Repulsive Fermi and Bose Polarons in Quantum Gases

TL;DR

This review discusses the properties and significance of repulsive Fermi and Bose polarons in ultracold quantum gases, emphasizing their theoretical and experimental understanding and highlighting open research questions.

Contribution

It provides a comprehensive overview of the current state of knowledge on repulsive polarons in fermionic and bosonic media, including recent advances and unresolved issues.

Findings

Summarizes experimental observations of repulsive polarons.

Reviews theoretical models describing polaron properties.

Identifies open questions for future research.

Abstract

Polaron quasiparticles are formed when a mobile impurity is coupled to the elementary excitations of a many-particle background. In the field of ultracold atoms, the study of the associated impurity problem has attracted a growing interest over the last fifteen years. Polaron quasiparticle properties are essential to our understanding of a variety of paradigmatic quantum many-body systems realized in ultracold atomic gases and in the solid state, from imbalanced Bose-Fermi and Fermi-Fermi mixtures to fermionic Hubbard models. In this topical review, we focus on the so-called repulsive polaron branch, which emerges as an excited many-body state in systems with underlying attractive interactions such as ultracold atomic mixtures, and is characterized by an effective repulsion between the impurity and the surrounding medium. We give a brief account of the current theoretical and experimental understanding of repulsive polaron properties, for impurities embedded in both fermionic and bosonic media, and we highlight open issues deserving future investigations.