotic ground states of spin imbalanced Fermi gas. Yet, an unambiguous experimental evidence of Sarma superfluid state has not been found. Here, we report the experimental discovery of the Sarma state in atomically thick FeSe films by a dilution-refrigerator scanning tunneling microscope under high magnetic fields. In the bilayer or trilayer FeSe films, we directly observe the key evidence of the entrance of the Sarma state: the inner Zeeman splitting coherence peaks cross the Fermi level under high in-plane magnetic fields. The angle dependent critical in-plane magnetic field of coherence peak crossing shows a two-fold symmetry due to the anisotropy of the in-plane g-factor of FeSe films. Moreover, in a superconducting FeSe monolayer of a lateral size of several hundred nanometers, the Sarma state can also be induced by strong out-of-plane magnetic fields. Our findings pave the way to explore the unusual physical properties and potential applications in superconducting spintronics of the spin-polarized Sarma superfluid state.