F. Graner

The Surface Tension of B.c.c. 3He Crystals

We have measured the surface tension γ of b.c.c. solid helium-3 from the visual observation of large single crystals, in equilibrium with their liquid phase, between 0.1 and 0.4 K. This first direct measurement gives a much larger result (γ = (0.060 ± 0.011) erg/cm2) than previously estimated from the maximum temperature at which facets were observed on growing crystals (0.10 K). We propose that quantum fluctuations reduce the step energy and broaden the roughening transition, so that dynamic roughening may occur far below the static roughening temperature.

The growth rate of3He crystals

We have measured the relaxation of the shape of3He crystals under the effect of gravity and surface tension, close to the minimum in their melting curve atTmin=0.32 K. A growth rate is deduced, which is found to be at a maximum atTmin when the latent heat is zero. We interpret this maximum value (k=0.18±0.04 sec/m) as the intrinsic mobility of the liquid-solid interface and compare it with existing theories. We also consider on which side the latent heat is released during growth, and how it may cross the liquid-solid interface.

Buckling of a Bidimensional Solid

We observe buckling, without collapsing, of a solid Langmuir film under uniaxial compression, like an usual elastic plate. The preliminary characterization of this instability shows that, at the threshold, surface tension is strictly positive, amplitude is continuous and wavelength is A, - 6 pm. Without adjustable parameter, we interpret these resulta as a second- order transition due to dipolar interactions between amphiphilic molecules.

Structure of bidimensional phospholipidic crystallites on formamide determined by X-ray diffraction

Abstract By using grazing incidence X-ray diffraction, we determined the structure of the solid phase of a C18ue5f8C18 phosphocholine monolayer spread at the air-formamide interface. The chains are organized on an oblique lattice, and their tilt, measured from Bragg rod scans, is about 35°. No structural change is observed when the surface pressure is varied from 1 to 38 mN/m. In the solid phase, the area per molecule is 46 A2, and the compression modulus is large. The phase stiffness is dominated by the interactions between the polar heads and the formamide, more than between the aliphatic chains.

Crystalline surfaces in helium

Abstract The particular properties of helium at low temperature recently allowed important progress in the general understanding of the structure and dynamics of crystal surfaces. Excellent agreement has been found between experimental results in helium 4 and the renormalization group theories of the roughening transition, which predict that it is an infinite order (Kosterlitz-Thouless) phase transition. The dynamic roughening of a crystal surface, i.e. the influence of a finite growth rate, has been particularly clarified. The dominant growth process of smooth faces close to their roughening transition has been shown to be the two-dimensional nucleation of terraces. In both helium 3 and helium 4, it also appeared possible to measure and analyze the three interface coefficients which describe the dissipation at rough crystal surfaces is non-equilibrium situations. The latter studies provide interesting information on the elementary processes (heat and mass flow) which occur in quantum liquids during their crystallization. A general review of all these phenomena is given.

The roughening transition and the growth of helium crystals

Abstract We present a review on the roughening transition and the surface of helium crystals, whose behaviour is well described by the modern or “critical” theory of roughening and whose study brings important information on the microscopic details of crystal growth processes.