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

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 and higher angular momentum of the Cooper pair wavefunction. A common microscopic mechanism that is discussed in view of high temperature superconductors is the spin-fluctuation mediated pairing where the formation of Cooper pairs is possible by avoiding the repulsive Coulomb interaction in space. This leads to an effective momentum-dependent pairing interaction which can drive sign-changing and momentum dependent electron pairing with d-wave form factor for the Fermi surface geometry in cuprates and s± form factor allowed by Fermi surface topology in iron based systems.

In this seminar, I will review these concepts of unconventional superconducting pairing and apply the spin-fluctuation formalism to two different systems: The infinite layer nickelates are considered to share properties with the cuprates, but in detail the system might be different in terms of its multiband nature and therefore Fermi surface topology. We show how correlation effects in conjunction with multiorbital pairing enhances the s± pairing compared to a d-wave instability and discuss connections to observations in penetration depth and tunneling experiments [1]. Second, 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 [2]. The tuneability with pressure opens the opportunity to study the interplay of conventional pairing with unconventional pairing.

[1] 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

[2] Sananda Biswas, Andreas Kreisel, Adrian Valadkhani, Matteo Dürrnagel, Tilman Schwemmer, Ronny Thomale, Roser Valentí, Igor I. Mazin arXiv:2211.01054 (2022) Hybrid s-wave superconductivity in CrB2