Hidden $(\pi,0)$ instability as an itinerant origin of bicollinear antiferromagnetism in Fe$_{1+x}$Te

TL;DR

This study demonstrates that the bicollinear antiferromagnetic order in Fe$_{1+x}$Te originates from orbital-dependent itinerant electron instabilities at wave vectors (0,$\pi$)/($\pi$,0), independent of Fermi surface nesting.

Contribution

It reveals an orbital modulation mechanism for magnetic instabilities in Fe$_{1+x}$Te, emphasizing the role of excess Fe and orbital-specific susceptibilities in magnetic ordering.

Findings

Magnetic instabilities occur at (0,$\pi$)/($\pi$,0) wave vectors in specific orbitals.

Excess Fe enhances the bicollinear antiferromagnetic order.

Orbital-dependent susceptibilities evolve differently with doping and structural parameters.

Abstract

By calculating orbitally resolved Pauli susceptibilities within maximally localized Wannier orbital basis transformed from first principles band structures, we find that magnetism in Fe$_{1+x}$Te still has its itinerant origin even without Fermi surface nesting, provided orbital modulation of particle-hole excitations are considered. This leads to strong magnetic instabilities at wave vector (0,$\pi$)/($\pi$,0) in d$_{xz}$/d$_{yz}$ orbitals that are responsible for the bicollinear antiferromagnetic state as extra electrons donated from excess Fe are considered. Magnetic exchange coupling between excess Fe and in-plane Fe further stabilizes the bicollinear antiferromagnetic order. Our results reveal that magnetism and superconductivity in iron chalcogenides may have different orbital origin, as Pauli susceptibilities of different orbitals evolve differently as a function of concentration of excess Fe and height of the chalcogen atom measured from the iron plane.