Seminarium Fizyki Materii Skondensowanej

Weyl semimetals have drawn a lot of attention in theoretical and experimental communities due to their remarkable properties: their energy spectra feature topological gapless states in both bulk and on the surface. This seems to contradict the main paradigm of topological insulators, which is that a gapped bulk spectrum is required to define a topological invariant. Another interesting feature of a Weyl semimetal is that in the presence of magnetic field a gapless chiral Landau level is formed, which is also protected due to Atiyah's index theorem. Such properties have motivated us to ask the following question: will this Landau level persist if the semimetal is subjected to induced superconductivity? A similar question was asked in the context of gapless Dirac fermions in d-wave superconductors by Schrieffer and Gor'kov, and independently Anderson in 1998, but was found to have a negative answer: the vortices in the superconducting order parameter spoil the Landau levels. We've shown that in the case of a Weyl superconductor this is no longer true and the Landau level survives. This new gapless state, impossible to obtain in any other class of materials, gives access to a direct observation of chiral anomaly -- an apparent violation of particle number conservation.