Nematic state of the FeSe superconductor

Ulrich Schwarz,Marco Scavini,Clemens Ritter,Gabriel Cuello,Ulrich K Rößler,Alexander A Tsirlin,Sahana Rößler,Cevriye Koz,Mauro Coduri,Steffen Wirth,Liudmila Muzica

doi:10.1103/physrevb.105.064505

Abstract

We study the crystal structure of the tetragonal iron selenide FeSe and its nematic phase transition to the low-temperature orthorhombic structure using synchrotron x-ray and neutron scattering analyzed in both real and reciprocal space. We show that in the local structure the orthorhombic distortion associated with the electronically driven nematic order is more pronounced at short length scales. It also survives up to temperatures above 90 K where reciprocal-space analysis suggests tetragonal symmetry. Additionally, the real-space pair distribution function analysis of the synchrotron x-ray diffraction data reveals a tiny broadening of the peaks corresponding to the nearest Fe-Fe, nearest Fe-Se, and the next-nearest Fe-Se bond distances as well as the tetrahedral torsion angles at a short length scale of 20 angstr\"om. This broadening appears below 20 K and is attributed to a pseudogap. However, we did not observe any further reduction in local symmetry below orthorhombic down to 3 K. Our results suggest that the superconducting gap anisotropy in FeSe is not associated with any symmetry-lowering short-range structural correlations.

Highlights

The discovery of superconductivity in FeSe [1] with a critical temperature Tc ≈ 8 K prompted intense research into this remarkable family of materials [2,3,4]
We study the crystal structure of the tetragonal iron selenide FeSe and its nematic phase transition to the low-temperature orthorhombic structure using synchrotron x-ray and neutron scattering analyzed in both real space and reciprocal space
We show that in the local structure the orthorhombic distortion associated with the electronically driven nematic order is more pronounced at short length scales

Summary

Introduction

The discovery of superconductivity in FeSe [1] with a critical temperature Tc ≈ 8 K prompted intense research into this remarkable family of materials [2,3,4]. In spite of the expectations, more than a decade of research proved FeSe is a rather unusual superconductor with several fascinating properties in the normal state, i.e., above Tc. A structural transition from the parent tetragonal (P4/nmm, No 129) crystal structure to the orthorhombic Cmma (new symbol Cmme, No 67) space group occurring at a temperature Ts ≈ 90 K is considered to be driven by electronic degrees of freedom and is referred to as a nematic transition.

Results

Discussion

Conclusion