Phase transitions and phase diagrams of K and Cs overlayers on a reconstructed and unreconstructed Cu(110) surface

Abstract

Low-energy electron-diffraction experiments have been carried out to study the phase transitions of K and Cs on a Cu(110) surface and the copper substrate reconstruction under adsorption within the temperature range between 80 and 400 K. The K and Cs adatoms on an 80-K Cu(110) substrate form quasihexagonally (QH) ordered structures when the nearest atomic spacing between the adatoms reaches the value \ensuremath{\simeq}8 A\r{} (coverage \ensuremath{\Theta}\ensuremath{\simeq}0.17). The disordering transition temperature of the QH structures for both K and Cs overlayers increases as coverage increases. The substrate surface reconstruction induced by the K and Cs overlayers starts at T\ensuremath{\simeq}150 K and \ensuremath{\Theta}\ensuremath{\simeq}0.05. The relationships between the QH phases of alkali overlayers and the (1\ifmmode\times\else\texttimes\fi{}3) and (1\ifmmode\times\else\texttimes\fi{}2) phases of the surface reconstruction are described quantitatively in phase diagrams for the K and Cs systems. A continuous phase transition (${T}_{c}$>260 K) from a well-ordered (1\ifmmode\times\else\texttimes\fi{}2)-reconstructed surface to the (1\ifmmode\times\else\texttimes\fi{}3)-reconstructed surface is also described as a function of excess K or Cs deposition (\ensuremath{\Delta}\ensuremath{\Theta}).