Person:

Silvera, Isaac

X-ray-diffraction studies are described for a new phase of carbon called collapsed fullerite (CF) that was produced by application of high pressure to fullerite (C({60})). At (\sim)20 GPa there is an irreversible transition to a phase that has neither the (111) Bragg peak of diamond nor any of the Bragg peaks associated with the fcc phase of C({60}). The spectrum of CF is flat and featureless in the range of study.

The free surface of thin films of liquid helium adsorbed on a solid substrate has been studied using x-ray reflectivity. The film thickness and interfacial profile are extracted from the angular dependence of measured interference between signals reflected from the liquid-vapor and liquid-substrate interfaces. Polished silicon wafers, chemically cleaned and passivated, were used as substrates. Results are reported for measurements for 4He films 35 to 130 Å thick in the temperature range 0.44 to 1.3 K. The 10%/90% interfacial width for temperature T=0.45 K varies from 5.3±0.5 Å for 36±1.5 Å thick films to 6.5±0.5 Å for 125±1.5 Å thick films. The profile width at zero temperature should not differ significantly from that measured at T=0.45 K. For T=1.22 K, the width is 7.8±1.0 Å.

X-ray specular reflectivity measurements of a saturated film of helium absorbed onto an atomically flat silicon substrate have been made at several temperatures. The thickness of the film has been determined and some preliminary data on the structure of the (^4He) liquid-vapor interface are reported.

Films of isotopic mixtures of helium have been studied using x-ray specular reflectivity techniques. In contrast with superfluid 4He films, x-ray exposure causes a reduction in the thickness of 4He films above the superfluid transition as well as films of pure 3He and 3He/4He mixtures. One proposed model that could account for this effect is a charging model, in which thinning is caused by electrostatic pressure of free charges that accumulate on the helium surface. Unfortunately, this model is not fully consistent with all of the experimental observations. A localized heating model, in which indirect heating of the film causes it to thin would explain the data if there were dissipative film flow in the 3He/4He mixtures at temperatures where the bulk is superfluid. We argue that various published experimental results suggest such an effect. In this model, film thinning data for dilute 3He/4He films indicates dissipation that is linear in 3He content of the film over two orders of magnitude.

The helium liquid-vapor interfacial density profile has been measured with x-ray specular reflectivity. Measurements were performed on thick films of helium adsorbed onto atomically flat silicon substrates. Both the amplitude and the phase of the complex scattering amplitude of the helium-vapor interface were obtained from measured interference between reflections from the helium liquid-vapor interface and the silicon-helium interface. Films whose thickness varied from 15 Å to 220 Å over a range of temperatures from 1.1 K to 3.0 K were studied. At T=1.13 K the film thickness is 215 Å and the interfacial width is 9.2 (\pm) 1 Å. No significant variation was seen in the interfacial widths measured at temperatures between 1.1 K and 1.8 K. Analysis of these measurements indicates that the interface is asymmetric, with the decay of the density into the vapor having the sharper falloff. The zero-K interfacial width extrapolated from the finite-temperature measurements with a quantized capillary-wave theory is 7.6(\pm^{1}_{2}) Å.

The average liquid-vapor density profiles 〈ρ(z)〉 of thick (^4)He films adsorbed onto a silicon substrate were measured using x-ray reflectivity. The results are well represented by a 90%-10% interfacial width of 9.2(\pm)1 Å at 1.13 K which extrapolates to a T=0 K, 90%-10% interfacial width of 7.6(\pm^{1}_{2}) Å. The sensitivity of the measurement to the width, shape, and asymmetry of the density profile is discussed.