Observation of Ising spin-nematic order and its close relationship to the superconductivity in FeSe single crystals

Abstract

Superconducting FeSe single crystals of (001) orientation are synthesized via a hydrothermal ion-release route. An Ising spin-nematic order is identified by our systematic measurements of in-plane angular-dependent magnetoresistance (AMR) and static magnetization. The turn-on temperature of anisotropic AMR signifies the Ising spin-nematic ordering temperature ${T}_{\mathrm{sn}}$, below which a twofold rotational symmetry is observed in the iron plane. A downward curvature appears below ${T}_{\mathrm{sn}}$ in the temperature dependence of static magnetization for the weak in-plane magnetic field as reported previously. Remarkably, we find a universal linear relationship between ${T}_{\mathrm{c}}$ and ${T}_{\mathrm{sn}}$ among various superconducting samples, indicating that the spin nematicity and the superconductivity in FeSe have a common microscopic origin.