Fermi liquid instabilities in the spin channel

TL;DR

This paper investigates Fermi surface instabilities in the spin triplet channel with high orbital partial waves, classifying resulting phases and predicting experimental signatures such as resonance peaks in spin wave spectra.

Contribution

It introduces a classification of Pomeranchuk instabilities into $ ext{alpha}$ and $eta$ phases, analyzing their properties and collective excitations in detail.

Findings

Alpha phases show anisotropic Fermi surface distortions.

Beta phases have topologically non-trivial spin configurations.

Resonance peaks in spin wave spectra are detectable via neutron scattering.

Abstract

We study the Fermi surface instabilities of the Pomeranchuk type in the spin triplet channel with high orbital partial waves ($F_{l}^a ~(l>0)$). The ordered phases are classified into two classes, dubbed the $\alpha$ and $\beta$-phases by analogy to the superfluid $^3$He-A and B-phases. The Fermi surfaces in the $\alpha$-phases exhibit spontaneous anisotropic distortions, while those in the $\beta$-phases remain circular or spherical with topologically non-trivial spin configurations in momentum space. In the $\alpha$-phase, the Goldstone modes in the density channel exhibit anisotropic overdamping. The Goldstone modes in the spin channel have nearly isotropic underdamped dispersion relation at small propagating wavevectors. Due to the coupling to the Goldstone modes, the spin wave spectrum develops resonance peaks in both the $\alpha$ and $\beta$-phases, which can be detected in inelastic neutron scattering experiments. In the p-wave channel $\beta$-phase, a chiral ground state inhomogeneity is spontaneously generated due to a Lifshitz-like instability in the originally nonchiral systems. Possible experiments to detect these phases are discussed.