An extremely polarized mixture of an ultracold Fermi gas is expected to reduce to a Fermi polaron system, which consists of a single impurity immersed in the Fermi sea of majority atoms. By developing a many-body T-matrix theory, we investigate spectral properties of the polarized mixture in experimentally relevant regimes in which the system of finite impurity concentration at nonzero temperature is concerned. We explicitly demonstrate presence of polaron physics in the polarized limit and discuss effects of many polarons in an intermediate regime in a selfconsistent manner. By analyzing the spectral function at finite impurity concentration, we extract the attractive and repulsive polaron energies. We find that a renormalization of majority atoms via an interaction with minority atoms is of significance to depict the many-polaron regime.An extremely polarized mixture of an ultracold Fermi gas is expected to reduce to a Fermi polaron system, which consists of a single impurity immersed in the Fermi sea of majority atoms. By developing a many-body T-matrix theory, we investigate spectral properties of the polarized mixture in experimentally relevant regimes in which the system of finite impurity concentration at nonzero temperature is concerned. We explicitly demonstrate presence of polaron physics in the polarized limit and discuss effects of many polarons in an intermediate regime in a selfconsistent manner. By analyzing the spectral function at finite impurity concentration, we extract the attractive and repulsive polaron energies. We find that a renormalization of majority atoms via an interaction with minority atoms is of significance to depict the many-polaron regime.

Jan 29, 20180 citations

Attractive Fermi polarons at nonzero temperatures with a finite impurity concentration

Hui Hu, Brendan C. Mulkerin, Jia Wang +1 more

We theoretically investigate how quasiparticle properties of an attractive Fermi polaron are affected by nonzero temperature and finite impurity concentration in three dimensions and in free space. By applying both non-self-consistent and self-consistent many-body $T$ -matrix theories, we calculate the polaron energy (including decay rate), effective mass, and residue, as functions of temperature and impurity concentration. The temperature and concentration dependencies are weak on the BCS side with a negative impurity-medium scattering length. Toward the strong attraction regime across the unitary limit, we find sizable dependencies. In particular, with increasing temperature the effective mass quickly approaches the bare mass and the residue is significantly enhanced. At temperature $T\ensuremath{\sim}0.1{T}_{F}$, where ${T}_{F}$ is the Fermi temperature of the background Fermi sea, the residual polaron-polaron interaction seems to become attractive. This leads to a notable down-shift in the polaron energy. We show that, by taking into account the temperature and impurity concentration effects, the measured polaron energy in the first Fermi polaron experiment [Schirotzek , Phys. Rev. Lett. 102, 230402 (2009)] could be better theoretically explained.We theoretically investigate how quasiparticle properties of an attractive Fermi polaron are affected by nonzero temperature and finite impurity concentration in three dimensions and in free space. By applying both non-self-consistent and self-consistent many-body $T$-matrix theories, we calculate the polaron energy (including decay rate), effective mass, and residue, as functions of temperature and impurity concentration. The temperature and concentration dependencies are weak on the BCS side with a negative impurity-medium scattering length. Toward the strong attraction regime across the unitary limit, we find sizable dependencies. In particular, with increasing temperature the effective mass quickly approaches the bare mass and the residue is significantly enhanced. At temperature $T\ensuremath{\sim}0.1{T}_{F}$, where ${T}_{F}$ is the Fermi temperature of the background Fermi sea, the residual polaron-polaron interaction seems to become attractive. This leads to a notable down-shift in the polaron energy. We show that, by taking into account the temperature and impurity concentration effects, the measured polaron energy in the first Fermi polaron experiment [Schirotzek , Phys. Rev. Lett. 102, 230402 (2009)] could be better theoretically explained.

arXiv:1708.03410Physical Review AJul 25, 201855 citations

Polaron Problems in Ultracold Atoms: Role of a Fermi Sea across Different Spatial Dimensions and Quantum Fluctuations of a Bose Medium

Hiroyuki Tajima, Junichi Takahashi, S. I. Mistakidis +2 more

The notion of a polaron, originally introduced in the context of electrons in ionic lattices, helps us to understand how a quantum impurity behaves when being immersed in and interacting with a many-body background. We discuss the impact of the impurities on the medium particles by considering feedback effects from polarons that can be realized in ultracold quantum gas experiments. In particular, we exemplify the modifications of the medium in the presence of either Fermi or Bose polarons. Regarding Fermi polarons we present a corresponding many-body diagrammatic approach operating at finite temperatures and discuss how mediated two- and three-body interactions are implemented within this framework. Utilizing this approach, we analyze the behavior of the spectral function of Fermi polarons at finite temperature by varying impurity-medium interactions as well as spatial dimensions from three to one. Interestingly, we reveal that the spectral function of the medium atoms could be a useful quantity for analyzing the transition/crossover from attractive polarons to molecules in three-dimensions. As for the Bose polaron, we showcase the depletion of the background Bose-Einstein condensate in the vicinity of the impurity atom. Such spatial modulations would be important for future investigations regarding the quantification of interpolaron correlations in Bose polaron problems.The notion of a polaron, originally introduced in the context of electrons in ionic lattices, helps us to understand how a quantum impurity behaves when being immersed in and interacting with a many-body background. We discuss the impact of the impurities on the medium particles by considering feedback effects from polarons that can be realized in ultracold quantum gas experiments. In particular, we exemplify the modifications of the medium in the presence of either Fermi or Bose polarons. Regarding Fermi polarons we present a corresponding many-body diagrammatic approach operating at finite temperatures and discuss how mediated two- and three-body interactions are implemented within this framework. Utilizing this approach, we analyze the behavior of the spectral function of Fermi polarons at finite temperature by varying impurity-medium interactions as well as spatial dimensions from three to one. Interestingly, we reveal that the spectral function of the medium atoms could be a useful quantity for analyzing the transition/crossover from attractive polarons to molecules in three-dimensions. As for the Bose polaron, we showcase the depletion of the background Bose-Einstein condensate in the vicinity of the impurity atom. Such spatial modulations would be important for future investigations regarding the quantification of interpolaron correlations in Bose polaron problems.

arXiv:2101.07643AtomsMar 9, 202130 citations

Fermi spin polaron and dissipative Fermi-polaron Rabi dynamics

Hui Hu, Xia-Ji Liu

We consider a spin impurity with multiple energy levels moving in a noninteracting Fermi sea and theoretically solve this Fermi-spin-polaron problem at nonzero temperature by using a non-self-consistent many-body $T$ -matrix theory. We focus on the simplest case with spin $\frac{1}{2}$ , where the two energy states of the impurity are coupled by a Rabi flip term. At small Rabi coupling, the impurity exhibits damped Rabi oscillations, where the decoherence is caused by the interaction with the Fermi sea, as recently reported in Fermi-polaron experiments with ultracold atoms. We investigate the dependence of Rabi oscillations on the Rabi coupling strength and examine the additional nonlinear damping due to large Rabi coupling. At finite temperature and nonzero impurity concentration, the impurity can acquire a pronounced momentum distribution. We show that the momentum or thermal average can sizably reduce the visibility of Rabi oscillations. We compare our theoretical predictions to recent experimental data and find good agreement without any adjustable parameters.We consider a spin impurity with multiple energy levels moving in a noninteracting Fermi sea and theoretically solve this Fermi-spin-polaron problem at nonzero temperature by using a non-self-consistent many-body $T$-matrix theory. We focus on the simplest case with spin $\frac{1}{2}$, where the two energy states of the impurity are coupled by a Rabi flip term. At small Rabi coupling, the impurity exhibits damped Rabi oscillations, where the decoherence is caused by the interaction with the Fermi sea, as recently reported in Fermi-polaron experiments with ultracold atoms. We investigate the dependence of Rabi oscillations on the Rabi coupling strength and examine the additional nonlinear damping due to large Rabi coupling. At finite temperature and nonzero impurity concentration, the impurity can acquire a pronounced momentum distribution. We show that the momentum or thermal average can sizably reduce the visibility of Rabi oscillations. We compare our theoretical predictions to recent experimental data and find good agreement without any adjustable parameters.

arXiv:2206.02317Physical Review ADec 19, 20236 citations

Polaron problems in ultracold atoms: Role of the medium across different spatial dimensions

Hiroyuki Tajima, Junichi Takahashi, S. I. Mistakidis +2 more

The notion of a polaron, originally introduced in the context of electrons in ionic lattices, helps us to understand how a quantum impurity behaves when being immersed in and interacting with a many-body background. We discuss the impact of the impurities on the medium particles by considering feedback effects from polarons that can be realized in ultracold quantum gas experiments and in particular we exemplify the modifications of the medium either in the presence of Fermi or Bose polarons. Regarding Fermi polarons we present a corresponding many-body diagrammatic approach operating at finite temperatures and discuss how mediated two- and three-body interactions are implemented within this framework. Utilizing this approach, we analyze the behavior of the spectral function of Fermi polarons at finite temperature by varying impurity-medium interactions as well as spatial dimensions from three to one. Interestingly, we reveal that the spectral function of the medium atoms could be a useful quantity for analyzing the transition/crossover from attractive polarons to molecules in three-dimensions. As for the Bose polaron, we showcase the depletion of the background Bose-Einstein condensate in the vicinity of the impurity atom. Such spatial modulations would be important for future investigations regarding the quantification of interpolaron correlations in Bose polaron problems.The notion of a polaron, originally introduced in the context of electrons in ionic lattices, helps us to understand how a quantum impurity behaves when being immersed in and interacting with a many-body background. We discuss the impact of the impurities on the medium particles by considering feedback effects from polarons that can be realized in ultracold quantum gas experiments and in particular we exemplify the modifications of the medium either in the presence of Fermi or Bose polarons. Regarding Fermi polarons we present a corresponding many-body diagrammatic approach operating at finite temperatures and discuss how mediated two- and three-body interactions are implemented within this framework. Utilizing this approach, we analyze the behavior of the spectral function of Fermi polarons at finite temperature by varying impurity-medium interactions as well as spatial dimensions from three to one. Interestingly, we reveal that the spectral function of the medium atoms could be a useful quantity for analyzing the transition/crossover from attractive polarons to molecules in three-dimensions. As for the Bose polaron, we showcase the depletion of the background Bose-Einstein condensate in the vicinity of the impurity atom. Such spatial modulations would be important for future investigations regarding the quantification of interpolaron correlations in Bose polaron problems.

Jan 19, 20210 citations

Thermodynamic signatures of the polaron-molecule transition in a Fermi gas

Meera M. Parish, Haydn S. Adlong, Weizhe Edward Liu +1 more

We consider the highly spin-imbalanced limit of a two-component Fermi gas, where there is a small density of $\ensuremath{\downarrow}$ impurities attractively interacting with a sea of $\ensuremath{\uparrow}$ fermions. In the single-impurity limit at zero temperature, there exists the so-called polaron-molecule transition, where the impurity sharply changes its character by binding a $\ensuremath{\uparrow}$ fermion at sufficiently strong attraction. Using a recently developed variational approach, we calculate the thermodynamic properties of the impurity, and we show that the transition becomes a smooth crossover at finite temperature due to the thermal occupation of excited states in the impurity spectral function. However, remnants of the single-impurity transition are apparent in the momentum-resolved spectral function, which can in principle be probed with Raman spectroscopy. We furthermore show that the Tan contact exhibits a characteristic nonmonotonic dependence on temperature that provides a signature of the zero-temperature polaron-molecule transition. For a finite impurity density, we argue that descriptions purely based on the behavior of the Fermi polaron are invalid near the polaron-molecule transition, since correlations between impurities cannot be ignored. In particular, we show that the spin-imbalanced system undergoes phase separation at low temperatures due to the strong attraction between $\ensuremath{\uparrow}\ensuremath{\downarrow}$ molecules induced by the Fermi sea. Thus, we find that the impurity spectrum and the induced impurity-impurity interactions are key to understanding the phase diagram of the spin-imbalanced Fermi gas.We consider the highly spin-imbalanced limit of a two-component Fermi gas, where there is a small density of $\ensuremath{\downarrow}$ impurities attractively interacting with a sea of $\ensuremath{\uparrow}$ fermions. In the single-impurity limit at zero temperature, there exists the so-called polaron-molecule transition, where the impurity sharply changes its character by binding a $\ensuremath{\uparrow}$ fermion at sufficiently strong attraction. Using a recently developed variational approach, we calculate the thermodynamic properties of the impurity, and we show that the transition becomes a smooth crossover at finite temperature due to the thermal occupation of excited states in the impurity spectral function. However, remnants of the single-impurity transition are apparent in the momentum-resolved spectral function, which can in principle be probed with Raman spectroscopy. We furthermore show that the Tan contact exhibits a characteristic nonmonotonic dependence on temperature that provides a signature of the zero-temperature polaron-molecule transition. For a finite impurity density, we argue that descriptions purely based on the behavior of the Fermi polaron are invalid near the polaron-molecule transition, since correlations between impurities cannot be ignored. In particular, we show that the spin-imbalanced system undergoes phase separation at low temperatures due to the strong attraction between $\ensuremath{\uparrow}\ensuremath{\downarrow}$ molecules induced by the Fermi sea. Thus, we find that the impurity spectrum and the induced impurity-impurity interactions are key to understanding the phase diagram of the spin-imbalanced Fermi gas.

arXiv:2011.08478Physical Review AFeb 9, 202119 citations

T

-matrix theories, we calculate the polaron energy (including decay rate), effective mass, and residue, as functions of temperature and impurity concentration. The temperature and concentration dependencies are weak on the BCS side with a negative impurity-medium scattering length. Toward the strong attraction regime across the unitary limit, we find sizable dependencies. In particular, with increasing temperature the effective mass quickly approaches the bare mass and the residue is significantly enhanced. At temperature $T\ensuremath{\sim}0.1{T}_{F}$, where

{T}_{F}

is the Fermi temperature of the background Fermi sea, the residual polaron-polaron interaction seems to become attractive. This leads to a notable down-shift in the polaron energy. We show that, by taking into account the temperature and impurity concentration effects, the measured polaron energy in the first Fermi polaron experiment [Schirotzek , Phys. Rev. Lett. 102, 230402 (2009)] could be better theoretically explained.

T

-matrix theory. We focus on the simplest case with spin

\frac{1}{2}

, where the two energy states of the impurity are coupled by a Rabi flip term. At small Rabi coupling, the impurity exhibits damped Rabi oscillations, where the decoherence is caused by the interaction with the Fermi sea, as recently reported in Fermi-polaron experiments with ultracold atoms. We investigate the dependence of Rabi oscillations on the Rabi coupling strength and examine the additional nonlinear damping due to large Rabi coupling. At finite temperature and nonzero impurity concentration, the impurity can acquire a pronounced momentum distribution. We show that the momentum or thermal average can sizably reduce the visibility of Rabi oscillations. We compare our theoretical predictions to recent experimental data and find good agreement without any adjustable parameters.