Department of Physics | University of Florida

Recent Highlights

Iron selenide: epitome or outlier?

Iron selenide is at once the simplest of the Fe-based superconductors, and one of the most mysterious. Its Tc is only 8K, but this increases under pressure or intercalation to 40K, and to 70K or above when deposited as a single layer on strontium titanate. It is not magnetic, unlike most of the Fe-based superconducting parent compounds, but displays a very strong electronic nematic order below a transition to an orthorhombic structure at 90K. In Ref. 1 we argued that the nematic ordering is due to magnetic correlations frustrated by ordering tendencies at several competing wavevectors. A microscopic model proposed in Refs. 2,3, using a standard 5-orbital Hubbard-Hund approach with phenomenological orbital ordering term, reproduces many features of experiment (ARPES,QO,STM,NMR), including most recently the detailed energy and temperature dependence of the inelastic neutron scattering intensity.

  1. ``Effect of magnetic frustration on superconductivity and nematicity in Fe chalcogenides", J. K. Glasbrenner, H. O. Jeschke, R. Valenti, P. J. Hirschfeld, R.M. Fernandes and I. I. Mazin, Nat. Phys. 11, 953 (2015).
  2. "Model of Electronic Structure and Superconductivity in Orbitally Ordered FeSe", S. Mukherjee, A. Kreisel, P.J. Hirschfeld and B.M. Andersen, Phys. Rev. Lett. 115, 026402 (2015).
  3. "Spin excitations in a model of FeSe with orbital ordering", A. Kreisel, S. Mukherjee, P. J. Hirschfeld and B.M. Andersen, arXiv:1510.02357.

FeSe neutron scattering intensity map at various energies and temperatures. Comparison of theory (Row 1 - 110K; row 2 - 10K. From Kreisel et al., Ref. 3 ) and experiment (Row 3 - 110K; row 4 - 4K. From Wang et al., arXiv:1511.02485).

Role of incipient bands in Fe-based superconductors

Superconductivity is normally considered to be a Fermi surface phenomenon, since the usual phonon mediated interaction is attractive only over a small energy range near the Fermi level. If a band does not cross the Fermi energy but is nevertheless within this attractive range, we call it "incipient". In the case of a single band, it has been known for many years that contributions to the critical temperature from incipient bands are very weak. In the paper below, we showed that in other cases where the pairing arises primarily from other bands at the Fermi level, the presence of an incipient band can substantially assist Tc, and lead to large gaps on incipient bands. This includes typical bulk Fe-based system like LiFeAs, where a significant gap on an incipient band has been seen experimentally, and more unusual systems like FeSe monolayers on strontium titanate, where a substrate phonon may provide part of the attractive interaction.

"Electron pairing in the presence of incipient bands in iron-based superconductors", Xiao Chen, S. Maiti, A. Linscheid, and P. J. Hirschfeld, arXiv:1508.04782, to be published in Phys. Rev. B, arXiv:1508.04782 .

Bootstrapping of phonon-mediated transition temperature Tph by spin fluctuation interaction arising from incipient hole band with band extremum E g. Actual Tc can be many times phonon Tc.

Order parameter collective modes in Fe-based superconductors

In various classes of unconventional superconductors, the possibility of time reversal symmetry breaking states, including s+id phases, has been raised. In particular, in the K-doped Ba122 Fe-based superconductor, the optimally doped system is isotropic, while the end point compound KFe2As2 is nodal (possibly d-wave), and an intermediate s+id phase has been claimed by some authors. Meanwhile, a collective mode peak has been observed in Raman experiments on this material, and identified as a Bardasis-Schrieffer mode, the collective mode of an s-wave superconductor gap oscillating as it couples to subdominant d-wave fluctuations. In the paper below, we calculated the collective mode spectrum in models of superconductors with attractive interactions in s-and d channels as a function of their relative strength, across the phase diagram. The Bardasis-Schrieffer mode is shown to have mixed symmetry in the s+id ground state, and should appear in multiple Raman polarization channels.

"Order parameter collective modes in superconductors with competing s- and d-wave interactions", S. Maiti and P.J. Hirschfeld, Phys. Rev. B 92, 094506 (2015).

Mode frequencies in s-wave system with attractive subdominant d-wave interaction ud The BS mode softens at the s+id phase boundaries, and does not propagate in the d-wave state.

Quasiparticle interference in multiband superconductors

Phase-sensitive measurements of the superconducting gap in Fe-based superconductors have proven more difficult than originally anticipated. While quasiparticle interference (QPI) measurements based on scanning tunneling spectroscopy are often proposed as definitive tests of gap structure, the analysis typically relies on details of the model employed. In the paper below we pointed out that the temperature dependence of momentum-integrated QPI data can be used to identify gap sign changes in a qualitative way, and presented explicit results for s+/- and s++ states in a system with typical Fe-pnictide Fermi surface.

"Robust determination of the superconducting gap sign structure via quasiparticle interference", P. J. Hirschfeld, D. Altenfeld, I. Eremin, and I.I. Mazin, Phys. Rev. B92, 184513 (2015) .

Quasiparticle interference pattern taken on LiFeAs by Chi et al, PRB 89, 104522 (2014). Note clearly separated spots at small (red) and large q (blue).

© 2015 Peter J. Hirschfeld. All rights reserved.

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