Electronic interactions and superconductivity with fragmented Fermi surface

DIPC Seminars

Magdalena Marganska
Institute for Theoretical Physics, University of Regensburg
Donostia International Physics Center (In person seminar)
Dario Bercioux
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Electronic interactions and superconductivity with fragmented Fermi surface

It has long been known that the superconducting pairing does not necessarily have to arise from an attractive interaction - starting from the Kohn-Luttinger proposal, through the high-Tc superconductors, up to the recent studies of superconducting pairing in the van der Waals material NbSe2 [1,2]. The Fermi surface in this material is split into three pockets, around the Gamma, K and K' points. The presence of different pockets splits the Coulomb scattering processes into subsets with different scattering ranges. The competing repulsion events at different ranges can result in the formation of Cooper pairs, provided that the short-range repulsion is stronger than the long-range one. Depending on which of the interaction types is dominant, we find that the material can support several superconducting gaps with different symmetries, both in the s and in the f pairing channel. We analyze the dI/dV characteristics of recently performed STM experiments on NbSe2 [3] and find that while they are consistent with both one and two gaps, the agreement is better when two gaps are considered. In order to gauge the strength of the interaction at different ranges we have calculated the screened Coulomb potential in this material, using the tight-binding model and the RPA approximation. We find that the dominant interaction is one that induces the predominantly s-wave gaps, two of them forming at the K/K' pockets and nearly degenerate, with the third one, around the Gamma pocket considerably larger. Far from being a spectator, the third Fermi pocket can be a powerful player, even to the point of changing the symmetry of the gaps in the K/K' valleys.
[1] Shaffer et al., PRB 101, 224503 (2020)
[2] Hoerhold et al., 2D Mater. 10, 025008 (2023)
[3] Wan et al., Adv. Mat. 34, 2206078 (2022)