Feshbach spectroscopy of aK−Rbatomic mixture

Abstract

We perform extensive magnetic Feshbach spectroscopy of an ultracold mixture of fermionic 40 K and bosonic 87 Rb atoms. The magnetic-field locations of 14 interspecies resonances is used to construct a quantum collision model able to predict accurate collisional parameters for all K-Rb isotopic pairs. In particular we determine the interspecies s-wave singlet and triplet scattering lengths for the 40 K-87 Rb mixture as (−111 ± 5)a0 and (−215 ± 10)a0 respectively. We also predict accurate scattering lengths and position of Feshbach resonances for the other K-Rb isotopic pairs. We discuss the consequences of our results for current and future experiments with ultracold K-Rb mixtures.PACS numbers: 34.50. 32.80.Pj; Quantum degenerate atomic mixtures [1,2,3,4,5,6] are promising for the study of a variety of novel physical phenomena, such as production of quantum gases of polar molecules [7], boson-induced superfluidity of fermions [8], and quantum phases of matter in optical lattices [9] or random potentials [10]. The study of these phenomena requires the use of magnetically tunable Feshbach resonances [11] to control the interaction. A detailed knowledge of resonances and collisional parameters is however necessary to achieve such control. Feshbach resonances have been deeply investigated in homonuclear systems, and recently observed also in some heteronuclear mixtures [12,13,14]. However, accurate Feshbach spectroscopy, has been performed so far only on homonuclear systems [15,16,17]. We report here an extensive experimental study of Feshbach resonances in a 40 K-87 Rb mixture. This is used to construct a quantum collisional model able to predict the relevant parameters for all K-Rb isotopic pairs, including both boson-fermion and boson-boson pairs of great experimental interest.In particular this study allows us to determine univocally the scattering lengths for the 40 K-87 Rb mixture, for which contrasting determinations have been reported. The values we find for the singlet a s and triplet a t scattering lengths are (−111 ± 5)a 0 and (−215 ± 10)a 0 , respectively. We also determine high-accuracy values for inter-isotope triplet and singlet scattering lengths for all other K-Rb pairs. In addition, we determine all relevant parameters of Feshbach resonances in the 40 K-87 Rb mixture and demonstrate the presence of several Feshbach resonances in other K-Rb isotopic pairs. Our findings have important consequences for both ongoing and future experiments with K-Rb mixtures.The apparatus and techniques used have already been presented in detail elsewhere [4] and are only briefly summarized here. We prepare samples of typically 10 5 K fermions and 5×10 5 Rb bosons at ultralow temperatures using two successive phases of laser and evaporative cooling. The mixture is magnetically trapped in the states |F = 9/2, m F = 9/2 for K and |2, 2 for Rb. The atomic sample is then adiabatically transferred to a purely optical trap, created by two off-resonance laser beams, at a wavelength of 830 nm, crossing in the horizontal plane. ...

Heteronuclear quantum gas mixtures

“…This upper limit coincides with the value deduced from the first 40 K- 87 Rb spectroscopy experiment at JILA and the corresponding resonance assignment. The identification of more Feshbach resonances and an upgraded collisional model performed at LENS as well as an improved magnetic field calibration has resulted in a scattering length of −215(10) a 0 [61], and the upper limit reported here is compatible with that value. Since the same argument is valid for any experiment, there is no contradiction between the observed stable particle number combinations and a F B = −215(10) a 0 .…”

“…While the coherence properties of the pure bosonic system (loss of coherence with increasing lattice depth) can be explained in terms of the superfluid to Mott-insulator transition and the associated many-body wavefunctions in the superfluid and the Mottinsulating regime, the nature of the observed shift of the coherence properties towards lower optical lattice depth in the mixture is still a point of intense discussions in the community. Possible scenarios include thermodynamic effects like adiabatic heating when ramping up the optical lattice, disorder-induced localization scenarios or a shift of the quantum critical point of the bosonic superfluid to Mott-insulator transition due to attractive interactions with the fermionic atoms (a FB = −215(10) a 0 [61]). In parallel to this work, similar studies and results have been reported at ETH Zürich [109].…”

“…The former features a scattering length which is independent of B over the magnetic field range studied in the experiment and small (given approximately by the background scattering length) * ; the energy of the latter strongly varies as a function of B across the resonance compared to the undisturbed |9/2, −9/2 energy level. The interaction energy (binding energy) of the Feshbachresonant state |9/2, −9/2 is thus given as the difference of the observed |9/2, −7/2 → * The closest known heteronuclear Feshbach resonances in this state are located at 522 G and 584 G [61] and have theoretical widths of below 1 G. |9/2, −9/2 transition frequency from the undisturbed transition frequency which can be calculated from the Breit-Rabi formula or extracted from a measurement where no bosons causing the collisional shift are present (see inset in fig. 20).…”

Collisional properties of sympathetically cooledK39

“…This amounts to producing a mixed MOT of the two species and then evaporatively cool 87 Rb allowing K to thermalize with it. With this technique it has been possible to reach quantum degeneracy of 40 K [9] and 41 K [10], but no attempts have been made to test this method on 39 K. For this mixture in fact, the inter-species cross section is again more than one order of magnitude smaller than the one of 87 Rb-40 K and 87 Rb-41 K [11,12].…”

“…Therefore the work presented in this paper is a critical step towards the production of a Bose-Einstein Condensate (BEC) of 39 K. Such a system is a promising candidate for the realization of a BEC with interaction tunable around zero, since a broad Feshbach resonance is predicted around 400 G [11,12].…”