Low-temperature evolution of the spectral weight of a spin-up carrier moving in a ferromagnetic background

TL;DR

This paper investigates how the spectral weight of a spin-up carrier in a ferromagnetic background changes at low temperatures, revealing spectral weight transfer outside the continuum and differences due to magnon gaps.

Contribution

It provides the first derivation of the lowest-temperature correction to the self-energy considering both gapless and gapped magnons.

Findings

Thermal broadening causes the quasiparticle peak to become a resonance.

Spectral weight is transferred outside the continuum at low temperatures.

Differences in spectral behavior are due to magnon gap presence.

Abstract

We derive the lowest-temperature correction to the self-energy of a spin-up particle injected in a ferromagnetic background. The background is modeled with both Heisenberg and Ising Hamiltonians so that differences due to gapless vs. gapped magnons can be understood. Beside the expected thermal broadening of the quasiparticle peak as it becomes a resonance inside a continuum, we also find that spectral weight is transferred to regions lying outside this continuum. We explain the origin of this spectral weight transfer and its low-temperature evolution.