# Strong-coupling magnetophononics: Self-blocking, phonon-bitriplons, and spin-band engineering

## TL;DR

This paper explores how strong spin-phonon coupling in quantum magnets leads to a self-blocking resonance effect, resulting in novel hybrid states and enabling control over spin excitations via ultrafast phonon driving.

## Contribution

It introduces the concept of self-blocking in magnetophononics, linking it to collective spin-phonon states and providing a framework for controlling spin spectra in quantum materials.

## Key findings

- Self-blocking limits phonon energy absorption in driven quantum magnets.
- Formation of hybrid phonon-triplon states under strong coupling.
- Potential for ultrafast control of spin excitations in experiments.

## Abstract

Magnetophononics, the modulation of magnetic interactions by driving infrared-active lattice excitations, is emerging as a key mechanism for the ultrafast dynamical control of both semiclassical and quantum spin systems by coherent light. We demonstrate that, in a quantum magnet with strong spin-phonon coupling, resonances between the driven phonon and the spin excitation frequencies exhibit an intrinsic self-blocking effect, whereby only a fraction of the available laser power is absorbed by the phonon. Using the quantum master equations governing the nonequilibrium steady states of the coupled spin-lattice system, we show how self-blocking arises from the self-consistent alteration of the resonance frequencies. We link this to the appearance of mutually repelling collective spin-phonon states, which in the regime of strong hybridization become composites of a phonon and two triplons. We then identify the mechanism and optimal phonon frequencies by which to control a global nonequilibrium renormalization of the lattice-driven spin excitation spectrum and demonstrate that this effect should be observable in ultrafast THz experiments on a number of known quantum magnetic materials.