According to Anderson's theorem, non-magnetic disorder does not change the energy gap of a conventional superconductor, although it does reduce the superfluid weight, which measures the ability to carry supercurrent. Kryštof Kolář, Tero Heikkilä and Päivi Törmä show that the opposite holds in flat-band superconductors.

There, disorder does reduce the order parameter, but its direct effect on the superfluid weight is mostly negligible. Analytically, the effect of disorder is given to lowest order by the difference between the intraband and interband parts of the localisation functional of impurity wavefunctions, which the authors find typically vanishes. The work is a collaboration between QMAT's Condensed Matter Theory group at the University of Jyväskylä and its Quantum Dynamics group at Aalto University.

Image: QMAT