JCMS: Inhomogeneity and disorder in unconventional superconductors

by Prof. Andreas Kreisel (Uppsala University)

Monday 8 Jun 2026, 10:30 → 12:00 Europe/Stockholm

Albano 3: 6228 - Mega (22 seats) (Albano Building 3)

Abstract: Disorder is a common theme in unconventional superconductors due to imperfections in the growth process when multiple elements are involved, or even inevitable because the material needs to be doped away from nominal charge neutrality to reach the superconducting state.

In any case, disorder is an opportunity to learn about the superconducting state or specifically probe its response to disorder. In this seminar, I will review common results on disordered unconventional superconductors and then move to two case studies where new twists of the superconducting response is apparent.

The first example is unconventional superconductivity on the kagome and Lieb lattice where it turns out that the superconducting order parameter that sums to zero, still can be robust against point like disorder. In the same way, these systems show a Hebel Slichter peak in the temperature-dependent spin-relaxation rate, a property that was previously only assigned to conventional non-sign changing order parameters.

In the second example, I will challenge the general notion that only overdoped cuprates approach the well defined Landau-BCS paradigm at large hole doping. With this work we connect to recent experimental data from tunneling spectroscopy and angle-resolved photo emission spectroscopy where gap-filling and a prominent shoulder at a second energy scale has been observed. The key ingredient for the theory is an inhomogeneous pairing interaction on the nanometer length scale which produces these features independent of the details of the modeling. We use traditional BCS mean-field calculations and time-dependent Ginzburg-Landau calculations to obtain the order parameter and density of states finding that the spectral gaps fill, rather than close. The result is a local spectrum that remains inhomogeneous at the critical temperature T_c: the local density of states is mostly given by the normal state, but with isolated superconducting islands characterized by a spectral pseudogap.