We observe two consecutive heteronuclear Efimov resonances in an ultracold Li-Cs mixture by measuring three-body loss coefficients as a function of magnetic field near a Feshbach resonance. The first resonance is detected at a scattering length of a (0) − = −320(10) a0 corresponding to ∼ 7(∼ 3) times the Li-Cs (Cs-Cs) van der Waals range. The second resonance appears at 5.8(1.0) a (0) − close to the unitarity-limited regime at the sample temperature of 450 nK. Indication of a third resonance is found in the atom loss spectra. The scaling of the resonance positions is close to the predicted universal scaling value of 4.9 for zero temperature. Deviations from universality might be caused by finite-range and temperature effects, as well as magnetic field dependent Cs-Cs interactions.The control of interactions in ultracold atomic systems via magnetically tunable Feshbach resonances opens up new pathways for the investigation of few-and manybody physics [1]. One intriguing example is the access to the universal regime, which is characterized by a magnitude of the scattering length a exceeding all other length scales of the system. In the limit of at least two resonant pairwise interactions, an infinite series of three-body bound-states, the so called Efimov states, exists [2][3][4]. Counterintuitively, these trimers persist even for a < 0, where the two body potential does not support a boundstate. The ratio between two subsequent trimer energies follows a discrete scale invariance with a universal scaling factor of exp(−2π/s 0 ). Here, s 0 only depends on the quantum statistics of the constituent atoms, their mass ratio, and the number of resonant interactions [3, 5]. This scale invariance is also reflected in those values of a where the energy of the bound-states coincides with the threshold of three free atoms for a < 0, resulting in enhanced three-body loss. When the position of the first resonance is given by a − only depends on the characteristic range r 0 of the interatomic van der Waals potential [6][7][8][9][10][11]. The universal scaling factor acquires a value of 22.7 for equal mass constituents and features a drastic reduction in heteronuclear massimbalanced systems of two heavy and one light particle [3, 5], resulting e.g. in a factor of 4.9 for a 6 Li-133 Cs mixture.In ultracold atom experiments, Efimov resonances become evident in the three-body loss coefficient L 3 in the rate equation for atom lossṅ = −L 3 n 3 . Here, n denotes the number density of atoms, and L 3 ∝ C(a)a 4 . The Efimov physics are contained in the dimensionless, logperiodic function C(a). Thus far, Efimov resonances have been studied in several equal mass systems [6,7,[12][13][14][15][16][17][18], where the scaling between different resonances is predicted to follow C(a) = C(22.7a). This large scaling factor demands a level of temperature and magnetic field control which makes the observation of an excited Efimov states highly involved. There had been indication of such an excited state in a three-component Fermi gas of 6 Li atoms ...

We report on the observation of nineteen interspecies Feshbach resonances in an optically trapped ultracold Bose-Fermi mixture of 133 Cs and 6 Li in the two energetically lowest spin states. We assign the resonances to s-and p-wave molecular channels by a coupled-channels calculation, resulting in an accurate determination of LiCs ground state potentials. Fits of the resonance position based on the undressed Asymptotic Bound State model do not provide the same level of accuracy as the coupled-channels calculation. Several broad s-wave resonances provide prospects to create fermionic LiCs molecules with a large dipole moment via Feshbach association followed by stimulated Raman passage. Two of the s-wave resonances overlap with a zero crossing of the Cs scattering length which offers prospects for the investigation of polarons in an ultracold Li-Cs mixture. [4][5][6], which gives access to the study of many-body physics, quantum chemistry and precision measurements [7,8]. With a permanent electric dipole moment of 5.5 Debye [9,10], the largest among all alkali-metal dimers, a system of LiCs molecules in their energetically lowest states [11] is considered to be an excellent candidate for the investigation of dipolar quantum gases [12]. Another application of the precise tunability close to a FR is the study of Efimov trimers [13]. The large mass ratio of m Cs /m Li = 22 results in an advantageous universal scaling factor of 4.88 instead of 22.7 as found for a system of equal masses [14], resulting in excellent conditions for observing a series of several Efimov resonances, which, so far, has not been achieved. Interspecies FR can be also used to control the interaction between an impurity and a Bose-Einstein Condensate (BEC). Such a system can directly be mapped to the Fröhlich polaron Hamiltonian [15][16][17], which describes the interaction of an electron gas with the charged lattice atoms in a crystal. As the excitations in a Bose-Einstein condensate represent the lattice phonons, a FR allows the precise adjustment of the modeled phonon-electron coupling strength α, thus allowing one to explore fundamental solid-state systems.FRs between different alkaline species have been identified for a variety of Bose-Bose [18][19][20][21] Cs were studied by means of thermalization measurements at zero magnetic field [40], however, the tunability of the interspecies scattering properties via tuning the magnetic field has yet remained unexplored. Here, we report on the observation of nineteen interspecies loss features in different spin channels of an optically trapped 6 Li -133 Cs mixture, by scanning a homogeneous magnetic field (Feshbach spectroscopy). The magnetic field positions and widths of the observed FRs are analyzed in a full coupled-channels calculation, allowing a consistent assignment of the resonances.We have realized an all-optical preparation scheme to simultaneously trap an ultracold 6 Li -133 Cs mixture with a magnetic field control up to 1300 G by sequentially transferring Li and Cs atoms into an optic...

We investigate theoretically and experimentally the heteronuclear Efimov scenario for a threebody system that consists of two bosons and one distinguishable particle with positive intraspecies scattering lengths. The three-body parameter at the three-body scattering threshold and the scaling factor between consecutive Efimov resonances are found to be controlled by the scattering length between the two bosons, approximately independent of short-range physics. We observe two excitedstate Efimov resonances in the three-body recombination spectra of an ultracold mixture of fermionic 6 Li and bosonic 133 Cs atoms close to a Li-Cs Feshbach resonance, where the Cs-Cs interaction is positive. Deviation of the obtained scaling factor of 4.0(3) from the universal prediction of 4.9 and the absence of the ground state Efimov resonance shed new light on the interpretation of the universality and the discrete scaling behavior of heteronuclear Efimov physics.

We study the interspecies scattering properties of ultracold Li-Cs mixtures in their two energetically lowest spin channels in the magnetic field range between 800 and 1000 G. Close to two broad Feshbach resonances (FR) we create weakly bound LiCs dimers by radio-frequency association and measure the dependence of their binding energy on the external magnetic field strength. Based on the binding energies and complementary atom loss spectroscopy of three other Li-Cs s-wave FRs we construct precise molecular singlet and triplet electronic ground state potentials using a coupled-channels calculation. We extract the Li-Cs interspecies scattering length as a function of the external field and obtain almost a ten-fold improvement in the precision of the values for the pole positions and widths of the s-wave FRs as compared to our previous work (Pires et al 2014 Phys. Rev. Lett. 112 250404). We discuss implications on the Efimov scenario and the universal geometric scaling for LiCsCs trimers.

We study Efimov resonances via three-body loss in an ultracold two-component gas of fermionic 6 Li and bosonic 133 Cs atoms close to a Feshbach resonance at 843 G, extending results reported previously [Pires et al., Phys. Rev. Lett. 112, 250404 (2014)] to temperatures around 120 nK. The experimental scheme for reaching lower temperatures is based upon compensating the gravityinduced spatial separation of the mass-imbalanced gases with bichromatic optical dipole traps. We observe the first and second excited Li-Cs-Cs Efimov resonance in the magnetic field dependence of the three-body event rate constant, in good agreement with the universal zero-range theory at finite temperature [Petrov and Werner, Phys. Rev. A 92, 022704 (2015)]. Deviations are found for the Efimov ground state, and the inelasticity parameter η is found to be significantly larger than those for single-species systems.

We provide a comprehensive comparison of a coupled channel calculation, the asymptotic bound-state model (ABM), and the multichannel quantum defect theory (MQDT (2013)]. We demonstrate how the accuracy of the ABM can be stepwise improved by including magnetic dipole-dipole interactions and coupling to a nondominant virtual state. We present a MQDT calculation, where magnetic dipole-dipole and second-order spin-orbit interactions are included. A frame transformation formalism is introduced, which allows the assignment of measured FRs with only three parameters. All three models achieve a total rms error of <1 G on the observed FRs. We critically compare the different models in view of the accuracy for the description of FRs and the required input parameters for the calculations.

We report on hyperfine-resolved spectroscopic measurements of the electric-dipole forbidden 5p 3/2 → 8p 1/2 transition in a sample of ultracold 87 Rb atoms. The hyperfine selection rules enable the weak magnetic-dipole (M1) contribution to the transition strength to be distinguished from the much stronger electric-quadrupole (E2) contribution. An upper limit on the M1 transition strength is determined that is about 50 times smaller than an earlier experimental determination. We also calculate the expected value of the M1 matrix element and find that it is less than the upper limit extracted from the experiment.

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