Transition from a polaronic condensate to a degenerate Fermi gas of heteronuclear molecules

Abstract

The interplay of quantum statistics and interactions in atomic Bose-Fermi mixtures leads to a phase diagram markedly different from pure fermionic or bosonic systems. However, investigating this phase diagram remains challenging when bosons condense. Here, we observe evidence for a quantum phase transition from a polaronic to a molecular phase in a density-matched degenerate Bose-Fermi mixture. The condensate fraction, representing the order parameter of the transition, is depleted by interactions and the build-up of strong correlations results in the emergence of a molecular Fermi gas. By driving through the transition, we ultimately produce a quantumdegenerate sample of sodium-potassium molecules exhibiting a large molecule-frame dipole moment of 2.7 Debye. The observed phase transition represents a new phenomenon complementary to the paradigmatic BEC-BCS crossover observed in Fermi systems. DEGENERATE BOSE-FERMI MIXTURES IN THE DENSITY-MATCHED REGIMEA simplified phase diagram of Bose-Fermi mixtures is illustrated in Fig. 1a as a function of the ratio of boson to fermion density n B /n F and the dimensionless interaction strength 1/k i a BF . Here, a BF denotes the bosonfermion scattering length and the wave vector k i is deter-

Quantum control of molecules for fundamental physics

“…Both methods utilize fields, either optical or slowly-varying magnetic, to couple atomic scattering states to bound molecular states, in either excited or ground electronic potentials. Since the associated molecules inherit the phase-space density of its atomic constituents, molecular gases created this way hold the record for the highest phase-space densities to date [27][28][29][30][31]. Three decades of progress in laser cooling and trapping of atoms have now enabled single diatomic molecules to be assembled atom-by-atom in the ground motional state of an optical tweezer (Fig.…”

Long-lived fermionic Feshbach molecules with tunable $p$-wave interactions

“…The surprisingly strong suppression of dimer-dimer loss in the non-halo regime suggests that after molecule formation, dimer loss can be strongly reduced by quenching the magnetic field into the regime of large binding energies. This has, for example, been essential in our recent work on the generation of degenerate Fermi gases of NaK molecules [29,30].…”

A High Phase-Space Density Gas of NaCs Feshbach Molecules

“…Typically, numbers have ranged from 1,000 to 20,000 molecules and temperatures from 200 to 1200 nK. Recently, it has been shown that degenerate Fermi gases of KRb [29] and NaK [30] can be directly created through optimized Feshbach association from the initial Bose-Fermi mixture, reaching temperatures of 50 nK and 100 nK, respectively. The equivalent for heteronuclear bosonic molecules, the creation of a molecular BEC, has not been achieved yet.…”