Emergent Nesting of the Fermi Surface from Local-Moment Description of Iron-Pnictide High-Tc Superconductors

TL;DR

This paper models the low-energy electronic structure of iron-pnictide superconductors using a t-J model, revealing emergent Fermi surface nesting and quantum critical behavior related to magnetic frustration and Hund coupling.

Contribution

It introduces a new theoretical framework combining Schwinger-boson-slave-fermion mean-field theory and exact diagonalization to explain Fermi surface nesting and quantum criticality in iron-pnictides.

Findings

Emergent Fermi surface pockets at cSDW momenta near a QCP.

Hole-pocket Fermi surfaces at zero momentum can be pushed below the Fermi level with Hund coupling.

The model explains recent experimental observations of electronic structure in iron-selenide superconductors.

Abstract

We uncover the low-energy spectrum of a t-J model for electrons on a square lattice of spin-1 iron atoms with 3dxz and 3dyz orbital character by applying Schwinger-boson-slave-fermion mean-field theory and by exact diagonalization of one hole roaming over a 4 x 4 x 2 lattice. Hopping matrix elements are set to produce hole bands centered at zero two-dimensional (2D) momentum in the free-electron limit. Holes can propagate coherently in the t-J model below a threshold Hund coupling when long-range antiferromagnetic order across the d+ = 3d(x+iy)z and d- = 3d(x-iy)z orbitals is established by magnetic frustration that is off-diagonal in the orbital indices. This leads to two hole-pocket Fermi surfaces centered at zero 2D momentum. Proximity to a commensurate spin-density wave (cSDW) that exists above the threshold Hund coupling results in emergent Fermi surface pockets about cSDW momenta at a quantum critical point (QCP). This motivates the introduction of a new Gutzwiller wavefunction for a cSDW metal state. Study of the spin-fluctuation spectrum at cSDW momenta indicates that the dispersion of the nested band of one-particle states that emerges is electron-type. Increasing Hund coupling past the QCP can push the hole-pocket Fermi surfaces centered at zero 2D momentum below the Fermi energy level, in agreement with recent determinations of the electronic structure of mono-layer iron-selenide superconductors.