Abstract
The Fermi-polaron problem of a mobile impurity interacting with fermionic medium emerges in various contexts, ranging from the foundations of Landau’s Fermi-liquid theory to electron-exciton interaction in semiconductors, to unusual properties of high-temperature superconductors. While classically the medium provides only a dissipative environment to the impurity, the quantum picture of polaronic dressing is more intricate and arises from the interplay of few- and many-body aspects of the problem. The conventional expectation for the dynamics of Fermi polarons is that it is dissipative in character, and any excess energy is rapidly emitted away from the impurity as particle-hole excitations. Here we report a strikingly different type of polaron dynamics in a one-dimensional system of the impurity interacting repulsively with the fermions. When the total momentum of the system equals the Fermi momentum, there emerges a sharp collective mode corresponding to long-lived oscillations of the polaronic cloud surrounding the impurity. This mode can be observed experimentally with ultracold atoms using Ramsey interferometry and radio-frequency spectroscopy.
- Received 4 December 2020
- Revised 18 July 2021
- Accepted 12 August 2021
DOI:https://doi.org/10.1103/PhysRevX.11.041015
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Polarons—quasiparticles composed of an impurity particle surrounded by a screening cloud of atoms—are ubiquitous in solid-state and cold-atom systems. In particular, “polaronic dressing,” which describes how the surrounding medium modifies properties of the impurity, is a hallmark of many-body interactions and is fundamental for understanding a wide range of materials. Usually, one does not examine internal excitations between the particle and the surrounding cloud, but rather considers the polaron as a robust static object. In this work, we demonstrate that this picture of polaronic dressing is incomplete and, under certain conditions, the particle and its cloud can form a long-lived collective excitation.
Specifically, by considering a 1D gas of fermions interacting repulsively with an impurity particle, we show that this collective excitation manifests as a narrow peak in the spectrum of sound modes of the fermionic medium in the presence of a mobile impurity. A possible way to visualize this sharp excitation is that it represents long-lived oscillations of the fermionic cloud surrounding the impurity.
Our discovery of this collective equilibrium mode provides an intuitive interpretation of recent numerical studies of far-from-equilibrium impurity dynamics. A key remaining challenge is the experimental detection of the predicted collective excitation. To this end, our work offers several experimental protocols feasible for state-of-the-art cold-atom platforms. Our theoretical approach provides a distinct way to understand far-from-equilibrium dynamics in many-body systems.
