Surface melting of gallium single crystals.

Abstract

Optical properties of the Ga(010), Ga(111), and Ga(112) surfaces have been studied. Single crystals have been grown at UHV conditions and in situ ellipsometric techniques have been applied. On the Ga(112) surface the formation of a quasiliquid layer has been detected at temperatures below the bulk melting point ${\mathit{T}}_{\mathit{m}}$. The optical properties of the layer are close to the ones of the crystal bulk. The thickness of the quasiliquid layer increases with increasing temperature T. The functional character of the growth law of the quasiliquid layer of the Ga(112) surface is in agreement with the prediction of mean-field theory. In the temperature range 0.5${\mathit{T}}_{\mathit{m}}$-T3.5 K the increase in layer thickness can be described by a logarithmic growth law with a correlation length \ensuremath{\xi}=0.8\ifmmode\pm\else\textpm\fi{}0.2 nm. For temperatures 0${\mathit{T}}_{\mathit{m}}$-T0.2 K the increase in layer thickness can be described by a power-law growth with a Hamaker constant W=(4.8\ifmmode\pm\else\textpm\fi{}2.0)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ mJ. There is a crossover from a logarithmic to a power-law growth of the layer in the temperature range 0.2${\mathit{T}}_{\mathit{m}}$-T0.5 K. The crossover thickness is comparable with the correlation length within the disordered or quasiliquid layer: ${\mathit{l}}_{\mathit{c}}^{\mathrm{*}}$=0.7\ifmmode\pm\else\textpm\fi{}0.1 nm. Ga(010) and Ga(111) surfaces are stable against thermal disordering up to ${\mathit{T}}_{\mathit{m}}$. The highest stability has been observed at the Ga(010) surface. On the Ga(111) surface, changes in the refractive index have been detected while cycling the temperature up to the bulk melting temperature, whereas the extinction coefficient remains constant. Close to the melting point no drastical changes in n or k have been observed indicating that no enhanced disordering takes place close to ${\mathit{T}}_{\mathit{m}}$.