Quantum matter in high magnetic fields

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Resurgence of superconductivity and the role of dxy hole band in FeSe1−xTex

Communications Physics volume 6, Article number: 362

Authors:

Archie B. Morfoot, Timur K. Kim, Matthew D. Watson, Amir A. Haghighirad, Shiv J. Singh, Nick Bultinck & Amalia I. Coldea

Abstract:

Iron-chalcogenide superconductors display rich phenomena caused by orbital-dependent band shifts and electronic correlations. Additionally, they are potential candidates for topological superconductivity due to the band inversion between the Fe d bands and the chalcogen pz band. Here we present a detailed study of the electronic structure of the nematic superconductors FeSe1−xTex (0 < x < 0.4) using angle-resolved photoemission spectroscopy to understand the role of orbital-dependent band shifts, electronic correlations and the chalcogen band. We assess the changes in the effective masses using a three-band low energy model, and the band renormalization via comparison with DFT band structure calculations. The effective masses decrease for all three-hole bands inside the nematic phase, followed by a strong increase for the band with dxy orbital character. Interestingly, this nearly-flat dxy band becomes more correlated as it shifts towards the Fermi level with increasing Te concentrations and as the second superconducting dome emerges. Our findings suggests that the dxy hole band, which is very sensitive to the chalcogen height, could be involved in promoting an additional pairing channel and increasing the density of states to stabilize the second superconducting dome in FeSe1−xTex. This simultaneous shift of the dxy hole band and enhanced superconductivity is in contrast with FeSe1−xSx.
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Signatures of a Quantum Griffiths Phase close to an Electronic Nematic Quantum Phase Transition

Authors:

Pascal Reiss, David Graf, Amir A Haghighirad, Thomas Vojta, Amalia I Coldea

Abstract:

In the vicinity of a quantum critical point, quenched disorder can lead to a quantum Griffiths phase, accompanied by an exotic power-law scaling with a continuously varying dynamical exponent that diverges in the zero-temperature limit. Here, we investigate a nematic quantum critical point in the iron-based superconductor FeSe$_{0.89}$S$_{0.11}$ using applied hydrostatic pressure. We report an unusual crossing of the magnetoresistivity isotherms in the non-superconducting normal state which features a continuously varying dynamical exponent over a large temperature range. We interpret our results in terms of a quantum Griffiths phase caused by nematic islands that result from the local distribution of Se and S atoms. At low temperatures, the Griffiths phase is masked by the emergence of a Fermi liquid phase due to a strong nematoelastic coupling and a Lifshitz transition that changes the topology of the Fermi surface.
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