Superconductivity in FeSe: The Role of Nematic Order

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Abstract

Bulk FeSe is a special iron-based material in which superconductivity emerges inside a well-developed nematic phase. We present a microscopic model for this nematic superconducting state, which takes into account the mixing between s-wave and d-wave pairing channels and the changes in the orbital spectral weight promoted by the sign-changing nematic order parameter. We show that nematicity only weakly affects Tc, but gives rise to cos2θ variation of the pairing gap on the hole pocket, whose magnitude and size agrees with angle resolved photoemission spectroscopy and STM data. We further show that nematicity increases the weight of the dxz orbital on the hole pocket, and increases (reduces) the weight of the dxy orbital on the Y (X) electron pocket.

Original languageEnglish (US)
Article number267001
JournalPhysical review letters
Volume120
Issue number26
DOIs
StatePublished - Jun 26 2018

Bibliographical note

Funding Information:
In this Letter we argued that the experimentally observed anisotropy of the superconducting gap in bulk FeSe can be explained within the low-energy model for nematic order, without adding phenomenologically different quasiparticle weights for the d x z / d y z orbitals. Our key result is that T c is not strongly affected by the nematic order, but nematicity mixes s -wave and d -wave pairing channels and gives rise to a cos 2 θ h gap anisotropy on the hole pocket The sign of the cos 2 θ h term is determined by the interplay between the nematic order parameters on hole and electron pockets, which are of different sign, and the relative strength of s -wave and d -wave components of the pairing interaction. On the Z pocket, we found a sizable cos 2 θ h gap anisotropy with the gap maximum along the X direction, in agreement with the data. In our calculations the gap on the Γ pocket is smaller and less anisotropic. On the peanutlike X pocket, the gap is found to be maximal along the minor axis, which is also in agreement with the data. We also argued that nematicity decreases the weight of the d x y orbital on the X pocket and increases it on the Y pocket. This may potentially explain why the Y pocket is less visible in STM and in some ARPES data. We are thankful to B. Andersen, L. Bascones, L. Benfatto, S. Borisenko, A. Coldea, M. Eschrig, P. Hirschfield, A. Kreisel, C. Meingast, L. Rhodes, J. C. Séamus Davis, O. Vafek, M. Watson, and Y. Y. Zhao for useful discussions. J. K. was supported by the National High Magnetic Field Laboratory through NSF Grant No. DMR-1157490 and the State of Florida. R. M. F. and A. V. C. were supported by the Office of Basic Energy Sciences, U.S. Department of Energy, under Awards No. DE-SC0012336 (R. M. F.) and No. DE-SC0014402 (A. V. C.). J. K. thanks FTPI at the University of Minnesota for hospitality during the completion of this work. 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