Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2610.37654

Mediated Interactions between Fermi Polarons in the Strongly Interacting Regime

Renato Pessoa, S. A. Vitiello, Luis A. Peña Ardila

cond-mat.quant-gasphysics.atom-phquant-phSubmitted Oct 27, 2026• Announced Oct 28, 2026• 0 citations
arXivHTMLPDF

Abstract

Mediated interactions between quasiparticles lie at the heart of Landau Fermi-liquid theory, yet their behavior in the strongly correlated regime is not fully understood. Motivated by recent experiments [Nat. Phys. 20, 68--73 (2024)], we investigate mediated interactions between quasiparticles emerging from a Fermi sea---Fermi polarons---using zero-temperature quantum Monte Carlo methods. Our main result is that the Landau interaction between polarons remains repulsive in the strongly interacting regime for any coupling strength in the polaronic branch, in contrast to the downward trend observed experimentally near resonance. Deep in the short-range universal regime, our results are insensitive to the microscopic details of the interaction potential, providing robust evidence for the sign of the ground-state induced interaction. In addition, we determine the real-space interaction between static impurities, recovering a Ruderman-Kittel-Kasuya-Yosida (RKKY)-like behavior at weak coupling and the nonperturbative Born-Oppenheimer result at strong coupling. More broadly, our results establish a microscopic benchmark for how Landau quasiparticle interactions survive beyond the perturbative regime of strongly correlated quantum matter.

AI Summary

The paper asks whether strong equilibrium correlations can explain the downturn in experimentally measured interactions between fermionic polarons near resonance. It studies a dilute mixture of heavy fermionic impurities and a light Fermi bath at zero temperature, using the experimental mass ratio $m_\downarrow/m_\uparrow=6.8$. Fixed-node diffusion Monte Carlo energies are fitted to a Landau–Pomeranchuk concentration expansion, separating the impurity Fermi-pressure term proportional to $\mathcal C^{2/3}$ from the interaction term $\bar f n_\uparrow\mathcal C/2$. Fits use six concentrations spanning $0.03\leq\mathcal C\leq0.07$; checks extend to approximately $0.15$. The extracted Landau interaction remains positive throughout the attractive polaron branch studied, including near resonance, rather than reproducing the experimental downturn. Zero-range boundary conditions and two short-range potentials give consistent results within numerical uncertainties. Single-polaron energies extracted from the fits agree with independent one-impurity calculations. Separately, energy differences for two fixed impurities determine a spatially resolved induced potential. This agrees with perturbative RKKY behavior at weak coupling and the nonperturbative Born–Oppenheimer prediction at strong coupling, including attractive regions and suppressed long-distance oscillations. These attractive regions concern a different limit from the positive interaction between mobile fermionic quasiparticles. The calculations impose plane-wave Slater-determinant nodes, and repulsive-branch results describe a constrained metastable state rather than the ground state. Comparison with experiment is limited by different impurity degeneracy, a bath temperature $T/T_F\sim0.25$, finite resonance range, and nonequilibrium preparation. The calculations isolate a zero-temperature, fixed-density polaronic contribution but do not determine which additional mechanism produces the measured downturn.

Some signs of AI-generated textStrong assumptionsOverclaims

Central claim may not hold: The effective mass needed to separate Fermi pressure from the Landau interaction is unspecified, and its uncertainty is not propagated into the reported interaction sign. The displayed Pöschl–Teller potential has its first s-wave zero-energy resonance at v0 = 2, not the stated v0 = 1, leaving the calibration of that universality check ambiguous. The reported positive interaction is obtained with untested plane-wave fixed nodes, and neither agreement between potential models nor upper bounds on total energies establish that the exact ground-state interaction has the same sign.

Written by gpt-6.1-sol from the paper's full text. Automated: it can be wrong.

Comments
5 pages + 5 pages(supplemental material) and 5 figures

Related papers

Mediated interactions between Fermi polarons and the role of impurity quantum statistics

Cosetta Baroni, Bo Huang, Isabella Fritsche +7 more

Abstract The notion of quasi-particles is essential for understanding the behaviour of complex many-body systems. A prototypical example of a quasi-particle is a polaron, formed by an impurity strongly interacting with a surrounding medium. Fermi polarons, created in a Fermi sea, provide a paradigmatic realization of this concept. Importantly, such quasi-particles interact with each other via the modulation of the medium. However, although quantum simulation experiments with ultracold atoms have substantially improved our understanding of individual polarons, the detection of their interactions has so far remained elusive. Here we report the observation of mediated interactions between Fermi polarons consisting of K impurities embedded in a Fermi sea of Li atoms. Our results confirm two predictions of Landau’s Fermi-liquid theory: the shift in polaron energy due to mediated interactions, which is linear in the concentration of impurities; and its sign inversion with impurity quantum statistics. For weak-to-moderate interactions between the impurities and the medium, our results agree with the static predictions of Fermi-liquid theory. For stronger impurity–medium interactions, we show that the observed behaviour at negative energies can be explained by a more refined many-body treatment including retardation and dressed molecule formation.

arXiv:2305.04915•Nature Physics•Oct 26, 2023•62 citations

The moving Fermi polaron

Johanna Hennebichler, Ruben Erlenstedt, Erich Dobler +5 more

The Fermi polaron, formed by an impurity interacting with a surrounding Fermi sea, exemplifies the canonical quasiparticle concept as a cornerstone in our description of quantum many-body systems across a wide range of energy scales. Experiments on atomic quantum gases have provided profound insights into the universal nature of the Fermi polaron. While most previous studies have focused on the case of zero impurity momentum, finite-momentum properties have remained largely uncharted. Here, we investigate the moving Fermi polaron by combining a novel Raman acceleration scheme with high-precision radio-frequency spectroscopy, exploring the quasiparticle dispersion relation over a wide range of momenta. We compare our measurements of energy shifts and spectral linewidths with a microscopic theory and reach quantitative agreement for all momenta. For low momenta, we find the energy of the moving polaron to be fully consistent with the Fermi liquid picture of a dressed particle with a constant effective mass. At high momenta, the polaron approaches the behavior of a weakly interacting bare particle, featuring small energy shifts and weak broadening. For intermediate momenta, broadening is generally larger and, most strikingly, the behavior differs for attractive and repulsive polarons. While the repulsive polaron exhibits a smooth connection between both regimes along with a monotonic change of the energy shift, the attractive case shows a peculiar non-monotonic behavior. With increasing momentum, the attractive polaron enters a regime where its energy deviates from the constant effective mass expression and broadening suddenly increases. By comparing this observation with theory, we show that this abrupt behavior coincides with the attractive polaron entering a molecule-hole continuum, where it is no longer the ground state. We interpret this as a motion-induced polaron-molecule transition.

cond-mat.quant-gasCross-listedarXiv:2606.21567•Jun 22, 2026•0 citations

Finite-range effects in the two-dimensional repulsive Fermi polaron

Raúl Bombín, Viktor Cikojević, Juan Sánchez-Baena +1 more

We study the repulsive Fermi polaron in a two-component, two-dimensional system of fermionic atoms inspired by the results of a recent experiment with $^{173}\mathrm{Yb}$ atoms [N. Darkwah Oppong et al., Phys. Rev. Lett. 122, 193604 (2019)]. We use the diffusion Monte Carlo method to report properties such as the polaron energy and the quasiparticle residue that have been measured in that experiment. To provide insight into the quasiparticle character of the problem, we also report results for the effective mass. We show that the effective range, together with the scattering length, is needed in order to reproduce the experimental results. Using different model potentials for the interaction between the Fermi sea and the impurity, we show that it is possible to establish a regime of universality, in terms of these two parameters, that includes the whole experimental regime. This illustrates the relevance of quantum fluctuations and beyond mean-field effects to correctly describe the Fermi polaron problem.

arXiv:2008.10510•Physical Review A•Apr 8, 2021•6 citations

Exact polaron-polaron interactions between heavy impurities in a quantum Hall fluid

Jia Wang, Xia-Ji Liu, Hui Hu

We present an exact solution for effective polaron-polaron interactions between heavy impurities, mediated by a sea of noninteracting light fermions in the quantum Hall regime with highly degenerate Landau levels. For weak attraction between impurities and fermions, where only the manifold of lowest Landau levels is relevant, we obtain an analytical expression of mediated polaron-polaron interactions. Remarkably, polaron interactions are exactly zero when fermions in the lowest Landau levels outnumber heavy impurities. For strong attraction, different manifolds of higher Landau levels come into play and we derive a set of equations that can be used to numerically solve the mediated polaron interaction potential. We find that the potential vanishes when the distance R between impurities is larger than the magnetic length, but strongly diverges at short range following a Coulomb form − 1 / R . Our exact results of polaron-polaron interactions might be examined in cold-atom setups, where a system of Fermi polarons in the quantum Hall regime is realized with a synthetic gauge field or under fast rotation. Our predictions could also be useful to understand the effective interaction between exciton-polarons in electron-doped semiconductors under strong magnetic field.

arXiv:2408.15007•Physical review. B./Physical review. B•Jul 14, 2025•1 citation

Quasiparticle Lifetime of the Repulsive Fermi Polaron

Haydn S. Adlong, Weizhe Edward Liu, Francesco Scazza +5 more

We investigate the metastable repulsive branch of a mobile impurity coupled to a degenerate Fermi gas via short-range interactions. We show that the quasiparticle lifetime of this repulsive Fermi polaron can be experimentally probed by driving Rabi oscillations between weakly and strongly interacting impurity states. Using a time-dependent variational approach, we find that we can accurately model the impurity Rabi oscillations that were recently measured for repulsive Fermi polarons in both two and three dimensions. Crucially, our theoretical description does not include relaxation processes to the lower-lying attractive branch. Thus, the theory-experiment agreement demonstrates that the quasiparticle lifetime is dominated by many-body dephasing within the upper repulsive branch rather than by relaxation from the upper branch itself. Our findings shed light on recent experimental observations of persistent repulsive correlations, and have important consequences for the nature and stability of the strongly repulsive Fermi gas.

arXiv:2005.00235•Physical Review Letters•Sep 24, 2020•53 citations

Precision thermodynamics of the Fermi polaron at strong coupling

S. Ramachandran, S. Jensen, Y. Alhassid

The Fermi polaron problem, which describes a mobile impurity that interacts with a spin-polarized Fermi sea, is a paradigmatic system in quantum many-body physics and has been challenging to address quantitatively in its strong coupling regime. We present the first controlled thermodynamic calculations for the Fermi polaron at strong coupling using finite-temperature auxiliary-field quantum Monte Carlo (AFMC) methods in the framework of the canonical ensemble. Modeled as a spin-imbalanced system, the Fermi polaron has a Monte Carlo sign problem, but we show that it is moderate over a wide range of temperatures and coupling strengths beyond the unitary limit of the BCS-BEC crossover. We calculate the contact, a quantity which measures the strength of the short-range correlations, as a function of temperature at unitarity and as a function of the coupling strength at fixed temperature and find good agreement with a variational approach based on one particle-hole excitation of the Fermi sea. We compare our results for the contact with recent experiments and find good agreement at unitarity (within error bars) but discrepancies away from unitarity on the BEC side of the crossover. We also calculate the thermal energy gap at unitarity as a function of temperature.

arXiv:2410.00886•Physical Review A•Jun 11, 2025•0 citations
Mediated interactions between quasiparticles lie at the heart of Landau Fermi-liquid theory, yet their behavior in the strongly correlated regime is not fully understood. Motivated by recent experiments [Nat. Phys. 20, 68--73 (2024)], we investigate mediated interactions between quasiparticles emerging from a Fermi sea---Fermi polarons---using zero-temperature quantum Monte Carlo methods. Our main result is that the Landau interaction between polarons remains repulsive in the strongly interacting regime for any coupling strength in the polaronic branch, in contrast to the downward trend observed experimentally near resonance. Deep in the short-range universal regime, our results are insensitive to the microscopic details of the interaction potential, providing robust evidence for the sign of the ground-state induced interaction. In addition, we determine the real-space interaction between static impurities, recovering a Ruderman-Kittel-Kasuya-Yosida (RKKY)-like behavior at weak coupling and the nonperturbative Born-Oppenheimer result at strong coupling. More broadly, our results establish a microscopic benchmark for how Landau quasiparticle interactions survive beyond the perturbative regime of strongly correlated quantum matter.
The paper asks whether strong equilibrium correlations can explain the downturn in experimentally measured interactions between fermionic polarons near resonance. It studies a dilute mixture of heavy fermionic impurities and a light Fermi bath at zero temperature, using the experimental mass ratio m↓/m↑=6.8m_\downarrow/m_\uparrow=6.8m↓​/m↑​=6.8.
Fixed-node diffusion Monte Carlo energies are fitted to a Landau–Pomeranchuk concentration expansion, separating the impurity Fermi-pressure term proportional to C2/3\mathcal C^{2/3}C2/3 from the interaction term fˉn↑C/2\bar f n_\uparrow\mathcal C/2fˉ​n↑​C/2. Fits use six concentrations spanning 0.03≤C≤0.070.03\leq\mathcal C\leq0.070.03≤C≤0.07; checks extend to approximately 0.150.150.15. The extracted Landau interaction remains positive throughout the attractive polaron branch studied, including near resonance, rather than reproducing the experimental downturn. Zero-range boundary conditions and two short-range potentials give consistent results within numerical uncertainties. Single-polaron energies extracted from the fits agree with independent one-impurity calculations.
Separately, energy differences for two fixed impurities determine a spatially resolved induced potential. This agrees with perturbative RKKY behavior at weak coupling and the nonperturbative Born–Oppenheimer prediction at strong coupling, including attractive regions and suppressed long-distance oscillations. These attractive regions concern a different limit from the positive interaction between mobile fermionic quasiparticles.
The calculations impose plane-wave Slater-determinant nodes, and repulsive-branch results describe a constrained metastable state rather than the ground state. Comparison with experiment is limited by different impurity degeneracy, a bath temperature T/TF∼0.25T/T_F\sim0.25T/TF​∼0.25, finite resonance range, and nonequilibrium preparation. The calculations isolate a zero-temperature, fixed-density polaronic contribution but do not determine which additional mechanism produces the measured downturn.
Abstract The notion of quasi-particles is essential for understanding the behaviour of complex many-body systems. A prototypical example of a quasi-particle is a polaron, formed by an impurity strongly interacting with a surrounding medium. Fermi polarons, created in a Fermi sea, provide a paradigmatic realization of this concept. Importantly, such quasi-particles interact with each other via the modulation of the medium. However, although quantum simulation experiments with ultracold atoms have substantially improved our understanding of individual polarons, the detection of their interactions has so far remained elusive. Here we report the observation of mediated interactions between Fermi polarons consisting of K impurities embedded in a Fermi sea of Li atoms. Our results confirm two predictions of Landau’s Fermi-liquid theory: the shift in polaron energy due to mediated interactions, which is linear in the concentration of impurities; and its sign inversion with impurity quantum statistics. For weak-to-moderate interactions between the impurities and the medium, our results agree with the static predictions of Fermi-liquid theory. For stronger impurity–medium interactions, we show that the observed behaviour at negative energies can be explained by a more refined many-body treatment including retardation and dressed molecule formation.
The Fermi polaron, formed by an impurity interacting with a surrounding Fermi sea, exemplifies the canonical quasiparticle concept as a cornerstone in our description of quantum many-body systems across a wide range of energy scales. Experiments on atomic quantum gases have provided profound insights into the universal nature of the Fermi polaron. While most previous studies have focused on the case of zero impurity momentum, finite-momentum properties have remained largely uncharted. Here, we investigate the moving Fermi polaron by combining a novel Raman acceleration scheme with high-precision radio-frequency spectroscopy, exploring the quasiparticle dispersion relation over a wide range of momenta. We compare our measurements of energy shifts and spectral linewidths with a microscopic theory and reach quantitative agreement for all momenta. For low momenta, we find the energy of the moving polaron to be fully consistent with the Fermi liquid picture of a dressed particle with a constant effective mass. At high momenta, the polaron approaches the behavior of a weakly interacting bare particle, featuring small energy shifts and weak broadening. For intermediate momenta, broadening is generally larger and, most strikingly, the behavior differs for attractive and repulsive polarons. While the repulsive polaron exhibits a smooth connection between both regimes along with a monotonic change of the energy shift, the attractive case shows a peculiar non-monotonic behavior. With increasing momentum, the attractive polaron enters a regime where its energy deviates from the constant effective mass expression and broadening suddenly increases. By comparing this observation with theory, we show that this abrupt behavior coincides with the attractive polaron entering a molecule-hole continuum, where it is no longer the ground state. We interpret this as a motion-induced polaron-molecule transition.
We study the repulsive Fermi polaron in a two-component, two-dimensional system of fermionic atoms inspired by the results of a recent experiment with 173Yb^{173}\mathrm{Yb}173Yb atoms [N. Darkwah Oppong et al., Phys. Rev. Lett. 122, 193604 (2019)]. We use the diffusion Monte Carlo method to report properties such as the polaron energy and the quasiparticle residue that have been measured in that experiment. To provide insight into the quasiparticle character of the problem, we also report results for the effective mass. We show that the effective range, together with the scattering length, is needed in order to reproduce the experimental results. Using different model potentials for the interaction between the Fermi sea and the impurity, we show that it is possible to establish a regime of universality, in terms of these two parameters, that includes the whole experimental regime. This illustrates the relevance of quantum fluctuations and beyond mean-field effects to correctly describe the Fermi polaron problem.
We present an exact solution for effective polaron-polaron interactions between heavy impurities, mediated by a sea of noninteracting light fermions in the quantum Hall regime with highly degenerate Landau levels. For weak attraction between impurities and fermions, where only the manifold of lowest Landau levels is relevant, we obtain an analytical expression of mediated polaron-polaron interactions. Remarkably, polaron interactions are exactly zero when fermions in the lowest Landau levels outnumber heavy impurities. For strong attraction, different manifolds of higher Landau levels come into play and we derive a set of equations that can be used to numerically solve the mediated polaron interaction potential. We find that the potential vanishes when the distance R between impurities is larger than the magnetic length, but strongly diverges at short range following a Coulomb form − 1 / R . Our exact results of polaron-polaron interactions might be examined in cold-atom setups, where a system of Fermi polarons in the quantum Hall regime is realized with a synthetic gauge field or under fast rotation. Our predictions could also be useful to understand the effective interaction between exciton-polarons in electron-doped semiconductors under strong magnetic field.
We investigate the metastable repulsive branch of a mobile impurity coupled to a degenerate Fermi gas via short-range interactions. We show that the quasiparticle lifetime of this repulsive Fermi polaron can be experimentally probed by driving Rabi oscillations between weakly and strongly interacting impurity states. Using a time-dependent variational approach, we find that we can accurately model the impurity Rabi oscillations that were recently measured for repulsive Fermi polarons in both two and three dimensions. Crucially, our theoretical description does not include relaxation processes to the lower-lying attractive branch. Thus, the theory-experiment agreement demonstrates that the quasiparticle lifetime is dominated by many-body dephasing within the upper repulsive branch rather than by relaxation from the upper branch itself. Our findings shed light on recent experimental observations of persistent repulsive correlations, and have important consequences for the nature and stability of the strongly repulsive Fermi gas.
The Fermi polaron problem, which describes a mobile impurity that interacts with a spin-polarized Fermi sea, is a paradigmatic system in quantum many-body physics and has been challenging to address quantitatively in its strong coupling regime. We present the first controlled thermodynamic calculations for the Fermi polaron at strong coupling using finite-temperature auxiliary-field quantum Monte Carlo (AFMC) methods in the framework of the canonical ensemble. Modeled as a spin-imbalanced system, the Fermi polaron has a Monte Carlo sign problem, but we show that it is moderate over a wide range of temperatures and coupling strengths beyond the unitary limit of the BCS-BEC crossover. We calculate the contact, a quantity which measures the strength of the short-range correlations, as a function of temperature at unitarity and as a function of the coupling strength at fixed temperature and find good agreement with a variational approach based on one particle-hole excitation of the Fermi sea. We compare our results for the contact with recent experiments and find good agreement at unitarity (within error bars) but discrepancies away from unitarity on the BEC side of the crossover. We also calculate the thermal energy gap at unitarity as a function of temperature.