Cold atoms
Description:
Brownian Motors and Ratchets
Ratchets are systems which create directed motion of particles (e.g. atoms) from periodic potentials giving zero average force. A Brownian Motor is a ratchet where random motion (usually thermal fluctuations) are important for the driving mechanism. This may see to violate the second law of thermodynamics, which we obviously cannot allow. The trick is that the systems are driven out of thermal equilibrium. This kind of motors can be very important for transport at the micro-scale (where ordinary motors based on thermal gradients are hard to realize).
The various mechanisms which give ratchet effects are quite general. One physical system where several different ratchet schemes has been realized is cold atoms in periodic light potentials (optical lattices). Together with Anders Kastberg's group in Umeå, I have investigated a quite interesting such realization, built on two phase-shifted symmetric potentials, and operational in all three dimensions.
Currently I am working together with Paul Halkyard, on ratchet mechanisms in the quantum domain (also using cold atoms). In fact it is not completely trivial to give a precise definition of a ratchet in the quantum domain. We are just now looking at how ratchet effects may occur at special resonant parameters, and how this is related to the symmetries of the system.
Few-body Physics
Efimov states are rather peculiar bound states of three particles, predicted by Vitaly Efimov in 1970. An Efimov state may exist even though two atoms cannot bind together. They also have remarkable scaling properties, completely independent of which kind of particles the Efimov state consists of. If all lengths are re-scaled by a factor 22.7 (a number which can be derived analytically), and energies by 22.7^2=585, then another Efimov state appears. I find this kind universality of the three-body interaction very fascinating. Recently (2006) Rudi Grimm and his group in Innsbruck showed that Efimov states are not just a theorists fiction, when they detected the first such state in ultracold Caesium. Since then, several other groups have followed suite.
Web address:
http://www.atom.physto.se/~jonsell/Research.html
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