Strain effects at solid surfaces near the melting point

We investigate the effects of strain on a crystal surface close to the bulk melting temperature T_m, where surface melting usually sets in. Strain lowers the bulk melting point, so that at a fixed temperature below but close to T_m the thickness of the quasi-liquid film is expected to grow with strain, irrespective of sign. In addition, a strain-induced solid surface free energy increase/decrease takes place, favoring/disfavoring surface melting depending on the sign of strain relative to surface stress. In the latter case one can produce a strain-induced prewetting transition, where for increasing temperature the liquid film suddenly jumps from zero to a finite thickness. This phenomenology is illustrated by a realistic molecular dynamics simulation of strained Al(110).Comment: Acceped for publication on Surface Scienc

AC Driven Jumps Distribution on a Periodic Substrate

A driven Brownian particle (e.g. an adatom on a surface) diffusing on a low-viscosity, periodic substrate may execute multiple jumps. In the presence of an additional periodic drive, the jump lengths and time durations become statistically modulated according to a syncronyzation mechanism reminiscent of asymmetric stochastic resonance. Here, too, bistability plays a key role, but in a dynamical sense, inasmuch as a particle switches between locked and running states.Comment: 4 pages, 4 figures, RevTeX, to be published in Surface Science Letter

Surface melting of methane and methane film on magnesium oxide

Experiments on surface melting of several organic materials have shown contradictory results. We study the Van der Waals interactions between interfaces in surface melting of the bulk CH_4 and interfacial melting of the CH_4 film on the MgO substrate. This analysis is based on the theory of Dzyaloshinskii, Lifshitz, and Pitaevskii for dispersion forces in materials characterized by the frequency dependent dielectric functions. These functions for magnesium oxide and methane are obtained from optical data using an oscillator model of the dielectric response. The results show that a repulsive interaction between the solid-liquid and liquid-vapor interfaces exists for the bulk methane. We also found that the van der Waals forces between two solid-liquid interfaces are attractive for the CH_4 film on the MgO substrate. This implies that the van der Waals forces induce the presence of complete surface melting for the bulk methane and the absence of interfacial melting for CH_4 on the MgO substrate.Comment: 11 pages, 4 ps figure

Temperature dependence of surface-melting-induced faceting of surfaces vicinal to Pb(111)

FWN – Publicaties zonder aanstelling Universiteit Leide

Surface melting of nanoscopic epitaxial films

By introducing finite size surface and interfacial excess quantities, interactions between interfaces are shown to modify the usual surface premelting phenomenon. It is the case of surface melting of a thin solid film s deposited on a planar solid substrate S. More precisely to the usual wetting condition of the solid s by its own melt l, necessary for premelting (wetting factor F<0), is adjoined a new quantity G describing the interactions of the l/s interface with the s/S interface. When G>0 this interface attraction boosts the premelting so that a two stage boosted surface premelting is foreseen: a continuous premelting, up to roughly half the deposited film, is followed by an abrupt first order premelting. When G<0 these interfaces repell each other so that premelting is refrained and the film remains partly solid above the bulk melting point (overheating) what is called astride melting. Elastic stress modifies both types of melting curves. Bulk and surface stresses have to be distinguished.Comment: 65 pages, 16 figure

Premelting of Thin Wires

Recent work has raised considerable interest on the nature of thin metallic wires. We have investigated the melting behavior of thin cylindrical Pb wires with the axis along a (110) direction, using molecular dynamics and a well-tested many-body potential. We find that---in analogy with cluster melting---the melting temperature $T_m (R)$ of a wire with radius $R$ is lower than that of a bulk solid, $T_m^b$, by $T_m (R) = T_m^b -c/R$. Surface melting effects, with formation of a thin skin of highly diffusive atoms at the wire surface, is observed. The diffusivity is lower where the wire surface has a flat, local (111) orientation, and higher at (110) and (100) rounded areas. The possible relevance to recent results on non-rupturing thin necks between an STM tip and a warm surface is addressed.Comment: 10 pages, 4 postscript figures are appended, RevTeX, SISSA Ref. 131/94/CM/S

Crystal-face dependence of surface melting

FWN – Publicaties zonder aanstelling Universiteit Leide

Microscopic View on Short-Range Wetting at the Free Surface of the Binary Metallic Liquid Gallium-Bismuth: An X-ray Reflectivity and Square Gradient Theory Study

We present an x-ray reflectivity study of wetting at the free surface of the binary liquid metal gallium-bismuth (Ga-Bi) in the region where the bulk phase separates into Bi-rich and Ga-rich liquid phases. The measurements reveal the evolution of the microscopic structure of wetting films of the Bi-rich, low-surface-tension phase along different paths in the bulk phase diagram. A balance between the surface potential preferring the Bi-rich phase and the gravitational potential which favors the Ga-rich phase at the surface pins the interface of the two demixed liquid metallic phases close to the free surface. This enables us to resolve it on an Angstrom level and to apply a mean-field, square gradient model extended by thermally activated capillary waves as dominant thermal fluctuations. The sole free parameter of the gradient model, i.e. the so-called influence parameter, $\kappa$, is determined from our measurements. Relying on a calculation of the liquid/liquid interfacial tension that makes it possible to distinguish between intrinsic and capillary wave contributions to the interfacial structure we estimate that fluctuations affect the observed short-range, complete wetting phenomena only marginally. A critical wetting transition that should be sensitive to thermal fluctuations seems to be absent in this binary metallic alloy.Comment: RevTex4, twocolumn, 15 pages, 10 figure