ISSN:
1573-7357
Source:
Springer Online Journal Archives 1860-2000
Topics:
Physics
Notes:
Abstract We have studied melting-freezing waves propagating at low temperature (40〈T〈500mK) on vicinal surfaces of hep helium 4 crystals, which are tilted by a small angle ø with respect to c facets. We have first obtained the experimental evidence of a crossover angle øc≈ 2.5 °, where the surface properties change from stepped and anisotropic to rough and Isotropic. This result confirms our previous prediction1 that such a crossover should occur at the small angle where the large step width is comparable to the average distance between steps. It also confirms the hypothesis that crystal surfaces are weakly coupled to the lattice in helium. In the ø→ 0 limit, we observed a clear stepped behaviour: the longitudinal component of the surface stiffness vanishes while the transverse component diverges. A quantitative analysis of these two components allowed us to measure the step energy and the interactions between steps. Good agreement is found with the prediction that step interactions result from the combination of elastic and entropic effects. We also found evidence that helium 3 impurities adsorb on the liquid-solid interface and lower the step energy when ordinary helium 4 (130 ppb of3He) is used instead of an ultrapure sample (0.4ppb). Furthermore, from the damping of the waves, we could study the dynamics of vicinal surfaces, i.e. their mobility as a function of temperature, angle and frequency. Here too, a crossover is observed from stepped to rough behavior. The dynamics is sensitive to the existence of steps up to higher angles than the stiffness. We show that a true stepped behavior is observed only if two conditions are fulfilled: the distance between steps must be much larger than the step width, and also larger than the mean wavelength of thermal phonons. By changing the frequency, we could finally confirm that the surface mobility increases when the phonon mean free path becomes smaller than the wavelength of the melting-freezing waves. We conclude with some suggestions for further theoretical and experimental studies.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00753563
Permalink