Spin wave amplification through superradiance

TL;DR

This paper demonstrates spin wave superradiance at an interface between normal and pumped ferromagnetic regions, where super-reflection occurs due to current-induced spectrum inversion, revealing a new way to amplify spin waves.

Contribution

It introduces a novel manifestation of superradiance in spin waves, showing how current-induced spectrum inversion leads to super-reflection at ferromagnetic interfaces.

Findings

Super-reflection coefficient exceeds 1 at the interface.

Refractive spin waves are activated and re-enter the normal FM region.

The process is analogous to black hole superradiance and Dicke superradiance.

Abstract

Superradiance is a phenomenon of multiple facets that occurs in classical and quantum physics under extreme conditions. Here we present its manifestation in spin waves under an easily realized condition. We show that an interface between a current-free (normal) ferromagnetic (FM) region and a current-flow (pumped) FM region can be a spin wave super-mirror whose reflection coefficient is larger than 1. The super-reflection is the consequence of current-induced spectrum inversion where phase and group velocities of spin waves are in the opposite directions. An incident spin wave activates a backward propagating refractive wave inside pumped FM region. The refractive spin wave re-enters the normal FM region to constructively interfere with the reflective wave. It appears that the pumped FM region coherently emits reflective waves, leading to a super-reflection. The process resembles superradiance of a spinning black hole through the Hawking radiation process, or Dicke superradiance of cavity photons inside population inverted media.