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Universal Loss Processes in Bosonic Atoms with Positive Scattering Lengths

Christian B. Langmack

Published Jan 1, 2013• 1 citation
OpenAlex

Abstract

In experiments with trapped ultracold gases, atoms can be lost through inelastic scattering processes.If the atoms have a scattering length that is much larger than the range of their interactions, then the system exhibits universal behavior that does not depend on details of their interactions.Dramatic enhancements in the loss rate are observed at special negative values of the scattering length for which there is a universal molecule at threshold.In some experiments, enhancements of the loss rate have also been observed at other positive values of the scattering length.

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In experiments with trapped ultracold gases, atoms can be lost through inelastic scattering processes.If the atoms have a scattering length that is much larger than the range of their interactions, then the system exhibits universal behavior that does not depend on details of their interactions.Dramatic enhancements in the loss rate are observed at special negative values of the scattering length for which there is a universal molecule at threshold.In some experiments, enhancements of the loss rate have also been observed at other positive values of the scattering length.
Atom loss resonances in ultracold trapped atoms have been observed at scattering lengths near atom-dimer resonances, at which Efimov trimers cross the atom-dimer threshold, and near two-dimer resonances, at which universal tetramers cross the dimer-dimer threshold. We propose a new mechanism for these loss resonances in a Bose-Einstein condensate of atoms. As the scattering length is ramped to the large final value at which the atom loss rate is measured, the time-dependent scattering length generates a small condensate of shallow dimers coherently from the atom condensate. The coexisting atom and dimer condensates can be described by a low-energy effective field theory with universal coefficients that are determined by matching exact results from few-body physics. The classical field equations for the atom and dimer condensates predict narrow enhancements in the atom loss rate near atom-dimer resonances and near two-dimer resonances due to inelastic dimer collisions.
Physical systems in reduced dimensions exhibit intriguing properties. For instance, the dependences of two-body and many-body physics on scattering lengths are distinct from their counterparts in three dimensions. Whereas many studies of ultracold atoms and molecules in reduced dimensions have been focusing on closed systems, two-body losses may occur in such systems. Here, we show that the two-body inelastic loss rate in reduced dimensions can be expressed in universal relations that are governed by contacts. These universal relations correlate the two-body decay rate with other physical observables at zero and finite temperatures and generic interaction strengths. Our results will provide experimentalists with a new protocol to study inelastic scatterings in both few- and many-body systems in reduced dimensions.
Recent measurements of Efimov resonances for a number of ultracold atom species have revealed an unexpected universality, in which three-body scattering properties are determined by the van der Waals length of the two-body interaction potential. To investigate whether this universality extends to heteronuclear mixtures, we measure loss rate coefficients in an ultracold trapped gas of 40K and 87Rb atoms. We find an Efimov-like resonance in the rate of inelastic collisions between 40K87Rb Feshbach molecules and 87Rb atoms. However, we do not observe any Efimov-related resonances in the rates of inelastic collisions between three atoms. These observations are compared to previous measurements by the LENS group of Efimov resonances in a 41K and 87Rb mixture as well as to recent predictions.
Ultracold quantum gases with large scattering length show resonant enhancement of three-body loss rates when an Efimov trimer is at the scattering threshold. We calculate the three-body loss rates in heteronuclear mixtures of atoms for the case of large scattering length between the unlike atoms. Using zero-range interactions, we present results from the numerical solution of the integral equations for the recombination amplitude in momentum space and extract expressions for the recombination rate constants. Moreover, we calculate the relative positions of loss features for different sign of the scattering length and a first comparison with available experimental data is shown.
Atoms with a large scattering length have universal low-energy properties that do not depend on the details of their structure or their interactions at short distances. In the 2-atom sector, the universal properties are familiar and depend only on the scattering length. In the 3-atom sector for identical bosons, the universal properties include the existence of a sequence of shallow triatomic molecules called Efimov trimers and log-periodic dependence of scattering observables on the energy and the scattering length. In this review, we summarize the universal results that are currently known. We also summarize the experimental information that is currently available with an emphasis on 3-atom loss processes.
We study the collisional properties of an ultracold mixture of cesium atoms and dimers close to a Feshbach resonance near 550 G in the regime of positive sss-wave scattering lengths. We observe an atom-dimer loss resonance that is related to Efimov's scenario of trimer states. The resonance is found at a value of the scattering length that is different from a previous observation at low magnetic fields. This indicates nonuniversal behavior of the Efimov spectrum for positive scattering lengths. We compare our observations with predictions from effective field theory and with a recent model based on the van der Waals interaction. We present additional measurements on pure atomic samples in order to check for the presence of a resonant loss feature related to an avalanche effect, as suggested by observations in other atomic species. We could not confirm the presence of such a feature.