Quantum Monte Carlo Simulations of High-Temperature FeSe Superconductors with no Sign Problem

Electron doping of a hidden spin-density wave (hSDW) state that exists over isotropic d+ = dxz + idyz and d- = dxz - idyz orbitals can lead to electron-type Fermi surface pockets at the corner of the two-iron Brillouin zone, but with no Fermi surface pockets at the center of the two-iron Brillouin zone[1].  The hSDW is a Mott insulator that results from nesting of bare electron-type and hole-type Fermi surfaces, respectively, at the center and at the corner of the one-iron Brillouin zone[2].   A recent Eliashberg theory analysis of a spin-fermion model based on the latter nested Fermi surfaces recovers the Lifshitz transition to electron-type Fermi surface pockets at the corner of the two-iron Brillouin zone, at weak electron doping[3].  The Eliashberg theory also finds an instability to S-wave Cooper pairing, with sign changes between the electron Fermi surface pockets and faint hole pockets that are also located at the corner of the two-iron Brillouin zone.  We study the same two-orbital spin-fermion model by carrying out determinant quantum Monte Carlo simulations that are free of the fermion sign problem[4].  The theoretical prediction of a Lifshitz transition to Fermi surface pockets at the corner of the Brillouin zone will be tested, while the competition between hSDW order, S-wave Cooper pairing, and D-wave Cooper paring will be examined.

[1] J.P. Rodriguez, Phys. Rev. B 95, 134511 (2017).

[2] J.P. Rodriguez and R. Melendrez, J. Phys. Commun. 2, 105011 (2018).

[3] J.P. Rodriguez, Phys. Rev. B 103, 184513 (2021).

[4] E. Berg, M.A. Metlitski, and S. Sachdev, Science 338, 1606 (2012).