Pressure Evolution of the Temperature Dependence of Strain in FeSe

The interplay between superconductivity and nematicity is a central issue in iron-based superconductors. Here we report simultaneous strain and electrical-resistivity measurements on FeSe under pressure, enabling direct tracking of structural, magnetic, and superconducting transitions through both lattice and transport responses. While the low-pressure nematic transition produces little anomaly in the bulk strain, the high-pressure magnetically driven orthorhombic transition is accompanied by a pronounced strain discontinuity. In addition, we discover a sharp hysteretic strain anomaly at a characteristic temperature below the resistively determined superconducting transition temperature. The anomaly exists only within the low-pressure nematic phase with little magnetic-field dependence, and disappears when the pressure-induced magnetic orthorhombic phase becomes dominant. The observed anomaly may be associated with the behavior of orthorhombic domains.