Master's Defense: Sofie Castro Holbæk

Superconductivity in kagome materials

It was recently discovered that a family of compounds, AV3Sb5 (A = K, Rb, Cs), becomes superconducting at low temperature. All three compounds feature layers of vanadium atoms organized in a two-dimensional kagome lattice. At present, there is no full consensus regarding the symmetry of the superconducting order parameter. Motivated by this, the aim of the current thesis is to study superconductivity on a kagome lattice with an emphasis on modelling impurity probes. We focus on the effects of impurities since, in many cases, they act as phase-sensitive probes.

We study the response to single point-like potential impurities. Due to the fact that a point-like potential impurity is located on a lattice site which has lower symmetry than the lattice itself, the electronic wave functions on the kagome lattice are found to only give non-zero weight to certain portions of the Fermi surface. This, combined with an even-parity order parameter like a d-wave gap, leads to a d-wave condensate reacting to impurities effectively like an s-wave superconductor. Consequently, no low-energy bound states are found within the full gap of a system with d + id superconductivity. This is the case although a d-wave superconductor conventionally would be sensitive to potential impurities. For comparison with the results on the kagome lattice, a single-impurity study on the square lattice is additionally presented.

Furthermore, the suppression of the critical temperature by an impurity average is studied and shows that an unconventional singlet order parameter with d-wave symmetry is less sensitive to potential impurities while unconventional triplet order parameters remain sensitive. This result is in agreement with our single-impurity study. Lastly, we discuss the simplifying assumptions made and the implications of our findings for other systems. The study presented in the thesis nuances the discussion of impurity effects in superconductors, and it shows that conventional results may not always apply if the site symmetry is smaller than the point group symmetry of the lattice.