Proposals to detect Bogoliubov Fermi surfaces

An exotic superconducting state with Fermi surfaces of Bogoliubov quasiparticles, Bogoliubov Fermi-surface (BG-FS), has recently been proposed in a centrosymmetric j=3/2 system with a Z₂ topological invariant. Several candidate materials of a BG-FS are proposed in Sr₂RuO₄ and doped FeSe, but its existence has not been confirmed yet. In this work, we provide theoretical proposals to pin down a BG-FS. First, we use the inversion instability of a BG-FS and suggest that second-harmonic-generation (SHG) experiments with a strain gradient to identify enhanced fluctuations of an inversion order parameter. [1] Second, a nontrivial current response of a Bogoliubov Fermi surface superconductivity is used. The importance of dissipation channels from a Fermi surface of Bogoliubov quasi-particles is emphasized. Specifically, we derive the expressions for the optical conductivity and compare them with that of conventional Bardeen-Cooper-Schrieffer superconductivity. Possible applications of our theory to iron-based superconductors and heavy fermion systems including FeSe will also be discussed.

[1] Hanbit Oh and Eun-Gook Moon,Phys. Rev. B 102, 020501(R)