Hydrodynamic Expansion of a Strongly Interacting Fermi-Fermi Mixture

Trenkwalder, Andreas; Kohstall, Christoph; Zaccanti, Matteo; Naik, Devang; Sidorov, Andrei; Schreck, Florian; Grimm, Rudolf

doi:10.1103/physrevlett.106.115304

Cited by 83 publications

(97 citation statements)

References 33 publications

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“…The value therefore somewhat deviates from the one given in Refs. [21,32]. In free space, without the light shift, the resonance center is located at 154.698(5) G. The character of the resonance is closed-channel dominated [20].…”

Section: Methodsmentioning

confidence: 98%

Metastability and coherence of repulsive polarons in a strongly interacting Fermi mixture

Kohstall

Zaccanti

Jag

et al. 2012

Nature

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485

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Ultracold Fermi gases with tuneable interactions represent a unique test bed to explore the many-body physics of strongly interacting quantum systems [1-4]. In the past decade, experiments have investigated a wealth of intriguing phenomena, and precise measurements of groundstate properties have provided exquisite benchmarks for the development of elaborate theoretical descriptions. Metastable states in Fermi gases with strong repulsive interactions [5-11] represent an exciting new frontier in the field. The realization of such systems constitutes a major challenge since a strong repulsive interaction in an atomic quantum gas implies the existence of a weakly bound molecular state, which makes the system intrinsically unstable against decay. Here, we exploit radio-frequency spectroscopy to measure the complete excitation spectrum of fermionic 40 K impurities resonantly interacting with a Fermi sea of 6 Li atoms. In particular, we show that a welldefined quasiparticle exists for strongly repulsive interactions. For this "repulsive polaron" [9, 12, 13] we measure its energy and its lifetime against decay. We also probe its coherence properties by measuring the quasiparticle residue. The results are well described by a theoretical approach that takes into account the finite effective range of the interaction in our system. We find that a non-zero range of the order of the interparticle spacing results in a substantial lifetime increase. This major benefit for the stability of the repulsive branch opens up new perspectives for investigating novel phenomena in metastable, repulsively interacting fermion systems.Landau's theory of a Fermi liquid [14] and the underlying concept of quasiparticles lay at the heart of our understanding of interacting Fermi systems over a wide range of energy scales, including liquid 3 He, electrons in metals, atomic nuclei, and the quark-gluon plasma. In the field of ultracold Fermi gases, the normal (non-superfluid) phase of a strongly interacting system can be interpreted in terms of a Fermi liquid [15][16][17][18]. In the population-imbalanced case, quasiparticles coined Fermi polarons are the essential building blocks and have been studied in detail experimentally [16] for attractive interactions. Recent theoretical work [9,12,13] has suggested a novel quasiparticle associated with repulsive interactions. The properties of this repulsive polaron are of fundamental importance for the prospects of repulsive many-body states. A crucial question for the feasibility of future experiments is the stability against decay into molecular excitations The vertical lines at 1/(κ F a) = ±1 indicate the width of the strongly interacting regime. The inset illustrates our rf spectroscopic scheme where the impurity is transferred from a noninteracting spin state |0 to the interacting state |1 . [11,12,19]. Indeed, whenever a strongly repulsive interaction is realized by means of a Feshbach resonance [20], a weakly bound molecular state is present into which the system may rapidly decay.Our system consi...

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Section: Methodsmentioning

confidence: 98%

Metastability and coherence of repulsive polarons in a strongly interacting Fermi mixture

Kohstall

Zaccanti

Jag

et al. 2012

Nature

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485

678

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“…(a) Various quantum-degenerate mixtures have been realized by multiple groups [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. (b) Recently, the localization of minority atoms by an optical lattice was demonstrated with an imbalanced Bose-Bose mixture of 87 Rb and 41 K atoms [95].…”

Section: Discussionmentioning

confidence: 99%

“…The basic ingredients required for the experimental implementation of our proposal are the following: (a) a quantum-degenerate Bose-Fermi [29][30][31][32][33][34][35][36] or Fermi-Fermi [37][38][39][40][41][42][43] mixture, (b) the ability to trap one type of atom by a strong optical-lattice potential, (c) the control of the interaction strength between the impurity and host atom, which is achieved by changing the impurity hyperfine state, and (d) the ability to achieve temperatures that are low enough to observe the OC.…”

Section: Discussionmentioning

confidence: 99%

Time-Dependent Impurity in Ultracold Fermions: Orthogonality Catastrophe and Beyond

et al. 2012

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The recent experimental realization of strongly imbalanced mixtures of ultracold atoms opens new possibilities for studying impurity dynamics in a controlled setting. In this paper, we discuss how the techniques of atomic physics can be used to explore new regimes and manifestations of Anderson's orthogonality catastrophe (OC), which could not be accessed in solid-state systems. Specifically, we consider a system of impurity atoms, localized by a strong optical-lattice potential, immersed in a sea of itinerant Fermi atoms. We point out that the Ramsey-interference-type experiments with the impurity atoms allow one to study the OC in the time domain, while radio-frequency (RF) spectroscopy probes the OC in the frequency domain. The OC in such systems is universal, not only in the long-time limit, but also for all times and is determined fully by the impurity-scattering length and the Fermi wave vector of the itinerant fermions. We calculate the universal Ramsey response and RF-absorption spectra. In addition to the standard power-law contributions, which correspond to the excitation of multiple particle-hole pairs near the Fermi surface, we identify a novel, important contribution to the OC that comes from exciting one extra particle from the bottom of the itinerant band. This contribution gives rise to a nonanalytic feature in the RF-absorption spectra, which shows a nontrivial dependence on the scattering length, and evolves into a true power-law singularity with the universal exponent 1=4 at the unitarity. We extend our discussion to spin-echo-type experiments, and show that they probe more complicated nonequilibirum dynamics of the Fermi gas in processes in which an impurity switches between states with different interaction strength several times; such processes play an important role in the Kondo problem, but remained out of reach in the solid-state systems. We show that, alternatively, the OC can be seen in the energy-counting statistics of the Fermi gas following a sudden quench of the impurity state. The energy distribution function, which can be measured in time-of-flight experiments, exhibits characteristic power-law singularities at low energies. Finally, systems in which the itinerant fermions have two or more hyperfine states provide an even richer playground for studying nonequilibrium impurity physics, allowing one to explore the nonequilibrium OC and even to simulate quantum transport through nanostructures. This provides a previously missing connection between cold atomic systems and mesoscopic quantum transport.

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“…Quite naturally, transport phenomena have also been studied [18]. More recently, transport experiments of an ultracold mass-balanced polarized Fermi gas [19] and an ultracold mass-imbalanced mixture with unequal populations [20] opened new opportunities to directly investigate nonequilibrium and dynamic properties of population-imbalanced Fermi systems.In this paper, we discuss nonequilibrium transport in the high polarization and low temperature regime, without restricting ourselves within small deviations from equilibrium. As a limiting case of high polarization and low temperature, we first consider a single minority atom moving in a zero-temperature Fermi sea of majority atoms.…”

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confidence: 99%

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Superdiffusive nonequilibrium motion of an impurity in a Fermi sea

Kim

Huse²

2012

Phys. Rev. A

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We treat the nonequilibrium motion of a single impurity atom in a low-temperature single-species Fermi sea, interacting via a contact interaction. In the nonequilibrium regime, the impurity does a superdiffusive geometric random walk where the typical distance traveled grows with time as ∼ t d/(d+1) for the d-dimensional system with d ≥ 2. For nonzero temperature T , this crosses over to diffusive motion at long times with diffusivity D ∼ T −(d−1)/2 . These results apply also to a nonzero concentration of impurity atoms as long as they remain dilute and nondegenerate.PACS numbers: 03.75. Ss, 67.85.Lm, In condensed matter physics, the behavior of a single impurity immersed in a sea of majority particles has been one of the simplest many-body problems and attracted much attention (see, e.g. [1,2]). Especially, transport of an impurity in Bosonic [3] and Fermionic [4,5] quantum liquids has been studied since a few decades ago. Recent realizations of ultra-cold polarized two-component atomic Fermi gases [6][7][8][9][10][11][12], with their remarkable controllability over parameters such as interaction strength and individual populations, have made it possible to directly access this type of quantum many-body system. Meanwhile, there have been a number of theoretical works on the single impurity problem in an ultracold Fermi gas [13][14][15][16][17], investigating the equilibrium and nearequilibrium properties. Quite naturally, transport phenomena have also been studied [18]. More recently, transport experiments of an ultracold mass-balanced polarized Fermi gas [19] and an ultracold mass-imbalanced mixture with unequal populations [20] opened new opportunities to directly investigate nonequilibrium and dynamic properties of population-imbalanced Fermi systems.In this paper, we discuss nonequilibrium transport in the high polarization and low temperature regime, without restricting ourselves within small deviations from equilibrium. As a limiting case of high polarization and low temperature, we first consider a single minority atom moving in a zero-temperature Fermi sea of majority atoms. We find that the impurity does a superdiffusive geometric random walk, where the time between collisions grows in proportion to time, and the impurity loses a fraction of order one of its energy in each collision. InAs is conventional, we call the majority species "up" ↑ and the minority (impurity) species "down" ↓. Note that in the regimes we study in this paper, the statistics of the minority atoms do not enter, so they may equally well be bosons or fermions. Assume the minority atom is initially near the origin in real space, with some probability distribution of its momentum Q ↓ with Q ↓ k F ↑ , where k F ↑ is the Fermi momentum of the majority Fermi sea. The minority atom is "dressed" either as a polaron or as a molecule, with effective mass m * and thus energy E ↓ ≈ 2 Q 2 ↓ 2m * ; we choose the rest energy of the dressed minority atom as its zero of energy. We assume that E ↓ is low enough so that no internal excitations of th...

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Hydrodynamic Expansion of a Strongly Interacting Fermi-Fermi Mixture

Cited by 83 publications

References 33 publications

Metastability and coherence of repulsive polarons in a strongly interacting Fermi mixture

Metastability and coherence of repulsive polarons in a strongly interacting Fermi mixture

Time-Dependent Impurity in Ultracold Fermions: Orthogonality Catastrophe and Beyond

Superdiffusive nonequilibrium motion of an impurity in a Fermi sea

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