Pancake prevents collapse

Instabilities in clouds of attracting matter are a well known phenomenon. In astrophysics, they lead to spectacular effects such as supernovae. But also in a gas, when it is made up of tiny atomic magnets, the magnetic forces lead to instabilities. Such a gas cloud implodes due to the attractive interaction between the magnetic atoms. The research group of Professor Tilman Pfau of the 5th Institute of Physics at the University of Stuttgart have recently measured the complete stability diagram of such a gas of magnets which maps exactly the border between stable and unstable states of the gas. These results have now been published in the latest issue of Nature Physics.

The finding that attracting matter is instable is known to everybody who has ever played with a bunch of magnets: they simply clump together. Even when they are aligned in parallel the same happens - unless the magnets are, at the same time, forced to stay in a plain. Such an arrangement in a pancake-like shape - in contrast to a spherical or cigar-like geometry - is stable. In the case of these spheres or cigars, only an additional repulsive interaction between the atoms that keeps them apart could prevent the whole system from collapsing (clumping together).

Tilman Pfau's group are investigating the properties of so called 'quantum gases' in the framework of their trans-regional collaborative research centre (SFB/TRR21 'Control of quantum correlations in tailored matter - Co.Co.Mat.' ). The gas in which the above experiments were performed consists of chromium atoms that are forced to undergo a phase transition to a so called Bose-Einstein condensate at extremely low temperatures. In this special state of quantum matter, the interactions as well as the shape of the trap that holds the atomic cloud and squeezes it into the desired shape can be controlled very precisely. Already ten years ago Tilman Pfau, together with a group of polish scientists, predicted theoretically the border between stable and unstable conditions. Only now could his group proof experimentally that the gas behaves as had been predicted and that it is indeed stabilised by forcing it into a plain pancake shape.

In their ongoing research project, the group are having a closer look at the dynamics of the collapse itself. Due to its similarity to a supernova, this collapse is sometimes called 'Bose Nova.' During the collapse the researchers expect for certain parameters the occurrence of new states of quantum matter caused by the magnetic interaction. An exactly controlled and triggered collapse could - on the other hand - also be used to deposit precisely focussed chromium atoms on a surface.