Bogoliubov-Fermi Surface With Inversion Symmetry And Electron-Electron Interactions: Relativistic Analogies And Lattice Theory

We show that the general low-energy Bogoliubov-de Gennes Hamiltonian in a multiband superconductor with broken time-reversal and preserved inversion symmetry is a generator of a real four-dimensional representation of SO(4). In the particular representation such an effective Hamiltonian is a purely imaginary matrix, and it is proportional to the antisymmetric tensor of a fictitious electromagnetic field which one can define in the momentum space. The quantum time evolution of the low-energy quasiparticle state becomes this way closely related to the classical relativistic motion of a charged particle in the presence of the Lorentz force that would be derived from such an electromagnetic field configuration. The condition for the emergence of a Bogoliubov-Fermi surface can then be understood as orthogonality of the fictitious electric and magnetic fields, which would allow zero Lorentz force. The corresponding zero-energy eigenstates are identified as the physical timelike and the unphysical spacelike solutions of the Lorentz force equation. We study the looming instability of the inversion-symmetric Bogoliubov-Fermi surface in the presence of electron-electron interaction by formulating a concrete interacting model on the Lieb lattice that features the requisite SO(4) kinetic energy term together with nearest-neighbor two-body repulsion. The latter is shown to favor dynamical breaking of the inversion symmetry. The inversion symmetry in our lattice model indeed becomes spontaneously broken at zero temperature at infinitesimal repulsion, with the original Bogoliubov-Fermi surface deformed and reduced in size. General features of this symmetry-breaking phenomenon are discussed and a comparison with other works in the literature is presented.