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arXiv:1507.06309 · Physical Review A

Universal clusters as building blocks of stable quantum matter

Shimpei Endo, Antonio M. Garcı́a-Garcı́a, Pascal Naidon

Published May 18, 2016• 26 citations
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Abstract

We present an exploratory study that suggests that Efimov physics, a leading research theme in few-body quantum physics, can also induce stable many-body ground states whose building blocks are universal clusters. We identify a range of parameters in a mass-and-density-imbalanced two-species fermionic mixture for which the ground state is a gas of Efimov-related universal trimers. An explicit calculation of the trimer-trimer interaction reveals that the trimer phase is an SU(3) Fermi liquid stable against recombination losses. We propose to experimentally observe this phase in a fermionic $^{6}\text{Li}\text{\ensuremath{-}}^{53}\text{Cr}$ mixture.

DOI
10.1103/physreva.93.053611

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We present an exploratory study that suggests that Efimov physics, a leading research theme in few-body quantum physics, can also induce stable many-body ground states whose building blocks are universal clusters. We identify a range of parameters in a mass-and-density-imbalanced two-species fermionic mixture for which the ground state is a gas of Efimov-related universal trimers. An explicit calculation of the trimer-trimer interaction reveals that the trimer phase is an SU(3) Fermi liquid stable against recombination losses. We propose to experimentally observe this phase in a fermionic 6Li\ensuremath-53Cr^{6}\text{Li}\text{\ensuremath{-}}^{53}\text{Cr}6Li\ensuremath-53Cr mixture.
This thesis investigates universal and non-universal aspects of few-body physics in a mass-imbalanced mixture of ultracold bosonic 133^{133}133Cs and fermionic 6^66Li atoms. For this purpose an experimental apparatus for the creation of quantum degenerate gases has been constructed. Using radio-frequency association of weakly-bound LiCs molecules and additional atom-loss spectroscopy we determine the two-body interaction properties between Li and Cs atoms at low collision energies. The analysis with a coupled-channels calculation yields precise singlet and triplet electronic ground state molecular potential curves, from which scattering lengths and positions of Fano-Feshbach resonances are extracted. These results represent almost a tenfold improvement over the previous determination, and are used to study few-body recombination processes and, for the first time, their dependence on the interaction strength between the heavy atoms close to two broad scattering resonances at various temperatures. Measurements of three-body loss rates reveal two series of three and two consecutive LiCs2_22​ Efimov resonances that demonstrate the universal scaling factor 4.9 between the first and second excited state resonances. The ground state resonance deviates from the discrete scaling law. By the use of hyperspherical formalism with either pairwise zero-range or Lennard-Jones model potentials we identify the van der Waals interaction between Cs atoms as a significant source of these modifications.
Efimov physics is renowned for the self-similar spectrum featuring the universal ratio of one eigenenergy to its neighbor. Even more esoteric is the numerically unveiled fact that every Efimov trimer is accompanied by a pair of tetramers. Here we demonstrate that this hierarchy of universal few-body clusters has a topological origin by identifying the numbers of universal three- and four-body bound states with the winding numbers of the renormalization-group limit cycle in theory space. The finding suggests a topological phase transition in mass-imbalanced few-body systems which should be tested experimentally.
We observe enhanced three-body recombination in a three-component ;{6}Li Fermi gas attributable to an excited Efimov trimer state intersecting the three-atom scattering threshold near 895 G. From measurements of the recombination rate we determine the Efimov parameters kappa_{*} and eta_{*} for the universal region above 600 G which includes three overlapping Feshbach resonances. The value of kappa_{*} also predicts the locations of loss features previously observed near 130 and 500 G [T. B. Ottenstein, Phys. Rev. Lett. 101, 203202 (2008)10.1103/PhysRevLett.101.203202; J. H. Huckans, Phys. Rev. Lett. 102, 165302 (2009)10.1103/PhysRevLett.102.165302] suggesting they are associated with a ground-state Efimov trimer near threshold. We also report on the realization of a degenerate three-component Fermi gas with approximate SU(3) symmetry.
Three interacting particles form a system which is well known for its complex physical behavior. A landmark theoretical result in few-body quantum physics is Efimov's prediction of a universal set of weakly bound trimer states appearing for three identical bosons with a resonant two-body interaction. Surprisingly, these states even exist in the absence of a corresponding two-body bound state and their precise nature is largely independent of the particular type of the two-body interaction potential. Efimov's scenario has attracted great interest in many areas of physics; an experimental test however has not been achieved. We report the observation of an Efimov resonance in an ultracold thermal gas of cesium atoms. The resonance occurs in the range of large negative two-body scattering lengths and arises from the coupling of three free atoms to an Efimov trimer. We observe its signature as a giant three-body recombination loss when the strength of the two-body interaction is varied near a Feshbach resonance. This resonance develops into a continuum resonance at non-zero collision energies, and we observe a shift of the resonance position as a function of temperature. We also report on a minimum in the recombination loss for positive scattering lengths, indicating destructive interference of decay pathways. Our results confirm central theoretical predictions of Efimov physics and represent a starting point from which to explore the universal properties of resonantly interacting few-body systems.
We study Efimov resonances via three-body loss in an ultracold two-component gas of fermionic 6Li^{6}\mathrm{Li}6Li and bosonic 133Cs^{133}\mathrm{Cs}133Cs atoms close to a Feshbach resonance at 843 G, extending results reported previously [Pires et al., Phys. Rev. Lett. 112, 250404 (2014)] to temperatures around 120 nK. The experimental scheme for reaching lower temperatures is based upon compensating the gravity-induced spatial separation of the mass-imbalanced gases with bichromatic optical dipole traps. We observe the first and second excited Li-Cs-Cs Efimov resonance in the magnetic field dependence of the three-body event rate constant, in good agreement with the universal zero-range theory at finite temperature [Petrov and Werner, Phys. Rev. A 92, 022704 (2015)]. Deviations are found for the Efimov ground state, and the inelasticity parameter $\ensuremath{\eta}$ is found to be significantly larger than those for single-species systems.
In these notes I provide an overview of ongoing theoretical and experimental research on ultracold atomic mixtures composed by two different fermionic species. First, I describe a general and simple framework that should allow also a non-expert reader to understand the rich few-body phenomena connected with such systems, and their possible impact at the many-body level. I then move to discuss the specific combination of fermionic lithium (6^66Li) and chromium (53^{53}53Cr) atoms, currently investigated in our lab, highlighting its peculiar properties with respect to other Fermi mixtures nowadays available. Finally, I summarize recent experimental progress achieved in producing and characterizing this novel system, providing an outlook for future studies based on ultracold 6^66Li-53^{53}53Cr Fermi mixtures.