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arXiv:1103.4630 · Physical Review A

Strongly interacting isotopic Bose-Fermi mixture immersed in a Fermi sea

Cheng-Hsun Wu, Ibon Santiago, Jee Woo Park, Peyman Ahmadi, Martin W. Zwierlein

Published Jul 13, 2011• 113 citations
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Abstract

We have created a triply quantum-degenerate mixture of bosonic $^{41}\mathrm{K}$ and two fermionic species $^{40}\mathrm{K}$ and $^{6}\mathrm{Li}$. The boson is shown to be an efficient coolant for the two fermions, spurring hopes for the observation of fermionic superfluids with imbalanced masses. We observe multiple heteronuclear Feshbach resonances, in particular a wide $s$-wave resonance for the combination $^{41}\mathrm{K}$-$^{40}\mathrm{K}$, opening up studies of strongly interacting isotopic Bose-Fermi mixtures. For large imbalance in the local densities of different species, we enter the polaronic regime of dressed impurities immersed in a bosonic or fermionic bath.

DOI
10.1103/physreva.84.011601

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The topic of the present lecture notes are two-species quantum mixtures composed of a deeply degenerate Fermi gas and a second component, the latter being fermionic or bosonic. A key ingredient is the possibility to tune the sss-wave interaction between the different species by means of magnetically controlled Feshbach resonances, which allow us to investigate regimes of strong interactions. In two case studies, we review our experiments on mixtures of 6^66Li fermions with fermionic 40^{40}40K or bosonic 41^{41}41K atoms and on mixtures of fermionic 161^{161}161Dy with 40^{40}40K atoms. We cover various topics of fermionic quantum many-body physics, ranging from impurity physics and quasiparticles over phase separation to the formation of ultracold molecules and progress towards novel superfluids.
This thesis describes the construction of a versatile apparatus to study ultracold quan-tum mixtures capable of simultaneously cooling fermionic 6Li and 40K, as well as the bosonic 41K. The main features of the experimental setup are presented, in particular the addition of a new species 23Na, which has enabled the study of the Bose-Fermi mixture 23Na-40K. Three main experimental benchmarks are outlined: first, the production of a Bose-Einstein Condensate of 41K is discussed and an evaluation of its properties as a coolant are analysed. Secondly, the creation of triply degenerate Bose-Fermi-Fermi of 41K-40K-6Li gas is presented. Simultaneous observation of Pauli Pressure and Bose Condensation in the triply degenerate gas is reported. In addition, interspecies Fes-hbach resonances between 41K-40K and 6Li-41K are observed, opening the way to the study of a strongly interacting isotopic Bose-Fermi mixture of 41K-40K, which have similar mass. Thirdly, the creation of a quantum degenerate Bose-Fermi mixture of 23Na-40K is discussed and over thirty Feshbach resonances are identified.
We present details of an experimental platform for studying Fermi polarons in a quantum-gas mixture. The system consists of about a thousand bosonic 133^{133}133Cs impurities immersed in a deeply degenerate Fermi gas (T/TF∼0.2T/T_F \sim 0.2T/TF​∼0.2) of approximately 2×1052\times 10^52×105 6^66Li atoms in a single hyperfine state, with interspecies interactions tunable via a Feshbach resonance. Using optical Raman spectroscopy without relative momentum transfer, we perform injection spectroscopy and thereby create the Fermi polaron. Owing to the large mass imbalance between the two species, the setup provides access to previously unexplored regimes of Fermi polarons.
This thesis presents studies of quantum degenerate atomic gases of fermionic 6Li and bosonic 7Li. Degeneracy is reached by evaporative cooling of 7Li in a strongly confining magnetic trap. Since at low temperatures direct evaporative cooling is not possible for a polarized fermionic gas, 6Li is sympathetically cooled by thermal contact with 7Li. In a first series of experiments both isotopes are trapped in their low-field seeking higher hyperfine states. A Fermi degeneracy of is achieved for 105 fermions. For more than ~300 atoms, the 7Li condensate collapses, due to the attractive interatomic interaction in this state. This limits the degeneracy reached for both species. To overcome this limit, in a second series of experiments 7Li and 6Li atoms are transferred to their low field seeking lower hyperfine states, where the boson-boson interaction is repulsive but weak. The inter-isotope collisions are used to thermalize the mixture. A 7Li Bose-Einstein condensate (BEC) of 104 atoms immersed in a Fermi sea is produced. The BEC is quasi-one-dimensional and the thermal fraction can be negligible. The measured degeneracies are . The temperature is measured using the bosonic thermal fraction, which vanishes at the lowest temperatures, limiting our measurement sensitivity. In a third series of experiments, the bosons are transferred into an optical trap and their internal state is changed to , the lowest energy state. A Feshbach resonance is detected and used to produce a BEC with tunable atomic interactions. When the effective interaction between atoms is tuned to be small and attractive, we observe the formation of a matter-wave bright soliton. Propagation of the soliton without spreading over a macroscopic distance of 1.1~mm is observed.