Objective
The surfaces and phase transitions of helium at low temperature have been studied, and optical and cryogenic techniques have been improved. A new dilution refrigerator was built. It works down to 7 mK despite an optical access through 4 sets of large optical windows, and has been used to observe crystallization waves propagating. A charge coupled device (CCD) camera has been designed and contructed, sensitive to the level of 10 photons per square millimetre per second. It has been used to obtain images of growing crystals of cubic helium-3 at 10 mK.
Experimental evidence was obtained showing that flat crystal surfaces are unstable under the application of sufficient uniaxial nonhydrostatic stress (The Grinfeld instability). Partial melting of the helium-4 crystal was observed under stress or strain.
Dendritic growth of helium-3 crystals was studied.
The existence of thermal and zero point vacancies in solid helium-4 wasd clarified. No evidence was found for superfluidity in solid helium-4.
The size and density of growing nuclei in a liquid mixture of helium-3 and helium-4 near the tricritical point were determined from light scattered at the interfaces between the droplets and the background phase. The nucleation data, when expressed in reduced units of time, length and temperature, lay on a universal curve.
The overlap of crystal steps on vicinal surfaces and the dynamic roughness of surfaces were investigated at very low temperature and very small tilt angle. The angular range in which the pure stepped behaviour occurs was found to be smaller than previously thought.
The wetting and annealing of thin solid hydrogen films have been studied. The films wet gold incompletely below the triple point. At temperatures below 2.6 K, hydrogen films were found to be stable on a time scale of hours. Between 2.6 and 2.8 K, rapid changes in film structure were observed, due to the onset of surface diffusion.
The physics of helium shows very clean examples of surfaces or phase transitions, where all thermodynamic parameters can be controlled with great precision. Three main experiments are proposed : - The 3He liquid-solid interface at very low temperature and high magnetic field, which is relevant to the general problems of interacting Fermi systems and crystal growth instabilities.
- The stepped surfaces of 4He crystals, where the structure, roughness, interactions, and mobilities of crystalline steps will be examined.
- The decomposition of solid mixtures of 3He and 4He, which opens interesting questions of nucleation, fluctuations, diffusion, segregation, etc.
For all these experiments, a combination of optical techniques and high performance cryogenics is needed. In most cases, electrical and magnetic techniques have also to be used. An intense collaboration is proposed between the research groups of S. Balibar (Paris), G G. Frossati (Leiden), and P. Leiderer (Mainz-Konstanz), who wish to exchange and improve their knowledge and experimental techniques in low temperature physics, magnetism, optics, crystal growth, and statistical physics.
Fields of science (EuroSciVoc)
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
- natural sciences chemical sciences inorganic chemistry noble gases
- engineering and technology materials engineering crystals
- engineering and technology electrical engineering, electronic engineering, information engineering electronic engineering sensors optical sensors
- natural sciences physical sciences optics
- natural sciences physical sciences theoretical physics particle physics photons
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Coordinator
75231 Paris
France
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