Realization of a two-dimensional ising system: Deuterium physisorbed on krypton-preplated graphite

Volumetric adsorption isotherm, calorimetric, and neutron diffraction measurements were used to characterize the quantum system D{sub 2} coadsorbed on graphite preplated by a monolayer of Kr. From the results obtained by these methods a detailed phase diagram of the complete submonolayer coverage range up to the initial stages of bilayer formation could be constructed. The dominant feature of the phase diagram is a commensurate (1x1)[(1/2)] structure, which was determined by neutron diffraction. Three phase transitions of this phase were studied: The order-disorder transition at the critical point which, according to the influence of the corrugation potential, occurs at a relatively high temperature (T{sub c}=25.68 K), the order-disorder transition at the tricritical point (T{sub TC}=17.88 K), and the commensurate-incommensurate (C-IC) transition at D{sub 2} fillings above 1.2x{radical}(3) monolayers. All these measurements gave convincing evidence that the system D{sub 2}/Kr/graphite can be regarded as realization of a two-dimensional (2D) Ising system. Critical exponents near the phase transitions were determined and good agreement with the theory obtained. Of particular interest was the investigation of the C-IC transition, which for a 2D Ising system could be studied. At this transition, the existence of a reentrant fluid was detected, which squeezes in between the C and IC phases down to at least 1.5 K, which is the lowest temperature to which a liquidlike phase of D{sub 2} has ever been found. This observation confirms the Kosterlitz-Thouless criterion and the theory of the C-IC transition.