# Avoided quasiparticle decay from strong quantum interactions

## Abstract

Quantum states of matter—such as solids, magnets and topological phases—typically exhibit collective excitations (for example, phonons, magnons and anyons)1. These involve the motion of many particles in the system, yet, remarkably, act like a single emergent entity—a quasiparticle. Known to be long lived at the lowest energies, quasiparticles are expected to become unstable when encountering the inevitable continuum of many-particle excited states at high energies, where decay is kinematically allowed. Although this is correct for weak interactions, we show that strong interactions generically stabilize quasiparticles by pushing them out of the continuum. This general mechanism is straightforwardly illustrated in an exactly solvable model. Using state-of-the-art numerics, we find it at work in the spin-$$1/2$$ triangular-lattice Heisenberg antiferromagnet (TLHAF). This is surprising given the expectation of magnon decay in this paradigmatic frustrated magnet. Turning to existing experimental data, we identify the detailed phenomenology of avoided decay in the TLHAF material2 Ba3CoSb2O9, and even in liquid helium3,4,5,6,7,8, one of the earliest instances of quasiparticle decay9. Our work unifies various phenomena above the universal low-energy regime in a comprehensive description. This broadens our window of understanding of many-body excitations, and provides a new perspective for controlling and stabilizing quantum matter in the strongly interacting regime.

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## Data availability

The data that support the findings of this study are available from the corresponding author upon request.

## Code availability

Details about the DMRG code are provided in the Methods and in the Supplementary Information.

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## Acknowledgements

The authors thank R. Coldea, S. Parameswaran and S. Chernyshev for discussions, and the latter for detailed comments on the manuscript. The authors also thank I. Khaymovich for pointing out the inspiring refs. 31,32. R.V. was supported by the German Research Foundation (DFG) through the Collaborative Research Center SFB 1143. F.P. acknowledges support from DFG Research Unit FOR 1807 through grant no. PO 1370/2-1, TRR80, Nanosystems Initiative Munich (NIM) by the German Excellence Initiative, the DFG under Germany’s Excellence Strategy EXC-2111-390814868, and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 771537). This research was supported in part by the National Science Foundation under grant no. NSF PHY-1748958 and by the Heising–Simons Foundation.

## Author information

Authors

### Contributions

All authors contributed equally to this work.

### Corresponding author

Correspondence to Ruben Verresen.

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### Competing interests

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## Supplementary information

### Supplementary Information

Supplementary Text and Supplementary Figs. 1–7.

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Verresen, R., Moessner, R. & Pollmann, F. Avoided quasiparticle decay from strong quantum interactions. Nat. Phys. 15, 750–753 (2019). https://doi.org/10.1038/s41567-019-0535-3

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