Phonon-mediated repulsion, sharp transitions and (quasi)self-trapping in the extended Peierls-Hubbard model

J. Sous,R. V. Krems,C. P. J. Adolphs,M. Berciu,M. Chakraborty

doi:10.1038/s41598-017-01228-y

J. Sous, R. V. Krems + Show 3 more

Open Access

https://doi.org/10.1038/s41598-017-01228-y

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Abstract

We study two identical fermions, or two hard-core bosons, in an infinite chain and coupled to phonons by interactions that modulate their hopping as described by the Peierls/Su-Schrieffer-Heeger (SSH) model. We show that exchange of phonons generates effective nearest-neighbor repulsion between particles and also gives rise to interactions that move the pair as a whole. The two-polaron phase diagram exhibits two sharp transitions, leading to light dimers at strong coupling and the flattening of the dimer dispersion at some critical values of the parameters. This dimer (quasi)self-trapping occurs at coupling strengths where single polarons are mobile. This illustrates that, depending on the strength of the phonon-mediated interactions, the coupling to phonons may completely suppress or strongly enhance quantum transport of correlated particles.

Highlights

Correlated quantum materials exhibit rich physics with many features yet to be understood
We showed that dressing interacting particles by phonons through SSH/Peierls couplings leads to very rich two-polaron physics, qualitatively different from what is known for conventional polaron models
We showed that the “pair-hopping” terms, which are mediated by phonon-exchange and can only arise in models with phonons modulating the particle hopping, play a major role, leading to sharp transitions of the bound dimer’s ground state

Summary

Introduction

Correlated quantum materials exhibit rich physics with many features yet to be understood. We consider polarons arising in the two-particle limit of an extended Hubbard model coupled to phonons through the SSH/Peierls couplings This is critical for understanding quantum transport of interacting excitons in devices based on organic semiconductors (such as low-temperature solar cells)[28, 29] and the prospects of observing the Mott-insulator/Peierls-insulator competition with highly controllable ultracold atoms/molecules systems, which require understanding of emergent interactions in the few-particle limit. Similar physics may arise in the context of interacting impurities in a Fermi degenerate gas[51,52,53] or Bose-Einstein condensates[54,55,56,57,58,59] of ultracold atoms Motivated by these experiments, we consider identical fermions/hard-core bosons and show that the interplay between particle statistics, particle interactions and coupling to phonons leads to unique features such as phonon-mediated repulsion and sharp transitions in the ground-state properties of dimers including one suggestive of self-trapping.

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