Fragile pressure-induced magnetism in FeSe superconductors with a thickness reduction

TL;DR

This study investigates how reducing the thickness of FeSe superconductors affects pressure-induced magnetism and superconductivity, revealing that magnetic fluctuations are crucial for high transition temperatures.

Contribution

The paper provides experimental evidence that magnetic fluctuations, rather than nematic fluctuations alone, are essential for enhancing $T_c$ in FeSe under pressure, especially in thin flakes.

Findings

Nematic phase boundary remains stable with decreasing thickness.

Magnetic order weakens significantly as thickness decreases.

Maximum $T_c$ is reduced with weakening magnetism and no high-$T_c$ phase in the thinnest samples.

Abstract

The emergence of high transition temperature ($T_c$) superconductivity in bulk FeSe under pressure is associated with the tuning of nematicity and magnetism. However, sorting out the relative contributions from magnetic and nematic fluctuations to the enhancement of $T_c$ remains challenging. Here, we design and conduct a series of high-pressure experiments on FeSe thin flakes. We find that, as the thickness decreases, the nematic phase boundary on temperature-pressure phase diagrams remains robust while the magnetic order is significantly weakened. A local maximum of $T_c$ is observed outside the nematic phase region, not far from the extrapolated nematic endpoint in all samples. However, the maximum $T_c$ value is reduced associated with the weakening of magnetism. No high-$T_c$ phase is observed in the thinnest sample. Our results strongly suggest that nematic fluctuations alone can only have a limited effect while magnetic fluctuations are pivotal on the enhancement of $T_c$ in FeSe.