Separate tuning of nematicity and spin fluctuations to unravel the origin of superconductivity in FeSe

Yoichi Tanabe,Jun Sung Kim,Katsumi Tanigaki,Saicharan Aswartham,Bernd Büchner,Igor Morozov,Jong Mok Ok,Dmitriy Chareev,Dmitri V Efremov,Seung-Ho Baek,Takahiro Urata

doi:10.1038/s41535-020-0211-y

Abstract

The interplay of orbital and spin degrees of freedom is the fundamental characteristic in numerous condensed matter phenomena, including high-temperature superconductivity, quantum spin liquids, and topological semimetals. In iron-based superconductors (FeSCs), this causes superconductivity to emerge in the vicinity of two other instabilities: nematic and magnetic. Unveiling the mutual relationship among nematic order, spin fluctuations, and superconductivity has been a major challenge for research in FeSCs, but it is still controversial. Here, by carrying out 77Se nuclear magnetic resonance (NMR) measurements on FeSe single crystals, doped by cobalt and sulfur that serve as control parameters, we demonstrate that the superconducting transition temperature Tc increases in proportion to the strength of spin fluctuations, while it is independent of the nematic transition temperature Tnem. Our observation therefore directly implies that superconductivity in FeSe is essentially driven by spin fluctuations in the intermediate coupling regime, while nematic fluctuations have a marginal impact on Tc.

Highlights

In correlated Fermi fluids, nematicity refers to the state in which rotational symmetry is spontaneously broken, while time-reversal invariance is preserved, and the symmetry of the crystal changes from tetragonal to orthorhombic.[1]
It is quite interesting to note that recent nuclear magnetic resonance (NMR) studies of FeSe under high pressure reveal the persistence of local nematicity at temperatures far above Tnem, which suggests a correlation between local nematicity and magnetism.[23,24]
Another interesting observation by NMR is the unusual suppression of ðT1TÞÀ1 at optimal pressure,[25] suggesting that the interplay of spin fluctuations (SFs) and superconductivity may undergo a critical change with high pressure

Summary

Introduction

In correlated Fermi fluids, nematicity refers to the state in which rotational symmetry is spontaneously broken, while time-reversal invariance is preserved, and the symmetry of the crystal changes from tetragonal to orthorhombic.[1]. FeSe has been a key platform for studying the origin of nematicity and its role in superconductivity,[12] as it exhibits nematic and SC orders at well-separated temperatures, Tnem ~ 90 K and Tc ~ 9 K, respectively, without involving magnetic order.

Results

Conclusion