W. F. Saam

Prediction of reentrant wetting of3He-4He mixtures on cesium

We examine the effect of3He impurities on the wetting behavior of4He on cesium, predicting a phase diagram which includes reentrant wetting transitions. This phase diagram is shown to be very sensitive to effects such as a theoretically predicted bound state of3He at the liquid-cesium interface, and the contact angle may be sensitive to interesting temperature dependences of the helium-cesium surface tension resulting from surface rotons or Rayleigh waves.

Phase transitions in multilayer films

Abstract Lattice gas models are constructed for multilayer physisorption and solved using Migdal-Kadanoff renormalization group transformations. The Ising lattice gas and solid-on-solid models as well as a more realistic case modeling Kr on graphite are treated. Wetting, roughening, and second-order solid-gas transitions are found and discussed. The effects of further-neighbor interactions in adsorbates are analyzed at zero temperature and are found to play a crucial role in certain situations. Contact is made with experiment, and new experiments are proposed.

Helium wetting and prewetting phenomena at finite temperatures

Helium wetting and prewetting phenomena at temperatures T>0 are addressed. Emphasis is given to the weak-binding substrates Cs and H2, which have been predicted and observed to be nonwet and wet, respectively, by4He at low temperatures. Calculations of the wetting temperature TW and the prewetting line near TW for Cs, as well as the prewetting line near T=0 for H2, are given. Predictions concerning the interplay between superfluid onset and prewetting are made, and prewetting critical temperatures are estimated.

BUNCHING TRANSITIONS ON VICINAL SURFACES AND QUANTUM N-MERS

We study vicinal crystal surfaces with the terrace-step-kink model on a discrete lattice. Including both a short-ranged attractive interaction and a long-ranged repulsive interaction arising from elastic forces, we discover a series of phases in which steps coalesce into bunches of n steps each. The value of n varies with temperature and the ratio of short to long range interaction strengths. We propose that the bunch phases have been observed in very recent experiments on Si surfaces. Within the context of a mapping of the model to a system of bosons on a 1D lattice, the bunch phases appear as quantum n-mers.

WETTING TEMPERATURE SHIFT OF HELIUM ON A LAYERED SUBSTRATE

We calculate the shift in wetting temperature TW of a He film on a layered substrate. The latter consists of an alkali metal layer film of thickness d, deposited on a semi-infinite medium. TW can change from nonzero to zero (Cs) or vice versa (Na) as d changes. The shift is an extremely sensitive probe of long range van der Waals potentials.

Step-bunching transitions on vicinal surfaces with attractive step interactions

Abstract We study vicinal crystal surfaces within the terrace–step–kink model on a discrete lattice. Including both a short-ranged attractive interaction and a long-ranged repulsive interaction arising from elastic forces, we discover a series of phases in which steps coalesce into bunches of n b steps each. The value of n b varies with temperature and the ratio of short- to long-range interaction strengths. For bunches with large number of steps, we show that, at T =0, our bunch phases correspond to the well-known periodic groove structure first predicted by Marchenko. An extension to T >0 is developed. We propose that the bunch phases have been observed in very recent experiments on Si surfaces, and we advance a conjecture explaining the exponent β ≈0.5 describing the shape of the observed phase transition curves. Further experiments are suggested. Within the context of a mapping of the model to a system of bosons on a 1D lattice, the bunch phases appear as quantum n -mers.We study vicinal crystal surfaces within the terrace-step-kink model on a discrete lattice. Including both a short-ranged attractive interaction and a long-ranged repulsive interaction arising from elastic forces, we discover a series of phases in which steps coalesce into bunches of

Weak adsorption and prewetting transitions

n_b

Theoretical study of third sound in a layered superfluid film

steps each. The value of

Wetting of3He-4He mixtures on alkali metal substrates

n_b

Wetting Phenomena in 3He-4He Mixtures on Weak and Superweak Substrates

varies with temperature and the ratio of short to long range interaction strengths. For bunches with large number of steps, we show that, at T=0, our bunch phases correspond to the well known periodic groove structure first predicted by Marchenko. An extension to