The "Higgs" Amplitude Mode in Weak Ferromagnetic Metals

Abstract

Using ferromagnetic Fermi liquid theory, Bedell and Blagoev derived the collective low-energy excitations of a weak ferromagnet. They obtained the well-known magnon (NambuGoldstone) mode and found a new gapped mode that was never studied in weak ferromagnetic metals. In this article we have identied this mode as the Higgs boson (amplitude mode) of a ferromagnetic metal. This is identied as the Higgs since it can be shown that it corresponds to a uctuation of the amplitude of the order parameter. We use this model to describe the itinerant-electron ferromagnetic material MnSi. By tting the model with the existing experimental results, we calculate the dynamical structure function and see well-dened peaks contributed from the magnon and the Higgs. Our estimates of the relative intensity of the Higgs amplitude mode suggest that it can be seen in neutron scattering experiments on MnSi.

Unconventional Hund metal in a weak itinerant ferromagnet

“…The loss of orbital coherence seemingly coincides with the divergence of the damped-magnon dispersion within the particle-hole continuum. This suggests the breakdown of coherent spin fluctuations due to the coupling of orbital and spin degrees of freedom as seen in the ferromagnetic Fermi-liquid theory 40,41 . Analysis of the DFT+DMFT orbital-resolved fluctuations (Supplementary Information) reveals that the coherence peaks are dominantly of itinerant e g ′ character in the spin channel as well as dominantly of e g − a 1g and e g ′ − a 1g character in the orbital channel.…”

“…An added decay channel is predicted when quasiparticle interactions are considered. The inclusion of higher-order Landau parameters developed for an interacting ferromagnetic Fermi liquid predicts the stabilization of a gapped mode comprised of transverse spin fluctuations 40,41 (which we label as the Δ-mode). Although the Δ-mode is difficult to measure directly, it can propagate (albeit damped) through the continuum, and when its dispersion crosses with that of the transverse spin fluctuations (shown in Supplementary Information), an additional damping channel is accessed and the damping rates of the excitations diverge.…”

“…Ferromagnetic Fermi-liquid calculations. We use the version of the ferromagnetic Fermi-liquid theory (FFLT), where the Fermi-liquid phenomenology is augmented with the Ginzburg-Landau theory for a small moment ferromagnet 40,41 . The transport and dynamical equations for the FFLT follow from the steady state and time evolution equations of the magnetization distribution function.…”

“…The transport and dynamical equations for the FFLT follow from the steady state and time evolution equations of the magnetization distribution function. This leads directly to the prediction of the existence of the amplitude mode when the l = 1 Fermi-liquid parameter is included we used other experimental measurements to get the magnitude of the amplitude mode for MnSi 40,41 . This is predicted to be about 50 meV at q = 0 and it drops to about 25 meV when the Goldstone mode crosses with the amplitude mode inside the particle-hole continuum.…”

Quantum spin transport and dynamics through a ferromagnetic/normal metal junction

“…is the steady state quasiparticle velocity given by Eq. ( 4), |v 0 p | = v f , and the dispersions of the p-h continuum for the ferromagnetic systems are [18], ω ± p-h (q) = ∓ 2 σ FM f a 0 + q • vp , where, vp = V FM 0 + v0 p , and, |v 0 p | = vf . For a given q, with the freedom of choosing v p over the entire Fermi surface, the dispersions of the p-h excitations form a continuum bounded by the maximum and minimum values of q • v p .…”