Observation of Efimov Resonances in a Mixture with Extreme Mass Imbalance

Abstract

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 ...

Three-body parameter for Efimov states inLi6

“…The scattering length has a pole at resonance, corresponding to a two-body bound state exactly at threshold. Signatures of Efimov states were first observed in an ultracold gas of cesium atoms [7] and have since been found in many other systems, including other bosonic gases [8][9][10][11][12][13], three-component fermionic spin mixtures [14][15][16][17], and mixtures of atomic species [18][19][20][21]. Moreover, extensions of the Efimov scenario to universal states of larger clusters [22][23][24] have been demonstrated in experiments [9,25,26], highlighting the general nature of universal few-body physics.…”

“…They require extremely low temperatures for experimental observation since the recombination peaks are less well defined when the de Broglie wavelengths are shorter than the scattering lengths [7,40,41]. Excited-state resonances have therefore been observed in only a very few experiments carried out with 6 Li [16], with 133 Cs [42], and with mixtures of 6 Li and 133 Cs [20,21]. Quantitative understanding of these resonances requires both very precise knowledge of the two-body scattering properties and an accurate theoretical description of finite-temperature effects.…”

Visualizing the Efimov Correlation in Bose Polarons

“…In cold atom system, one of the most intriguing three-body correlations lies in Efimov physics, which is characterized by an infinite number of trimer states nearby a two-body resonance and following a universal scaling law [1,2]. Efimov physics has been observed in a number of cold atoms experiments, while all of them are at the few-body level [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The manifestation of Efimov physics in the many-body system has yet to be observed.…”

Interspecies thermalization in an ultracold mixture of Cs and Yb in an optical trap

“…Such mixtures have been used to study Efimov physics [13][14][15], probe impurities in Bose gases [16], and entropically cool gases confined in an optical lattice [17]. Pairs of atoms in the mixtures can be combined using magnetically or optically tunable Feshbach resonances to create ultracold molecules [18][19][20][21][22][23][24][25][26].…”