(Un-) Conventional pairing in multiband superconductors: Role of Fermi surface geometry

by Prof. Andreas Kreisel (Niel Bohr Institute)

Wednesday 20 Sept 2023, 15:30 → 16:30 Asia/Shanghai

TDLI Meeting Room N600 (East Wing of Floor 6, North Building)

Abstract

Unconventional superconductivity is often referred to as originating from a pairing mechanism different from electron-phonon interactions and connected to an anisotropic superconducting order parameter with sign change of the Cooper pair wavefunction. A common microscopic mechanism is the spin-fluctuation mediated pairing where the formation of Cooper pairs is possible by avoiding the repulsive Coulomb interaction in space. In this seminar, I will review the concepts of unconventional superconducting pairing and examine superconducting instabilities in presence of spin-fluctuations in different multiband systems:
The superconductor Sr2RuO4 has been the subject of enormous interest over more than two decades, but the form of its order parameter has not been determined. We examine the superconducting instability from the perspective of a possible state with sign change as function of kz which has been proposed from momentum-dependent spin-orbit coupling [1] or as result of a large Hund’s exchange J driving an unusual interorbital pairing state [2].
The nickelate system adds another prototype of a multiband superconductor. By computing the leading superconducting instability from spin fluctuations in this system we find a transition from d-wave pairing symmetry to nodal s± superconductivity and discuss implications for the interpretation of recent experimental probes in this material [3].
Finally, in the CrB2 system, there are hints of having electron-phonon pairing and spin-fluctuation pairing with similar importance which can lead to a hybrid s+- superconductor. The tuneability with pressure opens the opportunity to study the interplay of conventional and unconventional pairing [4].

[1] Astrid T. Rømer, T. A. Maier, Andreas Kreisel, P. J. Hirschfeld, Brian M. Andersen, Pys. Rev. Research 4, 033011 (2022), Leading superconducting instabilihties in three-dimensional models for Sr2RuO4

[2] Mercè Roig, Astrid T. Rømer, Andreas Kreisel, P. J. Hirschfeld, Brian M. Andersen, Phys. Rev. B 106, L100501 (2022), Superconductivity in multiorbital systems with repulsive interactions: Hund's pairing vs. spin-fluctuation pairing

[3] Andreas Kreisel, Brian M. Andersen, Astrid T. Rømer, Ilya M. Eremin, Frank Lechermann, Phys. Rev. Lett. 129, 077002 (2022), Superconducting Instabilities in Strongly-Correlated Infinite-Layer Nickelates

[4] Sananda Biswas, Andreas Kreisel, Adrian Valadkhani, Matteo Dürrnagel, Tilman Schwemmer, Ronny Thomale, Roser Valentí, Igor I. Mazin, Phys. Rev. B 108, L020501 (2023), Hybrid s-wave superconductivity in CrB2

Biography

Andreas Kreisel is an assistant professor of Niel Bohr Institute.

Division

Condensed Matter

Other information

Zoom Meeting: https://cern.zoom.us/j/66853270637?pwd=cnBFQk05czhQS0grSlF6YTZpRVVkdz09
Meeting ID: 668 5327 0637   Passcode: 123456