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Magnetism, Superconductivity, and Spontaneous Orbital Order in Iron-Based Superconductors: Which Comes First and Why?

Andrey V. Chubukov, M. Khodas, and Rafael M. Fernandes
Phys. Rev. X 6, 041045 – Published 2 December 2016
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Abstract

Magnetism and nematic order are the two nonsuperconducting orders observed in iron-based superconductors. To elucidate the interplay between them and ultimately unveil the pairing mechanism, several models have been investigated. In models with quenched orbital degrees of freedom, magnetic fluctuations promote stripe magnetism, which induces orbital order. In models with quenched spin degrees of freedom, charge fluctuations promote spontaneous orbital order, which induces stripe magnetism. Here, we develop an unbiased approach, in which we treat magnetic and orbital fluctuations on equal footing. Key to our approach is the inclusion of the orbital character of the low-energy electronic states into renormalization group (RG) analysis. We analyze the RG flow of the couplings and argue that the same magnetic fluctuations, which are known to promote s+ superconductivity, also promote an attraction in the orbital channel, even if the bare orbital interaction is repulsive. We next analyze the RG flow of the susceptibilities and show that, if all Fermi pockets are small, the system first develops a spontaneous orbital order, then s+ superconductivity, and magnetic order does not develop down to T=0. We argue that this scenario applies to FeSe. In systems with larger pockets, such as BaFe2As2 and LaFeAsO, we find that the leading instability is either towards a spin-density wave or superconductivity. We argue that in this situation nematic order is caused by composite spin fluctuations and is vestigial to stripe magnetism. Our results provide a unifying description of different iron-based materials.

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  • Received 14 June 2016

DOI:https://doi.org/10.1103/PhysRevX.6.041045

Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Andrey V. Chubukov1, M. Khodas2,3, and Rafael M. Fernandes1

  • 1School of Physics and Astronomy, University of Minnesota, Minneapolis, Minneapolis 55455, USA
  • 2Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel
  • 3Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242, USA

Popular Summary

Magnetism, superconductivity, and nematic order are all observed in iron-based superconductors. It is commonly believed that either orbital or magnetic fluctuations function as “glue” that binds electrons together, thereby enabling superconductivity. In order to understand which order develops upon a decrease in temperature, one needs to treat magnetic, orbital, and superconducting fluctuations on an equal footing and analyze which susceptibility diverges first. We conduct this analysis here by exploring a technique known as renormalization group. This technique allows us to understand how a system’s properties change as one progressively integrates out fluctuations with higher energies.

Our theoretical work focuses on iron-based superconductors such as FeSe, BaFe2As2, and LaFeAsO. We consider various models, including ones with quenched orbital degrees of freedom and quenched spin degrees of freedom. Our model takes into account 14 topologically different interaction terms. At intermediate energies, we find that magnetic susceptibility is the largest, but the tendency toward magnetism competes with the tendency toward superconductivity. The competition strongly reduces the susceptibilities in both channels. As a consequence, the leading instability upon a decrease in temperature favors nematic order. The nematic channel is a distant third at intermediate energies, but it avoids the competition with magnetism and superconductivity and wins when the other two channels become weaker because of competition.

We expect that our findings will pave the way for studies of the behavior of other iron-based superconducting materials.

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Vol. 6, Iss. 4 — October - December 2016

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