Anisotropic magnon damping by zero-temperature quantum fluctuations in ferromagnetic CrGeTe$_3$

TL;DR

This study reveals that in 2D ferromagnetic CrGeTe3, zero-temperature quantum fluctuations cause anisotropic magnon damping and quasiparticle breakdown, indicating a novel dynamical spin-lattice coupling effect.

Contribution

It demonstrates the presence of anisotropic magnon damping caused by quantum fluctuations in a 2D ferromagnet, revealing a new form of dynamical spin-lattice coupling.

Findings

In-plane spin waves show anomalous dispersion and damping.

Magnons along the c axis behave as expected for a local moment ferromagnet.

Zero-temperature quantum fluctuations induce dynamical spin-lattice coupling.

Abstract

Spin and lattice are two fundamental degrees of freedom in a solid, and their fluctuations about the equilibrium values in a magnetic ordered crystalline lattice form quasiparticles termed magnons (spin waves) and phonons (lattice waves), respectively. In most materials with strong spin-lattice coupling (SLC), the interaction of spin and lattice induces energy gaps in the spin wave dispersion at the nominal intersections of magnon and phonon modes. Here we use neutron scattering to show that in the two-dimensional (2D) van der Waals honeycomb lattice ferromagnetic CrGeTe3, spin waves propagating within the 2D plane exhibit an anomalous dispersion, damping, and break-down of quasiparticle conservation, while magnons along the c axis behave as expected for a local moment ferromagnet. These results indicate the presence of dynamical SLC arising from the zero-temperature quantum fluctuations in CrGeTe3, suggesting that the observed in-plane spin waves are mixed spin and lattice quasiparticles fundamentally different from pure magnons and phonons.