We report on the experimental observation of multiorbital polarons in a two-dimensional Fermi gas of ^{173}Yb atoms formed by mobile impurities in the metastable ^{3}P_{0} orbital and a Fermi sea in the ground-state ^{1}S_{0} orbital. We spectroscopically probe the energies of attractive and repulsive polarons close to an orbital Feshbach resonance and characterize their coherence by measuring the quasiparticle residue. For all probed interaction parameters, the repulsive polaron is a long-lived quasiparticle with a decay rate more than 2 orders of magnitude below its energy. We formulate a many-body theory, which accurately treats the interorbital interactions in two dimensions and agrees well with the experimental results. Our work paves the way for the investigation of many-body physics in multiorbital ultracold Fermi gases.
We report on the experimental observation of multiorbital polarons in a two-dimensional Fermi gas of ^{173}Yb atoms formed by mobile impurities in the metastable ^{3}P_{0} orbital and a Fermi sea in the ground-state ^{1}S_{0} orbital. We spectroscopically probe the energies of attractive and repulsive polarons close to an orbital Feshbach resonance and characterize their coherence by measuring the quasiparticle residue. For all probed interaction parameters, the repulsive polaron is a long-lived quasiparticle with a decay rate more than 2 orders of magnitude below its energy. We formulate a many-body theory, which accurately treats the interorbital interactions in two dimensions and agrees well with the experimental results. Our work paves the way for the investigation of many-body physics in multiorbital ultracold Fermi gases.
This thesis reports on two experiments employing ultracold atoms to realize many-body physics in the presence of an orbital degree of freedom. In particular, we develop the experimental techniques to probe the Fermi polaron problem across an orbital Feshbach resonance and examine the relaxation dynamics in the one-dimensional mass-imbalanced Fermi-Hubbard model.
Quasiparticles like the Fermi polaron constitute an elementary part of Landau's Fermi liquid theory and find extensive application in the description of interacting electron systems. In the first part of this thesis, we exploit the recently observed orbital Feshbach resonance in ytterbium to produce multiorbital Fermi polarons in mixtures of ground state and clock state atoms. To this end, we employ clock-line spectroscopy and identify the repulsive as well as attractive polaron in the many-body spectrum of the two-dimensional impurity-bath system. We also determine other important quasiparticle properties, namely, the residue and lifetime. Our measurement results agree well with a tailored many-body theory, thereby confirming the relevance of orbital interactions for the Fermi polaron problem.
In the second experiment, we study the mass-imbalanced Fermi-Hubbard model with ytterbium atoms trapped in a state-dependent optical lattice for the ground and metastable clock state. Atomic populations are prepared in both of these states and correspond to heavy and light particles exhibiting vastly different dynamical time scales. Recently, such models have been extensively explored in the context of localization and thermalization in isolated quantum many-body systems. To probe non-equilibrium dynamics in our system, we record the density of the light species after adjusting the external trapping potential. In this way, we identify a strong suppression of transport at early times and slow relaxation at late times, with a significant dependence on the dynamical time scale and interaction strength of both species. Our results demonstrate the emergence of metastability and extremely slow thermalization due to dynamical constraints.
The implementations of multiorbital many-body physics established within this thesis pave the way for quantum simulators that could shed light on open questions in condensed-matter theory.
We characterize properties of the so-called repulsive polaron across the recently discovered orbital Feshbach resonance in alkaline-earth-metal-like atoms. Being a metastable quasiparticle excitation at the positive energy, the repulsive polaron is induced by the interaction between an impurity atom and a Fermi sea. By analyzing in detail the energy, the polaron residue, the effective mass, and the decay rate of the repulsive polaron, we reveal interesting features that are intimately related to the two-channel nature of the orbital Feshbach resonance. In particular, we find that the lifetime of the repulsive polaron is nonmonotonic in the Zeeman-field detuning between the two channels, and has a maximum on the BEC-side of the resonance. Further, by considering the stability of a mixture of the impurity and the majority atoms against phase separation, we show that the itinerant ferromagnetism may exist near the orbital Feshbach resonance at appropriate densities. Our results can be readily probed experimentally, and have interesting implications for the observation of itinerant ferromagnetism near an orbital Feshbach resonance.
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.
Polaron quasiparticles are formed when a mobile impurity is coupled to the elementary excitations of a many-particle background. In the field of ultracold atoms, the study of the associated impurity problem has attracted a growing interest over the last fifteen years. Polaron quasiparticle properties are essential to our understanding of a variety of paradigmatic quantum many-body systems realized in ultracold atomic gases and in the solid state, from imbalanced Bose–Fermi and Fermi–Fermi mixtures to fermionic Hubbard models. In this topical review, we focus on the so-called repulsive polaron branch, which emerges as an excited many-body state in systems with underlying attractive interactions such as ultracold atomic mixtures, and is characterized by an effective repulsion between the impurity and the surrounding medium. We give a brief account of the current theoretical and experimental understanding of repulsive polaron properties, for impurities embedded in both fermionic and bosonic media, and we highlight open issues deserving future investigations.
We study the impurity problem in a Fermi gas of 173Yb atoms near an orbital Feshbach resonance, where a single moving particle in the 3P0 state interacting with two background Fermi seas of particles in different nuclear states of the ground 1S0 manifold. By employing wave function ansatzs for molecule and polaron states, we investigate various properties of the molecule, the attractive polaron, and the repulsive polaron states. We find that in comparison to the case of only one Fermi sea is populated, the presence of an additional Fermi sea acts as an energy shift between the two channels of the orbital Feshbach resonance. Besides, the fluctuation around the Fermi level would also bring sizable effects to the attractive and repulsive polaron states.
We study the impurity problem in a gas of 173Yb atoms near the recently discovered orbital Feshbach resonance. In an orbital Feshbach resonance, atoms in the electronic ground state 1S0 interact with those in the long-lived excited 3P0 state with magnetically tunable interactions. We consider an impurity atom with a given hyperfine spin in the 3P0 state interacting with a single-component Fermi sea of atoms in the ground 1S0 manifold. Close to the orbital Feshbach resonance, the impurity can induce collective particle-hole excitations out of the Fermi sea, which can be regarded as the polaron state. As the magnetic field decreases, a molecular state becomes the ground state of the system. We show that a polaron to molecule transition exists in 173Yb atoms close to the orbital Feshbach resonance. Furthermore, due to the spin-exchange nature of the orbital Feshbach resonance, the formation of both the polaron and the molecule involve spin-flipping processes with interesting density distributions among the relevant hyperfine spin states. We show that the polaron to molecule transition can be detected using Raman spectroscopy.