Fermi operator expansion for the Hartree-Fock-Bogoliubov theory

A variety of phases in the inner crust of neutron stars are crucial for understanding the pulsar phenomena. However, the three-dimensional coordinate-space calculation of the phases is computationally demanding. We aim to generalize the Fermi operator expansion (FOE) method that is effective for finite-temperature coordinate-space simulation, from the Hartree-Fock theory to Hartree-Fock-Bogoliubov (HFB) theory including the pairing effects. Furthermore, the periodic structure with free neutrons in the inner crust requires us to treat the system with the band theory. We give a concise proof that the generalized density matrix in the HFB theory can be obtained with the FOE. The Chebyshev polynomial expansion is used for calculations of the HFB band theory. Using a model for a slab phase of the inner crust, the FOE method produces results in good agreement with those based on the diagonalization of the HFB Hamiltonian. The FOE method for the HFB band theory is a powerful tool for studying the nontrivial exotic structures in neutron stars. The FOE method is suitable for parallelization and further acceleration is possible with nearsightedness.