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Physical Review Research

Optically trapped Feshbach molecules of fermionic Dy161 and K40

Elisa Soave, Alberto Canali, Zhu-Xiong Ye, Marian Kreyer, Emil Kirilov, Rudolf Grimm

Published Aug 18, 2023• 11 citations
Publisher

Abstract

A pure sample of bosonic Feshbach molecules composed of fermionic Dy-161 and K-40 atoms has been prepared in an optical trap, with a phase-space density of about 0.1. Investigation of the molecular cloud decay in the presence of 1064-nm trapping light points to a one-photon coupling to electronically excited states as the main factor limiting the sample lifetime. This observation, together with a refined understanding of the molecule properties and behavior as a function of the magnetic field, offers crucial insights and experimental progress toward achieving quantum-degenerate samples of DyK molecules and novel superfluids based on mass-imbalanced fermion mixtures.

DOI
10.1103/physrevresearch.5.033117

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A pure sample of bosonic Feshbach molecules composed of fermionic Dy-161 and K-40 atoms has been prepared in an optical trap, with a phase-space density of about 0.1. Investigation of the molecular cloud decay in the presence of 1064-nm trapping light points to a one-photon coupling to electronically excited states as the main factor limiting the sample lifetime. This observation, together with a refined understanding of the molecule properties and behavior as a function of the magnetic field, offers crucial insights and experimental progress toward achieving quantum-degenerate samples of DyK molecules and novel superfluids based on mass-imbalanced fermion mixtures.
We report on the preparation of a pure ultracold sample of bosonic DyK Feshbach molecules, which are composed of the fermionic isotopes 161Dy and 40K. Employing a magnetic sweep across a resonance located near 7.3 G, we produce up to 5000 molecules at a temperature of about 50 nK. For purification from the remaining atoms, we apply a Stern-Gerlach technique based on magnetic levitation of the molecules in a very weak optical dipole trap. With the trapped molecules we finally reach a high phase-space density of about 0.1. We measure the magnetic field dependence of the molecular binding energy and the magnetic moment, refining our knowledge of the resonance parameters. We also demonstrate a peculiar anisotropic expansion effect observed when the molecules are released from the trap and expand freely in the magnetic levitation field. Moreover, we identify an important lifetime limitation that is imposed by the 1064-nm infrared trap light itself and not by inelastic collisions. The light-induced decay rate is found to be proportional to the trap light intensity and the closed-channel fraction of the Feshbach molecule. These observations suggest a one-photon coupling to electronically excited states to limit the lifetime and point to the prospect of loss suppression by optimizing the wavelength of the trapping light. Our results represent important insights and experimental steps on the way to achieve quantum-degenerate samples of DyK molecules and novel superfluids based on mass-imbalanced fermion mixtures.
We study the decay of a dense, ultracold sample of weakly bound DyK dimers stored in an optical dipole trap. Our bosonic dimers are composed of the fermionic isotopes Dy 161 and K 40 , which is of particular interest for experiments related to pairing and superfluidity in fermionic systems with mass imbalance. We have realized dipole traps with near-infrared laser light in four different wavelength regions between 1050 and 2002 nm . We have identified trap-light-induced processes as the overall dominant source of losses, except for wavelengths around 2000 nm , where light-induced losses appeared to be much weaker. In a trap near 1550 nm , we found a plateau of minimal light-induced losses, and by carefully tuning the wavelength we reached conditions where losses from inelastic collisions between the trapped dimers became observable. For very weakly bound dimers close to the center of a magnetically tuned Feshbach resonance, we demonstrate the Pauli suppression of collisional losses by about an order of magnitude.
The authors report the observation of Feshbach resonances in the Fermi-Fermi mixture of ultracold 161{}^{161}161Dy and 40{}^{40}40K. The authors show experimental evidence that Dy-K offers a Feshbach resonance at 7.29 G with properties that could make the study of resonantly interacting mass-imbalanced Fermi-Fermi mixtures possible. In addition, they demonstrate the formation of DyK Feshbach molecules.
Observation of low-field Feshbach resonances between 161^{161}161Dy and 40^{40}40K
We report on the observation of Feshbach resonances at low magnetic field strength (below 10 G) in the Fermi-Fermi mixture of 161^{161}161Dy and 40^{40}40K. We characterize five resonances by measurements of interspecies thermalization rates and molecular binding energies. As a case of particular interest for applications, we consider a resonance near 7.29 G, which combines accurate magnetic tunability and access to the universal regime of interactions with experimental simplicity. We show that lifetimes of a few 100 ms can be achieved for the optically trapped, resonantly interacting mixture. We also demonstrate the hydrodynamic expansion of the mixture in the strongly interacting regime and the formation of DyK Feshbach molecules. Our work opens up new experimental possibilities in view of mass-imbalanced superfluids and related phenomena.
We report on the production of a pure sample of up to 3 x 10(5) optically trapped molecules from a Fermi gas of 6Li atoms. The dimers are formed by three-body recombination near a Feshbach resonance. For purification, a Stern-Gerlach selection technique is used that efficiently removes all trapped atoms from the atom-molecule mixture. The behavior of the purified molecular sample shows a striking dependence on the applied magnetic field. For very weakly bound molecules near the Feshbach resonance, the gas exhibits a remarkable stability with respect to collisional decay.
We report on the realization of a Fermi-Fermi mixture of ultracold atoms that combines mass imbalance, tunability, and collisional stability. In an optically trapped sample of ^{161}Dy and ^{40}K, we identify a broad Feshbach resonance centered at a magnetic field of 217 G. Hydrodynamic expansion profiles in the resonant interaction regime reveal a bimodal behavior resulting from mass imbalance. Lifetime studies on resonance show a suppression of inelastic few-body processes by orders of magnitude, which we interpret as a consequence of the fermionic nature of our system. The resonant mixture opens up intriguing perspectives for studies on novel states of strongly correlated fermions with mass imbalance.