Possible spin-polarized Cooper pairing in high temperature FeSe superconductor

TL;DR

This paper reports the coexistence of high-temperature superconductivity and itinerant ferromagnetism in lithium-intercalated FeSe flakes, suggesting possible spin-polarized Cooper pairing and a novel uniform coexistence of these phenomena.

Contribution

It demonstrates the simultaneous presence of superconductivity at around 40 K and ferromagnetism in FeSe, indicating a new form of coexistence and potential spin-polarized pairing mechanisms.

Findings

Observation of anomalous Hall effect with hysteresis

Ferromagnetism persists below superconducting transition

Superconductivity is enhanced by in-plane magnetic field

Abstract

Superconductivity and long-range ferromagnetism hardly coexist in a uniform manner. The counter-example has been observed, in uranium-based superconductors for instance, with a coexisting temperature limited to about 1 K. Here, we report the coexistence of high temperature superconductivity and itinerant ferromagnetism in lithium intercalated FeSe flakes. In superconducting samples with transition temperature around 40 K, we observe the anomalous Hall effect with a hysteresis loop in transverse resistivity and a butterfly-like pattern of magneto-resistance. Intriguingly, such ferromagnetism persists down to a temperature at which the zero-field resistance fully vanishes. Furthermore, the superconductivity is enhanced under an in-plane magnetic field, suggestive of the participation of spin-polarized Cooper pairs. The surprising finding underscores a uniform coexistence of the two antagonistic phenomena on a record-high energy scale.