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

Abstract

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...

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

“…This should enable the Ramsey and spin-echo-type experiments that reveal the OC. Furthermore, experiments with strongly imbalanced Fermi-Fermi mixtures, which addressed the polaron dynamics (mobile impurities) [44,[46][47][48][49], demonstrated that the impurity-host atom interactions can be tuned in the full interaction range, from the strong-attractive to the strong-repulsive regime. This should allow for the exploration of different regimes of the OC, discussed above.…”

“…The localization of the impurity atoms at length scales of roughly 10% of the Fermi wavelength has been achieved at typical densities; thus, the impurity can be treated as pointlike, and one can neglect its excitations to the excited states of the trapping potentials, as in our analysis above. (c) RF pulses have been used to switch between the hyperfine states of the impurity atoms, which interact differently with the host fermions [44]. This should enable the Ramsey and spin-echo-type experiments that reveal the OC.…”

“…In our analysis below, we will present all results in terms of the scattering length a. Importantly, the strength of interactions between host atoms and a given hyperfine state of the impurities can be controlled by tuning the magnetic field [44,[46][47][48][49]. We consider a regime where the density of the impurity atoms is so low that scattering processes taking place on different impurities can be analyzed separately.…”

“…The two types of atoms can have very different polarizability; hence, it is possible to create an optical lattice that strongly localizes the impurity atoms while having very little effect on the host fermions. The hyperfine spin states j "i; j #i of the impurity atoms can be manipulated using radio-frequency (RF) fields, which allows one to switch between strongly and weakly interacting states with respect to the host fermions, and thus introduce time-dependent impurities in the Fermi gas [44]. The OC physics generally takes place when the two hyperfine states scatter the host fermions differently.…”

Species-selective confinement of atoms dressed with multiple radiofrequencies

“…Quantum simulation experiments that use mixtures of atomic species promise new insight into the behavior of impurities coupled to larger quantum systems. Experiments in this field have observed non-equilibrium dynamics [3,8], polaronic phenomena [9,10] and the disruption and localization of phases by scattered impurities [11], while control over individual impurities has led to the successful doping of cold gases with single atoms [12,13]. Using the impurities as a probe of the larger system presents many prospects for future experimental work, including the observation of impurity decoherence [14], Markovianity [14][15][16][17], and the non-destructive probing of reservoir excitations [18] and correlations [19].…”