Böhmer, Anna E.

Fanfarillo, Lara

Wang, Qisi

English

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Editorial: Nematicity iniron-based superconductorsQisi Wang1*, Lara Fanfarillo2,3* and Anna E. Böhmer4,5*1Physik-Institut, Universität Zürich, Zürich, Switzerland, 2International School for Advanced StudiesSISSA/ISAS, Department of Physics, Trieste, Italy, 3Department of Physics, University of Florida,Gainesville, FL, United States, 4Lehrstuhl für Experimentalphysik IV, Fakultät für Physik und Astronomie,Ruhr-Universität Bochum, Bochum, Germany, 5Institute for Quantum Materials and Technologies,Karlsruhe Institute of Technology, Karlsruhe, GermanyKEYWORDSiron-based superconductors, nematicity, electronic anisotropy, orbital nematicity,spin nematicity, orbital ordering, nematic susceptibility, nematic fluctuationsEditorial on the Research TopicNematicity in iron-based superconductorsIn iron-based materials, nematicity is a commonly observed symmetry-breaking statethat exists in proximity to superconductivity. The nematic instability is associated with astructural transition that lowers the symmetry of the lattice and characterized by both thedevelopment of anisotropy in transport and electronic properties as well as orbital-dependent splitting in the electronic bands. The variety of experimental signaturescharacterizing the nematic state allowed investigations with diverse experimentalprobes that throughout the past decade uncovered multiple surprising results. Yet,key questions regarding the origin of nematicity remain unsettled as the spin, orbital,and lattice degrees of freedom are intimately coupled [1].The current Research Topic compiles the latest works that tackle both the origin andcharacterization of the nematic phase in iron-based materials. The contributions mainlyfocus on the analysis of doped compounds of the “122” group (e.g., BaFe2As2) includingthe heavily hole-doped ones, in which a new type of nematic instability has been recentlyreported [2], and on FeSe, that provides the unique opportunity to study the nematicphase within a wide range of temperature in the absence of long-range magnetic ordering[3]. In this way, we highlight the forefront of the research on nematicity in iron-basedsuperconductors.The Research Topic presents three review papers and six experimental and theoreticalcontributions collected from the front of original research.Degiorgi reviews optical studies of FeSe and Ba0.6K0.4Fe2As2. The study of thebroadband optical anisotropy in FeSe suggests that spin fluctuations together with thehigh-energy orbital ordering assume a dominant role in the onset of nematicity. Foroptimally doped Ba0.6K0.4Fe2As2, it is shown that the stress-induced optical anisotropyoccurs only below the superconducting transition temperature. These findingsdemonstrate an intimate relation between spin fluctuations, orbital nematicity, andsuperconductivity.OPEN ACCESSEDITED AND REVIEWED BYJian-Xin Zhu,Los Alamos National Laboratory (DOE),United States*CORRESPONDENCEQisi Wang,qisiwang@physik.uzh.chLara Fanfarillo,laura.fanfarillo@sissa.itAnna E. Böhmer,boehmer@physik.rub.deSPECIALTY SECTIONThis article was submitted toCondensed Matter Physics,a section of the journalFrontiers in PhysicsRECEIVED 06 September 2022ACCEPTED 08 September 2022PUBLISHED 27 September 2022CITATIONWang Q, Fanfarillo L and Böhmer AE(2022), Editorial: Nematicity in iron-based superconductors.Front. Phys. 10:1038127.doi: 10.3389/fphy.2022.1038127COPYRIGHT© 2022 Wang, Fanfarillo and Böhmer.This is an open-access articledistributed under the terms of theCreative Commons Attribution License(CC BY). The use, distribution orreproduction in other forums ispermitted, provided the originalauthor(s) and the copyright owner(s) arecredited and that the originalpublication in this journal is cited, inaccordance with accepted academicpractice. No use, distribution orreproduction is permitted which doesnot comply with these terms.Frontiers in Physics frontiersin.org01TYPE EditorialPUBLISHED 27 September 2022DOI 10.3389/fphy.2022.1038127Rana and Furukawa present a mini-review of their 77Senuclear magnetic resonance (NMR) studies on FeSe, when thecompound is tuned by application of physical pressure, chemicalpressure, or a combination of both. Indeed, FeSe exhibits acomplex T-x-p phase diagram as physical pressure, p, isapplied or sulfur is substituted for selenium in FeSe1-xSx(chemical pressure), including magnetic order and abundantmagnetic fluctuations. Rana and Furukawa find that, whileantiferromagnetic fluctuations appear to enhance Tc ingeneral, the effect is stronger in the absence of nematicity.Rhodes et al. present a detailed review of the evolution of theFermi surface of FeSe in the nematic phase, addressing theproblematic issue of the “missing electron pocket.” It is stillunclear how the experimentally determined Fermi surface nearthe M point of the Brillouin zone evolves from having twoelectron pockets in the tetragonal state, to exhibiting just asingle electron pocket in the nematic state [4]. In this review,Rhodes et al. collect recent angle-resolved photoemissionspectroscopy (ARPES) and scanning tunneling microscopyworks to analyze the orbital dependent band-shifts in thenematic phase, as well as theoretical modeling based on theinclusion of an additional nematic order parameter having “xy”orbital character.Bötzel and Eremin analyze the magnetic anisotropy of FeSeusing the same phenomenological model reviewed in Rhodeset al. The model combines the usual nematic order parameterbased on the differentiation of the xz and yz orbitals with a non-local xy nematic order parameter. The interesting result is thatthe inclusion of the latter successfully describes not only theabsence of the Y-electron pocket, but also the temperaturedependence of the anisotropy on the spin susceptibility.Onari and Kontani discuss a unified picture of nematicity foriron-based superconductors presenting a theoretical descriptionbased on a self-consistent diagrammatic approximation. Themodel allows for the description of several experimentalsignatures of nematicity both in FeSe-based superconductorsand in heavily hole-doped “122”-type materials, which derivefrom BaFe2As2 by doping towards the end-members K/Rb/CsFe2As2.The controversial issue of nematicity in heavily hole-doped“122” materials is also the focus of Hong et al. that discuss theResearch Topic from the experimental point of view. Whereasnematic orders far from a magnetic instability have been claimedin (Ba,Rb)Fe2As2, it is still debated whether the elastoresistance ofthese compounds is a signature of a new nematic instability.Hong et al. present elastoresistance data for amultitude of heavilyhole-doped 122-systems, which show a divergence ofelastoresistance. However, they present a new interpretationbased on the well-known Lifshitz transition in the systemunrelated to nematicity. This work adds a new element to theinterpretation of elastoresistance, an experimental quantity thathas impacted heavily in the investigation of nematicity.Curro et al. present an NMRwork, on the distribution of spinfluctuations in doped pnictides and the effect of uniaxial strainand strain hysteresis. They find that the spin lattice relaxationrate is inhomogeneous, and the spatial distribution of local spinfluctuations correlates with the nematic susceptibility. Theirresults suggest that a nematic glass behavior is induced byquenched strain fields associated with doping atoms.Gong et al. present a systematic study of nematic fluctuationsin the non-superconducting BaFe1.9−xNi0.1CrxAs2 systemcombining electronic transport, ARPES, and inelastic neutronscattering measurements. By monitoring the evolution of thenematic fluctuations as a function of Cr doping, a strongcorrelation between the resistivity- and spin nematicity isrevealed, while the orbital anisotropy behaves differently.Their results suggest the importance of the interplay betweenlocal moments and itinerant electrons for understanding thenematic fluctuations.Kreisel et al. theoretically analyze the anisotropy of thespin excitations in FeSe, focusing on the high-energy range asthe one detected by resonant inelastic x-ray scattering (RIXS)experiments. They consider an itinerant model previouslyused to describe the spin-excitation anisotropy as measuredby neutron scattering measurements, with magneticfluctuations included within the random phaseapproximation. The calculated cross section exhibitsoverall agreement with the data of recent RIXSexperiments on FeSe [5], where a theoretical interpretationin terms of local moments was discussed. The work by Kreiselet al. proves again that nematic phenomenology presentssome aspects that can be described either via an itinerantor a local spin scenario, suggesting a non-trivial role ofelectronic correlations in affecting the metallic state ofiron-based superconductors [6].This editorial conveys the objectives of the above nine articlesthat represent the latest progress in the research of nematicity iniron-based superconductors. We wish to thank all the authorsand referees for their contributions and hope for more futurestudies on this Research Topic.Author contributionsAll authors listed have made a substantial, direct, andintellectual contribution to the work and approved it forpublication.Conflict of interestThe authors declare that the research was conducted in theabsence of any commercial or financial relationships that couldbe construed as a potential conflict of interest.Frontiers in Physics frontiersin.org02Wang et al. 10.3389/fphy.2022.1038127Publisher’s noteAll claims expressed in this article are solely those of theauthors and do not necessarily represent those of their affiliatedorganizations, or those of the publisher, the editors and thereviewers. Any product that may be evaluated in this article, orclaim that may be made by its manufacturer, is not guaranteed orendorsed by the publisher.References1. Fernandes RM, Chubukov AV, Schmalian J. What drives nematic order in iron-based superconductors? Nat Phys (2014) 10:97–104. doi:10.1038/nphys28772. Ishida K, Tsujii M, Hosoi S, Mizukami Y, Ishida S, Iyo A, et al..Novel electronicnematicity in heavily hole-doped iron pnictide superconductors. Proc Natl Acad SciU.S.A (2020) 117:6424–9. doi:10.1073/pnas.19091721173. Böhmer AE, Kreisel A. Nematicity, magnetism and superconductivity in FeSe.J Phys Condens Matter (2018) 30:023001. doi:10.1088/1361-648X/aa9caa4. Yi M, Pfau H, Zhang Y, He Y, Wu H, Chen T, et al..Nematic energy scale andthe missing electron pocket in FeSe. Phys Rev X (2019) 9:041049. doi:10.1103/physrevx.9.0410495. Lu X, ZhangW, Tseng Y, Liu R, Tao Z, Paris E, et al..Spin-excitation anisotropyin the nematic state of detwinned FeSe. Nat Phys (2022) 18:806–12. doi:10.1038/s41567-022-01603-16. Fanfarillo L. Go for a spin. Nat Phys (2022) 18:738–9. doi:10.1038/s41567-022-01624-wFrontiers in Physics frontiersin.org03Wang et al. 10.3389/fphy.2022.1038127

Editorial: Nematicity in iron-based superconductors

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Editorial: Nematicity in iron-based superconductors

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