Formation of ultracold fermionic NaLi Feshbach molecules

Abstract

We describe the formation of fermionic NaLi Feshbach molecules from an ultracold mixture of bosonic 23 Na and fermionic 6 Li. Precise magnetic field sweeps across a narrow Feshbach resonance at 745 G result in a molecule conversion fraction of 5% for our experimental densities and temperatures, corresponding to a molecule number of 5 × 10 4 . The observed molecular decay lifetime is 1.3 ms after removing free Li and Na atoms from the trap. The preparation and control of ultracold atoms has led to major advances in precision measurements and many-body physics. One current frontier is to extend this to diatomic molecules. Early experiments focused on homonuclear molecules, where highlights included the study of fermion pairs across the BEC-BCS crossover [1]. The preparation of heteronuclear molecules is more challenging because it requires a controlled reaction between two distinct atomic species. However, heteronuclear molecules can have a strong elecric dipole moment, which leads to a range of new scientific directions [2], including precision measurements, such as of the electron electric dipole moment [3], quantum computation mediated by dipolar coupling between molecular qubits [4] or in a hybrid system of molecules coupled to superconducting waveguides [5], many-body physics with anisotropic long-range interactions [6,7], and ultracold chemistry [8].A number of experiments have explored molecule formation in ultracold atoms using photoassociation and Feshbach resonances [2].Due to the lower abundance of fermionic alkali isotopes, only one heteronuclear fermionic molecule 40 K 87 Rb has been produced at ultracold temperatures [9]. Fermionic molecules are appealing due to Pauli suppression of s-wave collisions between identical fermions [10], as well as prospects for preparing fermions with long-range interactions as a model system for electrons with Coulomb interactions [6]. In this paper, we report the formation of a new fermionic heteronuclear molecule 23 Na 6 Li.NaLi has at least three unique features due to its constituents being the two smallest alkali atoms. First, its small reduced mass gives it a large rotational constant, which suppresses inelastic molecule-molecule collisions that occur via coupling between rotational levels [11]. Second, NaLi is reactive in its singlet X 1 Σ + ground state, meaning that the reaction NaLi + NaLi → Na 2 + Li 2 is energetically allowed [12], but with an unusually small predicted rate constant of 10 −13 cm 3 /s that is by far the lowest among all reactive heteronuclear alkali molecules [13] and should allow lifetimes > 1 s even without dipolar suppression [14]. This is related to NaLi having the smallest van der Waals C 6 coefficient of all heteronuclear alkali atom pairs [15], which results in weak scattering by the long-range potential. Finally, this slow collision rate, together with weak spin-orbit coupling in diatomic molecules with small atomic numbers Z of its constituents [16], may allow a long-lived triplet a 3 Σ + ground-state in NaLi. This state has nonzero ...

Fixed-node diffusion Monte Carlo study of the BCS-BEC crossover in a bilayer system of fermionic dipoles

“…(6). A simple choice of the single-particle states ϕ α (r) is provided by the plane waves complying with PBC in the box of size L. The wave vectors k α , α = 1, .…”

“…In the case of fermionic particles, the quest for ultracold polar molecules in their rovibrational ground state is actively being pursued using mixtures of 40 K 87 Rb [3,4], 23 Na 40 K [5], 23 Na 6 Li [6], and 133 Cs 6 Li [7]. The electric dipole moment of these heteronuclear molecules ranges from 0.56 D for 40 K 87 Rb to 5.5 D in the case of 133 Cs 6 Li. Another possibility to realize a dipolar Fermi gas is to bring highly magnetic atoms, with magnetic moments on the order of ten Bohr magnetons, to the regime of quantum degeneracy, as it has been successfully achieved with Dy [8] and Er [9] atoms.…”

Prospects of forming ultracold molecules in2Σstates by magnetoassociation of alkali-metal atoms with Yb

“…It will therefore be important to design a molecule creation experiment with very careful field control. In this context it is worth noting that Zürn et al [64] have recently carried out radio-frequency spectroscopy on Li 2 molecules at fields around 800 G with a field precision of ±1 mG, which is close to 1 part in 10 6 , while Heo et al [18] achieved molecule formation in 6 LiNa, using a resonance 10 mG wide at 745 G, with active feedback stabilization of the current to achieve field noise less than 10 mG [18].…”

“…In the latter, cold atomic clouds are subjected to time-dependent magnetic fields that convert atom pairs into molecules by adiabatic passage across zero-energy Feshbach resonances [9]. Recent years have seen substantial progress in producing ultracold molecules made up of pairs of alkali-metal atoms [10][11][12][13][14][15][16][17][18]. The molecules are left in high vibrational states and are susceptible to collisional trap loss.…”

Condensed phase of Bose-Fermi mixtures with a pairing interaction

“…For vanishing boson density, we prove that the bosonic condensate fraction reduces to the quasiparticle weight Z of the Fermi polaron studied in the context of polarized Fermi gases, unifying in this way two apparently unrelated quantities. Bose-Fermi (BF) mixtures with a tunable pairing interaction between bosons and fermions have been actively investigated in the context of ultracold gases , where the tunability of the BF interaction has been demonstrated and exploited in several experiments [23][24][25][26][27][28][29][30][31][32]. Previous work has shown that, even at zero temperature, a sufficiently strong BF attraction suppresses completely the boson condensate in mixtures where the number of bosons does not exceed the number of fermions [1,12,15].…”