Robust superconductivity and fragile magnetism induced by the strong Cu impurity scattering in the high-pressure phase of FeSe

TL;DR

This study reveals that high-pressure superconductivity in FeSe is resilient to Cu impurities, while magnetic phases are fragile, indicating a robust superconducting state with a potentially sign-preserving order parameter.

Contribution

It demonstrates that Cu impurity scattering does not suppress high-pressure superconductivity in FeSe, unlike its effect at ambient pressure, highlighting the robustness of the superconducting phase.

Findings

Superconductivity remains robust under Cu impurity at high pressure.

Magnetic phase signatures are suppressed by impurities at low pressure.

Magnetic anomalies appear under magnetic field despite the absence of magnetic signatures in zero field.

Abstract

Superconductivity in FeSe is strongly enhanced under applied pressure and it is proposed to emerge from anomalously coupled structural and magnetic phases. Small impurities inside the Fe plane can strongly disrupt the pair formation in FeSe at ambient pressure and can also reveal the interplay between normal and superconducting phases. Here, we investigate how an impurity inside the Fe plane induced by the Cu substitution can alter the balance between competing electronic phases of FeSe at high pressures. In the absence of an applied magnetic field, at low pressures the nematic and superconducting phases are suppressed by a similar factor. On the other hand, at high pressures, above 10 kbar, the superconductivity remains unaltered despite the lack of any signature in transport associated to a magnetic phase in zero-magnetic field. However, by applying a magnetic field, the resistivity displays an anomaly preceding the activated behaviour in temperature, assigned to a magnetic anomaly. We find that the high-pressure superconducting phase of FeSe is robust and remains enhanced in the presence of Cu impurity, whereas the magnetic phase is not. This could suggest that high-$T_{\rm c}$ superconductivity has a sign-preserving order parameter in a presence of a rather glassy magnetic phase.