Superconductivity

Singlet-triplet degeneracy in the quasi-1D limit of the Hubbard model

In [1], we proved the presence of an accidental degeneracy between singlet and triplet superconductivity in the quasi-1D limit of the Hubbard model on the square lattice, in the weak coupling regime. This provides a solvable microscopic model for which a mixed-parity superconducting order parameter can spontaneously appear, despite the presence of inversion symmetry. Interestingly, this could be relevant to the quasi-1D orbitals of Sr2RuO4, a superconductor for which the nature of the superconducting order is still under debate. We discuss how a mixed parity order parameter could reconcile a large set of past experiments.

[1] Thomas Scaffidi, "Degeneracy between even- and odd-parity superconductivity in the quasi-1D Hubbard model and implications for Sr2RuO4", arXiv:2007.13769

Tuning superconductivity in Sr2RuO4 with strain

The nature of superconductivity in Sr2RuO4 has remained a mystery for more than 20 years now (see [1] for a review). In an ongoing collaboration with the groups of Clifford Hicks and Andrew Mackenzie, we had previously shown (Science 2017 [2]) how in-plane pressure provides a way of drastically changing the superconducting properties of this material, all the way to a van Hove singularity at which the critical temperature more than doubles compared to the unstrained value. Surprisingly, as we show in a recent preprint [3], out-of-plane pressure lowers Tc instead of increasing it, which is hard to reconcile with most current candidates for the order parameter in this material. This calculation was made possible by the extension of our weak-coupling calculations to 3D, which is described in two earlier papers[4,5].

Topological superconductivity

One  of the reasons why Sr2RuO4 attracted so much attention was the early evidence for p+ip superconductivity, a topological state with robust quantum properties which could be used as a resource to process quantum information. Using a weak coupling calculation developed during my PhD, I investigated the possibility of p+ip superconductivity in this material and compared predictions with a number of experiments. 

One important aspect was the proper accounting of spin-orbit coupling, which led to a prediction of a superconducting gap of similar size on all three bands, in agreement with thermodynamic and STM data and in contrast to previous calculations [1]. We also showed that the superconducting order parameter favored in the weak coupling limit has a topological invariant (called Chern number) of -7, instead of the usually assumed +1 [2]. This order parameter could naturally reconcile the presence of a chiral superconducting phase in Sr2RuO4 with the absence of measurable edge currents. Moreover, we studied the fluctuations of the relative phase of the gap on the different bands (the so-called Leggett mode) and predicted some distinctive signatures in Raman spectroscopy [3]. We could also identify distinctive features in the quasi-particle interference spectra obtained by scanning tunneling spectroscopy on the normal state of Sr2RuO4 [4].


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