We employ radio-frequency spectroscopy to investigate a polarized spin-mixture of ultracold 6 Li atoms close to a broad Feshbach scattering resonance. Focusing on the regime of strong repulsive interactions, we observe well-defined coherent quasiparticles even for unitarity-limited interactions. We characterize the many-body system by extracting the key properties of repulsive Fermi polarons: the energy E+, the effective mass m * , the residue Z and the decay rate Γ. Above a critical interaction, E+ is found to exceed the Fermi energy of the bath while m * diverges and even turns negative, thereby indicating that the repulsive Fermi liquid state becomes energetically and thermodynamically unstable.Landau's idea of mapping the behavior of impurity particles interacting with a complex environment into quasiparticle properties [1] plays a fundamental role in physics and materials science, from helium liquids [2] and colossal magnetoresistive materials [3,4] to polymers and proteins [5,6]. In the field of ultracold gases, the impurity problem and the associated concept of polaron quasiparticle have attracted over the last decade a growing interest [7][8][9][10]. Initiated with the investigation of polarized Fermi gases in the BEC-BCS crossover [11][12][13][14][15][16], the study of polaron physics has been extended to mass-imbalanced [17,18], low-dimensional fermionic systems [19], and also to bosonic environments [20][21][22]. The polaron properties are fundamentally relevant for understanding the more complex scenario of partially-polarized and balanced Fermi mixtures: the impurity limit exhibits some of the critical points of the full phase diagram, whose topology we can thus learn about by investigating polarized systems [8,16].While researchers initially focused on attractive interactions [14,15], more recently they have explored novel quasiparticles associated with repulsive interactions: these repulsive polarons [23][24][25][26][27] are centrally important for realizing repulsive many-body states [23,24,28,29] and therein exploring itinerant ferromagnetism [30][31][32]. In particular, if the polaron energy exceeds the Fermi energy of the surrounding medium, a fullyferromagnetic phase is favored against the paramagnetic Fermi liquid [23][24][25]27]. However, short-ranged strong repulsion always require an underlying weakly-bound molecular state, into which the system may rapidly decay [31,33], making the repulsive polaron an excited manybody state, whose theoretical and experimental investigation are challenging. In three dimensions, repulsive Fermi polarons have been first unveiled in a 6 Li -40 K mixture at a comparatively narrow Feshbach resonance [17], but they lack observation in the universal, broad * scazza@lens.unifi.it resonance case, for which the decay rate is expected to be the largest [10].In this Letter we report on reverse radio-frequency (RF) spectroscopy [17,34,35] experiments to unveil the existence and characterize the properties of repulsive polarons in a polarized Fermi mixture of lithium ...
It has long been expected that quantum degenerate gases of molecules would open access to a wide range of phenomena in molecular and quantum sciences. However, the very complexity that makes ultracold molecules so enticing has made reaching degeneracy an outstanding experimental challenge over the past decade. We now report the production of a Fermi degenerate gas of ultracold polar molecules of potassium-rubidium (KRb). Through coherent adiabatic association in a deeply degenerate mixture of a rubidium Bose-Einstein condensate and a potassium Fermi gas, we produce molecules at temperatures below 0.3 times the Fermi temperature. We explore the properties of this reactive gas and demonstrate how degeneracy suppresses chemical reactions, making a long-lived degenerate gas of polar molecules a reality.Ultracold polar molecules have received attention as ideal candidates to realize a plethora of proposals in molecular and many-body physics. These include the development of chemistry in the quantum regime [1], the emulation of strongly interacting lattice spin models [2-6], the production of topological phases in optical lattices [7-10], the exploration of fundamental symmetries [11][12][13][14][15], and the study of quantum information science [16][17][18]. While magnetic atoms also exhibit long-ranged dipolar interactions and can be used to carry out these proposals [19,20], polar molecules offer more tunable, stronger interactions and additional degrees of freedom. A low-entropy, quantum degenerate sample is a prerequisite for many of these explorations.The intrinsic complexity of molecules relative to atoms, owing to the additional rotational and vibrational degrees of freedom, has made their cooling to ultralow temperatures one of the most significant experimental challenges in molecular physics [21]. While the direct laser cooling of certain diatomic molecules has progressed enormously in recent times so that magneto-optic [22][23][24][25] and pure optical [26] trapping have been demonstrated, phase space density in these systems remains many orders of magnitude away from degeneracy. To date, by far the coldest diatomic molecules have been made by cooling atoms to a few hundred nanokelvin (10 −9 K) and coherently associating the ultracold atoms into deeply bound molecules using a Fano-Feshbach resonance [27] fol-lowed by stimulated Raman adiabatic passage (STI-RAP) [28].
We report on the observation of the Josephson effect between two strongly interacting fermionic superfluids coupled through a thin tunneling barrier. We prove that the relative population and phase are canonically conjugate dynamical variables, coherently oscillating throughout the entire crossover from molecular Bose-Einstein condensates (BEC) to Bardeen-Cooper-Schrieffer (BCS) superfluids. We measure the plasma frequency and we extract the Josephson coupling energy, both exhibiting a non-monotonic behavior with a maximum near the crossover regime. We also observe the transition from coherent to dissipative dynamics, which we directly ascribe to the propagation of vortices through the superfluid bulk. Our results highlight the robust nature of resonant superfluids, opening the door to the study of the dynamics of superfluid Fermi systems in the presence of strong correlations and fluctuations.The Josephson effect is a pristine example of a macroscopic quantum phenomenon, disclosing the broken symmetry associated with the superfluid state [1]. On a very fundamental level, it allows to pinpoint the most elusive part of the superfluid order parameter, the phase, through a measurable quantity, a particle current [2]. Furthermore, being based on tunneling processes, Josephson dynamics provides fundamental insights into the microscopic properties of superfluids and their robustness against dissipative phenomena [3]. Since its discovery, Josephson effect has been demonstrated for a variety of fermionic and bosonic systems [3][4][5][6][7][8][9][10][11][12]. However, it has so far eluded observation in BEC-BCS crossover superfluids [13,14] realized by ultracold Fermi gas mixtures close to a Feshbach resonance [15,16]. The interest in these systems is twofold: on the one hand, they encompass the two paradigmatic aspects of superfluidity within a single framework: Bose-Einstein condensation of tightly bound molecules and BCS superfluidity of long-range fermion pairs [13]. Moreover, in the resonant regime where the pair size matches the interparticle spacing, they exhibit universal properties, sharing analogies with other exotic strongly-correlated fermionic superfluids, from cuprate superconductors to nuclear and quark matter [17,18].In this work, we report on the observation of the Josephson effect in ultracold gases of 6 Li atom pairs across the BEC-BCS crossover. Our Josephson junction consists of two superfluid reservoirs, weakly coupled through a thin tunneling barrier. For all interaction regimes, we detect coherent oscillations of both the pair population imbalance ∆N = N L −N R and the relative phase ϕ = ϕ L −ϕ R across * Permanent address: Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, 01000 México Distrito Federal, Mexico.the junction, measured in situ and after time-of-flight expansion respectively. We prove these two observables to be dynamically conjugate [2], directly unveiling macroscopic phase coherence in these strongly-correlated fermionic superfluids. We measure th...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.