Nematic fluctuations in iron-oxychalcogenide Mott insulators

Byron Freelon ,Bhupendra Karki,S Michelle Everett ,S Kamusella,Swagata Acharya,Alaa Alfailakawi,I P Swainson ,L Craco ,Kun Wang ,V Grinenko,Minghu Fang,M S Laad ,Bingbing Xu ,Z Yamani ,Roxana Flacau,F Brückner,Rajib Sarkar ,Benjamin A Frandsen ,Jöerg C Neuefeind ,Hangdong Wang,R J Birgeneau ,H.‐H Klauß

doi:10.1038/s41535-020-00302-5

Abstract

Nematic fluctuations occur in a wide range physical systems from biological molecules to cuprates and iron pnictide high-Tc superconductors. It is unclear whether nematicity in pnictides arises from electronic spin or orbital degrees of freedom. We studied the iron-based Mott insulators La2O2Fe2OM2M = (S, Se), which are structurally similar to pnictides. Nuclear magnetic resonance revealed a critical slowing down of nematic fluctuations and complementary Mössbauerr spectroscopy data showed a change of electrical field gradient. The neutron pair distribution function technique detected local C2 fluctuations while neutron diffraction indicates that global C4 symmetry is preserved. A geometrically frustrated Heisenberg model with biquadratic and single-ion anisotropic terms provides the interpretation of the low temperature magnetic fluctuations. The nematicity is not due to spontaneous orbital order, instead it is linked to geometrically frustrated magnetism based on orbital selectivity. This study highlights the interplay between orbital order and spin fluctuations in nematicity.

Highlights

Nematic phases occur in a variety of very different physical systems throughout nature
Two scenarios are proposed[5]: One proposal suggests that anisotropic spin fluctuations are necessary precursors of the nematic ordering that has been experimentally observed[7]; the other claims that ferro-orbital ordering involving dxz,yz orbitals is responsible for nematicity[8]
Orbital-selective Mott correlations are important for understanding magnetism in the Mott-insulating states of the oxychalcogenides. Such selective-Mott correlations have recently been the focus of attention in the iron-selenides[12,13,14,15], and the present study provides an example of the relevance of orbital-selective Mott physics in Fe-based materials

Summary

INTRODUCTION

Nematic phases occur in a variety of very different physical systems throughout nature. Two scenarios are proposed[5]: One proposal suggests that anisotropic spin fluctuations are necessary precursors of the nematic ordering that has been experimentally observed[7]; the other claims that ferro-orbital ordering involving dxz,yz orbitals is responsible for nematicity[8] Complicating this debate is the fact that various Fe-based materials exhibit different magnetic order. Orbital-selective Mott correlations are important for understanding magnetism in the Mott-insulating states of the oxychalcogenides Such selective-Mott correlations have recently been the focus of attention in the iron-selenides[12,13,14,15], and the present study provides an example of the relevance of orbital-selective Mott physics in Fe-based materials. At the section’s end, we present theoretical modeling that is helpful in interpreting our findings

RESULTS AND DISCUSSION

Freelon et al 3

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METHODS