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

**Authors:** M. Yarmohammadi, M. Krebs, G. S. Uhrig, B. Normand  
**arXiv:** [2303.00125](https://arxiv.org/abs/2303.00125)  
**Date:** 2023-02-28  
**Summary page:** https://www.summarizepaper.com/en/paper/2303.00125v1/

## Key points

- Magnetophonics involves modulation of magnetic interactions using infrared-active lattice excitations
- Resonances between driven phonon and spin excitation frequencies exhibit self-blocking effect
- Self-blocking arises from alteration of resonance frequencies due to strong hybridization
- Optimal phonon frequencies and mechanisms can control nonequilibrium renormalization of spin excitation spectrum
- Ultrafast terahertz experiments can observe this effect in quantum magnetic materials
- Advancements in technology for measuring quantities such as magnetic circular dichroism, magneto-optic Kerr effect, and second-harmonic polarimetry as time-resolved variants become available
- Ultrafast laser technology has enabled progress in studying nonequilibrium phenomena in correlated condensed matter systems
- Weak coupling between light and spin needs to be overcome for complex many-body states found in quantum magnetic materials
- Lindblad treatment of open quantum systems is adopted to model the problem of heating effects in quantum magnetic materials
- Dimerized spin chain is used as an example to analyze the non-equilibrium stationary state under continuous laser driving
- Strong feedback between spin and lattice sectors is considered in the presence of strong spin-phonon coupling
- Systematic scans of driving frequency are performed to account for shifts in characteristic mode energies caused by strong coupling

## Summary

The field of magnetophononics, which involves the modulation of magnetic interactions by driving infrared-active lattice excitations, is gaining importance as a mechanism for the ultrafast control of both semiclassical and quantum spin systems using coherent light. In this study, the authors 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. This means that only a fraction of the available laser power is absorbed by the phonon. By using quantum master equations to describe the nonequilibrium steady states of the coupled spin-lattice system, they show how self-blocking arises from the alteration of resonance frequencies. This is linked to mutually repelling collective spin-phonon states that become composites of a phonon and two triplons in the regime of strong hybridization. The authors identify optimal phonon frequencies and mechanisms to control a global nonequilibrium renormalization of the lattice-driven spin excitation spectrum. They propose that this effect can be observed in ultrafast terahertz experiments on various known quantum magnetic materials. The study also discusses advancements in technology for measuring quantities such as magnetic circular dichroism, magneto-optic Kerr effect, and second-harmonic polarimetry as time-resolved variants become available across a wider range of probing frequencies. The formalism presented in this study can be applied to analyze expectation values measured by these direct probes. The authors highlight that ultrafast laser technology has enabled significant progress in studying nonequilibrium phenomena in correlated condensed matter systems. However, extending these methods to complex many-body states found in quantum magnetic materials requires overcoming weak coupling between light and spin. Therefore, they investigate magnetophononic channels where driving lasers couple to infrared-active optical phonons and modulate magnetic interactions. Quantum magnetic materials typically have low characteristic energy scales, making them sensitive to heating effects. To model this problem, the authors adopt the Lindblad treatment of open quantum systems and analyze the non-equilibrium stationary state of a system subjected to continuous laser driving. They focus on a dimerized spin chain as a straightforward example of a gapped spin system, where triplons are treated as conventional bosons. The driven phonon is modeled as an infrared-active optical mode coupled to either strong or weak bonds of the dimerized chain. The study expands on previous work by considering the regime of strong feedback between the spin and lattice sectors in the presence of strong spin-phonon coupling. Systematic scans of the driving frequency are performed to account for shifts in characteristic mode energies caused by strong coupling. The authors acknowledge that current experiments deviate from the non-equilibrium steady state protocol, using ultrashort and ultra intense pulses with broad frequency ranges and high order response processes; however they argue that these directions are crucial steps towards understanding and controlling nonequilibrium phenomena in quantum magnetic materials.