Electron-magnon scattering in elementary ferromagnets from first principles: lifetime broadening and band anomalies

TL;DR

This paper presents a first-principles study of electron-magnon scattering in ferromagnets, revealing spin-dependent lifetime effects, band anomalies, and satellite features in Fe, Co, and Ni using many-body perturbation theory.

Contribution

It introduces a $GT$ self-energy framework that treats single-particle and collective excitations on equal footing, providing new insights into quasiparticle renormalization and band anomalies.

Findings

Strong spin-dependent lifetime effects near Fermi energy, especially in Fe

Observation of a 1.5 eV band anomaly in Fe linked to quasihole coupling

Identification of dispersion anomalies and satellite bands in Ni and Fe

Abstract

We study the electron-magnon scattering in bulk Fe, Co, and Ni within the framework of many-body perturbation theory implemented in the full-potential linearized augmented-plane-wave method. To this end, a $\mathbf{k}$-dependent self-energy ($GT$ self-energy) describing the scattering of electrons and magnons is constructed from the solution of a Bethe-Salpeter equation for the two-particle (electron-hole) Green function, in which single-particle Stoner and collective spin-wave excitations (magnons) are treated on the same footing. Partial self-consistency is achieved by the alignment of the chemical potentials. The resulting renormalized electronic band structures exhibit strong spin-dependent lifetime effects close to the Fermi energy, which are strongest in Fe. The renormalization can give rise to a loss of quasiparticle character close to the Fermi energy, which we attribute to electron scattering with spatially extended spin waves. This scattering is also responsible for dispersion anomalies in conduction bands of iron and for the formation of satellite bands in nickel. Furthermore, we find a band anomaly at a binding energy of 1.5~eV in iron, which results from a coupling of the quasihole with single-particle excitations that form a peak in the Stoner continuum. This band anomaly was recently observed in photoemission experiments. On the theory side, we show that the contribution of the Goldstone mode to the $GT$ self-energy is expected to (nearly) vanish in the long-wavelength limit. We also present an in-depth discussion about the possible violation of causality when an incomplete subset of self-energy diagrams is chosen.