Giant spatial anisotropy of magnon lifetime in altermagnets

Abstract

Altermagnets are a new class of magnetic materials with zero net magnetization (like antiferromagnets) but spin-split electronic bands (like ferromagnets) over a fraction of reciprocal space. As in antiferromagnets, magnons in altermagnets come in two flavours, that either add one or remove one unit of spin to the $S=0$ ground state. However, in altermagnets these two magnon modes are non-degenerate along some directions in reciprocal space. Here we show that the lifetime of altermagnetic magnons has a very strong dependence on both flavour and direction. Strikingly, coupling to Stoner modes leads to a complete suppression of magnon propagation along selected spatial directions. This giant anisotropy will impact electronic, spin, and energy transport properties and may be exploited in spintronic applications.

Figures (11)

• Figure 1: Schematic representation of the model altermagnet on a square lattice defined by primitive vectors $\vec{a}_1$ and $\vec{a}_2$, with $|\vec{a}_1|=|\vec{a}_1|=a$. The solid line connecting blue and red sites represents the nearest neighbor hopping $\tau$. Dashed and dotted lines represent the alternating second neighbor hoppings $\tau'(1\pm\delta)$.
• Figure 2: Dispersion relation for magnons in an insulating altermagnet in the strong coupling regime ($U=10\tau$). The Heisenberg model used to fit the RPA energies includes up to third-neighbor exchange.
• Figure 3: Top: spin excitation spectral densities in the metallic phase ($U=2.5\tau$), along $\vec{q}=(Q,Q)$ (a) and $\vec{q}=(Q,0)$ (b), as a function of energy, for selected wave numbers. To improve visualization, the spectral density has been multiplied by 100 for the three largest wavenumbers ($Q=0.3$, 0.4 and 0.5), by 50 for $Q=0.25$ and by 5 for $Q=0.2$. In (b), solid lines correspond to $\rho^{-+}$, associated with the $S^z=1$ spin excitations, and dashed lines correspond to $\rho^{+-}$, associated with the $S^z=-1$ spin excitations. Bottom: Lifetimes of the metallic altermagnons ($U=2.5\tau$) propagating along the $\vec{q}=(Q,Q)$ (c) and $\vec{q}=(Q,0)$ (d), as a function of wave number, for $S^z=-1$ (squares) and $S^z=1$ (stars) spin excitations.
• Figure 4: Top: Spectral densities for $S^z=-1$ ($\rho^{+-}$, left) and $S^z=1$ ($\rho^{-+}$, right) metallic altermagnons ($U=2.5\tau$) propagating along the $x$ direction, as a function of wave number and energy. Bottom: Spectral densities for $S^z=-1$ ($\bar{\rho}^{+-}$, left) and $S^z=1$ ($\bar{\rho}^{-+}$, right) Stoner excitations (single-particle spin flips) propagating along the $x$ direction, as a function of wave number and energy.
• Figure 5: Altermagnon spectral densities as functions of propagation angle, for a fixed wavelength ($\frac{10a}{3}$). The radial variable represents energy (in units of the nearest-neighbor hopping $\tau$). $\rho^{+-}_A$ corresponds to $S^z=-1$ magnons, $\rho^{-+}_B$ corresponds to $S^z=1$ magnons. Top panels: metallic phase ($U=2.5\tau$); bottom panels: doped insulating phase ($U=3.5\tau$, excess 0.1 electrons per unit cell).