Atomes dans monocristaux
Helium is a peculiar element : it has the smallest interactions with other atoms and has the smallest mass, except for hydrogen which however makes molecules. As a result quantum effects are more pronounced in condensed helium than in any other material. The simplest consequence is that the density of liquid helium is three times lower than that of a classical liquid with the same atom-atom interactions. At low temperature, zero point agitation of atoms prevents helium from becoming solid : it must be compressed up to 25 bars to undergo a liquid-solid transition.
We have recently succeeded in putting solid helium in a metaststable state, down to 4 bar below the melting pressure. This has been the PhD work of Fabien Souris, defended in january 2013 and could possibly lead to realizing the so-called Andreev, Lifshitz, Chester (ALC) scenario to supersolidity.
Previously, we have studied the spectroscopic properties of cesium atoms implanted in single crystals of helium. This has been the PhD work of Mathieu Melich, defended in september 2008.