Quasiparticles and their interactions are a key part of our understanding of quantum many-body systems. Quantum simulation experiments with cold atoms have in recent years advanced our understanding of isolated quasiparticles, but so far they have provided limited information regarding their interactions and possible bound states. Here, we show how exploring mobile impurities immersed in a Bose-Einstein condensate (BEC) in a two-dimensional lattice can address this problem. First, the spectral properties of individual impurities are examined, and in addition to the attractive and repulsive polarons known from continuum gases, we identify a new kind of quasiparticle stable for repulsive boson-impurity interactions. The spatial properties of polarons are calculated showing that there is an increased density of bosons at the site of the impurity both for repulsive and attractive interactions. We then derive an effective Schrödinger equation describing two polarons interacting via the exchange of density oscillations in the BEC, which takes into account strong impurity-boson two-body correlations. Using this, we show that the attractive nature of the effective interaction between two polarons combined with the two-dimensionality of the lattice leads to the formation of bound states - i.e. bipolarons. The wave functions of the bipolarons are examined showing that the ground state is symmetric under particle exchange and therefore relevant for bosonic impurities, whereas the first excited state is doubly degenerate and odd under particle exchange making it relevant for fermionic impurities. Our results show that quantum gas microscopy in optical lattices is a promising platform to explore the spatial properties of polarons as well as to finally observe the elusive bipolarons.
Cited by 3
Authors / Affiliations: mappings to Contributors and OrganizationsSee all Organizations.
- 1 Shanshan Ding,
- 2 3 G. A. Domínguez-Castro,
- 1 Aleksi Julku,
- 3 Arturo Camacho Guardian,
- 1 4 Georg M. Bruun
- 1 Aarhus Universitet / Aarhus University [AU]
- 2 Leibniz Universität Hannover / University of Hannover
- 3 Universidad Nacional Autónoma de México / National Autonomous University of Mexico [UNAM]
- 4 南方科技大学 / Southern University of Science and Technology [SUSTech]
- Consejo Nacional de Ciencia y Tecnología [CONACYT]
- Danmarks Grundforskningsfond / Danish National Research Foundation [DNRF]
- Deutsche Forschungsgemeinschaft / German Research FoundationDeutsche Forschungsgemeinschaft [DFG]
- Universidad Nacional Autónoma de México / National Autonomous University of Mexico [UNAM]