High-Tc Superconductivity in Iron Selenide by Proximity to Hidden Spin Density Wave

We study a possible parent groundstate for high-Tc superconductivity in iron selenide that is a hidden spin density wave (hSDW) at checkerboard wavenumber Q = (pi,pi). It is in the same class as the hSWD state studied by Berg, Metlitski and Sachdev in the context of copper-oxide high-Tc superconductors[1]. The magnetic moment of the present hSDW alternates in sign between d+ = dxz + i dyz and d- = dxz – i dyz iron orbitals, as well as between the “black” and “white” sites of the square lattice of iron atoms[2]. Such hidden magnetic order is a result of nesting between electron-type and hole-type Fermi surfaces at the center and at the corner of the unfolded Brillouin zone. The hSDW becomes energetically favorable in the presence of magnetic frustration, at weak enough Hund’s Rule coupling. In the critical hSDW state achieved by increasing the strength of Hund’s Rule, the interaction of electrons with hidden spin fluctuations results in a Lifshitz transition to electron/hole Fermi surface pockets at the corner of the folded Brillouin zone[3]. The electronic spectral weight at the Fermi level is vanishingly small, however, in such a Mott insulator. Weak electron doping about half filling results in a rigid shift in energy of the renormalized bands, with smaller hole Fermi surface pockets that exhibit yet smaller spectral weight, and with larger electron Fermi surface pockets that exhibit appreciable spectral weight[4]. The latter accounts for the observation by ARPES of electron Fermi surface pockets alone in high-Tc iron-selenide superconductors. These Fermi surfaces experience an S-wave Cooper pairing instability due to the exchange of hidden spin fluctuations that alternates in sign between the electron pockets and the hole pockets. This picture then potentially resolves the puzzling observations of an isotropic energy gap over the Fermi surface pockets in high-Tc iron-selenide superconductors.

[1] E. Berg, M.A. Metlitski, and S. Sachdev, Science 338, 1606 (2012).
[2] J.P. Rodriguez, Phys. Rev. B 95, 134511 (2017).
[3] J.P. Rodriguez and R. Melendrez, J. Phys. Commun. 2, 105011 (2018).
[4] J.P. Rodriguez, Phys. Rev. B 103, 184513 (2021).