Collisions between Tunable Halo Dimers:
Exploring an Elementary Four-Body Process with Identical Bosons

Institut für Experimentalphysik,
University of Innsbruck, and
Austrian Academy of Sciences,
Innsbruck, Austria

Tunable Halo Dimers

The advent of Feshbach association techniques provides unprecedented possibilities to create and study weakly bound two-body states with universal properties. These states are called halo dimers because of the large spatial extend of the wavefunction, far exceeding the range of the interaction potential (see Fig.1). Halo dimers can be found in various areas of physics, such as nuclear physics, molecular physics, and ultracold atomic physics. An early example is the deuteron, and other prominent examples are ground-state 4He2 dimers and 6Li2 Feshbach dimers in the last-bound vibrational level.

In the field of ultracold gases, Feshbach manipulation has allowed to introduce the concept of tunability to halo dimers. Both the size and the binding energy, which are directly connected to the scattering length a via universal scaling laws, can be conveniently controlled by magnetic bias fields. Such tunable halo dimers are a unique probe for universal few-body physics. A binary collision between two halo dimers can be seen as an elementary four-body process !

Cs2 halo dimers show extraordinary stability against collisions

One could expect weakly-bound and large molecules to be extremely fragile against fragmentation or vibrational quenching. In our experiment, we have observed exactly the opposite !
We have demonstrated that trapped halo dimers exhibit an unusual long lifetime. The inelastic collisional rate between tunable halo dimers shows a pronounced scattering length dependence, with a minimum in the loss coefficient as a most striking feature (see Fig.2). This structure is directly connected to the halo nature of the dimers, while non-universal molecules show no structure in their relaxation rate coefficient.

In addition, we have observed a clear decrease of the relaxation rate with decreasing temperature (see Fig.2), roughly following a square-root dependence. This surprising behaviour highlights that more complex processes, which go beyond simple two-body threshold mechanism, are involved and raises the question of the scattering behavior of halo dimers in the zero-temperature limit. We have measured at 40 nK the minimum loss rate coefficient to be about 7*10-12 cm3/s. This small value corresponds to at least an order of magnitude improvement in the stability with respect to other Feshbach molecules !!!

Fig.2: Loss rate coefficient for collisions between halo dimers as a function of the two-body scattering length and the temperature.

Collisions between Tunable Halo Dimers: Exploring an Elementary Four-Body Process with Identical Bosons
F. Ferlaino, S. Knoop, M. Mark, M. Berninger, H. Schöbel, H.-C. Nägerl, and R. Grimm
Phys. Rev. Lett. 101, 023201 (2008)



Preparation of a Pure Molecular Quantum Gas
J. Herbig, T. Kraemer, M. Mark, T. Weber, C. Chin, H.-C. Nägerl, and R. Grimm
Science 301, 1510 (2003)

Spectroscopy of ultracold, trapped cesium Feshbach molecules
M. Mark, F. Ferlaino, S. Knoop, J.G. Danzl, T. Kraemer, C. Chin, H.-C. Nägerl, and R. Grimm
Phys. Rev. A 76, 042514 (2007)



This work has been carried out in the LevT experiment.

Francesca Ferlaino (Lise-Meitner Fellow)
Steven Knoop (Marie Curie Fellow)
Michael Mark
Martin Berninger
Harald Schöbel
Hanns-Christoph Nägerl
Rudi Grimm



The experiment is supported by the Austrian Science Fund (FWF) within No. SFB 15 (project part 16).


last change: 18-07-08 by FF