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 and higher angular momentum of the Cooper pair wavefunction. In this case, the superconducting order parameter is momentum-compensated such that spectroscopic indications such as bound states at nonmagnetic impurities, inelastic neutron scattering in the superconducting state and nuclear magnetic resonance behave qualitatively different from conventional superconductors.
I will consider the case of the kagome and Lieb lattices which feature three atoms per unit cell, flat bands, and nontrivial momentum structure of the Bloch weights on some of the bands. The latter has strong consequences for the mentioned probes and leads to a more robust superconductivity to point-like disorder. For the NMR relaxation rate, we find a Hebel-Slichter peak and absence of in-gap bound states, despite the order parameters summing to zero over the whole Brillouin zone. I will explain these surprising results and discuss how experimental data has to be re-interpreted.

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