We present controlled numerical results for the ground-state spectral function of the resonant Fermi polaron in three dimensions. We establish the existence of a ``dark continuum''---a region of anomalously low spectral weight between the narrow polaron peak and the rest of the spectral continuum. The dark continuum develops when the $s$ -wave scattering length is of the order of the inverse Fermi wave vector, $a\ensuremath{\lesssim}1/{k}_{\mathrm{F}}$, i.e., in the absence of a small interaction-related parameter when the spectral weight is not expected to feature a near-perfect gap structure after the polaron peak.We present controlled numerical results for the ground-state spectral function of the resonant Fermi polaron in three dimensions. We establish the existence of a ``dark continuum''---a region of anomalously low spectral weight between the narrow polaron peak and the rest of the spectral continuum. The dark continuum develops when the $s$-wave scattering length is of the order of the inverse Fermi wave vector, $a\ensuremath{\lesssim}1/{k}_{\mathrm{F}}$, i.e., in the absence of a small interaction-related parameter when the spectral weight is not expected to feature a near-perfect gap structure after the polaron peak.

arXiv:1603.06963Physical Review ANov 23, 201652 citations

The Scattering and Coherence of a Fermi Polaron

Marko Cetina, Michael Jag, Rianne S. Lous +4 more

Bulletin of the American Physical SocietyJun 3, 20140 citations

Exact Quasiparticle Properties of a Heavy Polaron in BCS Fermi Superfluids

Jia Wang, Xia-Ji Liu, Hui Hu

We present the Ramsey response and radio-frequency spectroscopy of a heavy impurity immersed in an interacting Fermi superfluid, using the exact functional determinant approach. We describe the Fermi superfluid through the conventional Bardeen-Cooper-Schrieffer theory and investigate the role of the pairing gap on quasiparticle properties revealed by the two spectroscopies. The energy cost for pair breaking prevents Anderson's orthogonality catastrophe that occurs in a noninteracting Fermi gas and allows the existence of polaron quasiparticles in the exactly solvable heavy impurity limit. Hence, we rigorously confirm the remarkable features such as dark continuum, molecule-hole continuum, and repulsive polaron. For a magnetic impurity scattering at finite temperature, we predict additional resonances related to the subgap Yu-Shiba-Rusinov bound state, whose positions can be used to measure the superfluid pairing gap. For a nonmagnetic scattering at zero temperature, we surprisingly find undamped repulsive polarons. These exact results might be readily observed in quantum gas experiments with Bose-Fermi mixtures that have a large-mass ratio.We present the Ramsey response and radio-frequency spectroscopy of a heavy impurity immersed in an interacting Fermi superfluid, using the exact functional determinant approach. We describe the Fermi superfluid through the conventional Bardeen-Cooper-Schrieffer theory and investigate the role of the pairing gap on quasiparticle properties revealed by the two spectroscopies. The energy cost for pair breaking prevents Anderson's orthogonality catastrophe that occurs in a noninteracting Fermi gas and allows the existence of polaron quasiparticles in the exactly solvable heavy impurity limit. Hence, we rigorously confirm the remarkable features such as dark continuum, molecule-hole continuum, and repulsive polaron. For a magnetic impurity scattering at finite temperature, we predict additional resonances related to the subgap Yu-Shiba-Rusinov bound state, whose positions can be used to measure the superfluid pairing gap. For a nonmagnetic scattering at zero temperature, we surprisingly find undamped repulsive polarons. These exact results might be readily observed in quantum gas experiments with Bose-Fermi mixtures that have a large-mass ratio.

arXiv:2202.05366Physical Review LettersApr 26, 202228 citations

Spectral properties of the 2D Holstein polaron

H. Fehske, J. Loos, Gerhard Wellein

arXiv:cond-mat/9709011Zeitschrift für Physik B Condensed MatterDec 1, 199771 citations

Strong-coupling Bose polarons out of equilibrium: Dynamical renormalization-group approach

Fabian Grusdt, Kushal Seetharam, Yulia E. Shchadilova +1 more

When a mobile impurity interacts with a surrounding bath of bosons, it forms a polaron. Numerous methods have been developed to calculate how the energy and the effective mass of the polaron are renormalized by the medium for equilibrium situations. Here, we address the much less studied nonequilibrium regime and investigate how polarons form dynamically in time. To this end, we develop a time-dependent renormalization-group approach which allows calculations of all dynamical properties of the system and takes into account the effects of quantum fluctuations in the polaron cloud. We apply this method to calculate trajectories of polarons following a sudden quench of the impurity-boson interaction strength, revealing how the polaronic cloud around the impurity forms in time. Such trajectories provide additional information about the polaron's properties which are challenging to extract directly from the spectral function measured experimentally using ultracold atoms. At strong couplings, our calculations predict the appearance of trajectories where the impurity wavers back at intermediate times as a result of quantum fluctuations. Our method is applicable to a broader class of nonequilibrium problems. As a check, we also apply it to calculate the spectral function and find good agreement with experimental results. At very strong couplings, we predict that quantum fluctuations lead to the appearance of a dark continuum with strongly suppressed spectral weight at low energies. While our calculations start from an effective Fr\"ohlich Hamiltonian describing impurities in a three-dimensional Bose-Einstein condensate, we also calculate the effects of additional terms in the Hamiltonian beyond the Fr\"ohlich paradigm. We demonstrate that the main effect of these additional terms on the attractive side of a Feshbach resonance is to renormalize the coupling strength of the effective Fr\"ohlich model.When a mobile impurity interacts with a surrounding bath of bosons, it forms a polaron. Numerous methods have been developed to calculate how the energy and the effective mass of the polaron are renormalized by the medium for equilibrium situations. Here, we address the much less studied nonequilibrium regime and investigate how polarons form dynamically in time. To this end, we develop a time-dependent renormalization-group approach which allows calculations of all dynamical properties of the system and takes into account the effects of quantum fluctuations in the polaron cloud. We apply this method to calculate trajectories of polarons following a sudden quench of the impurity-boson interaction strength, revealing how the polaronic cloud around the impurity forms in time. Such trajectories provide additional information about the polaron's properties which are challenging to extract directly from the spectral function measured experimentally using ultracold atoms. At strong couplings, our calculations predict the appearance of trajectories where the impurity wavers back at intermediate times as a result of quantum fluctuations. Our method is applicable to a broader class of nonequilibrium problems. As a check, we also apply it to calculate the spectral function and find good agreement with experimental results. At very strong couplings, we predict that quantum fluctuations lead to the appearance of a dark continuum with strongly suppressed spectral weight at low energies. While our calculations start from an effective Fr\"ohlich Hamiltonian describing impurities in a three-dimensional Bose-Einstein condensate, we also calculate the effects of additional terms in the Hamiltonian beyond the Fr\"ohlich paradigm. We demonstrate that the main effect of these additional terms on the attractive side of a Feshbach resonance is to renormalize the coupling strength of the effective Fr\"ohlich model.

arXiv:1711.03478Physical Review AMar 19, 201842 citations

s

-wave scattering length is of the order of the inverse Fermi wave vector, $a\ensuremath{\lesssim}1/{k}_{\mathrm{F}}$, i.e., in the absence of a small interaction-related parameter when the spectral weight is not expected to feature a near-perfect gap structure after the polaron peak.