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

Abstract

Volumetric adsorption isotherm, calorimetric, and neutron diffraction measurements were used to characterize the quantum system ${\mathrm{D}}_{2}$ 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 $(1\ifmmode\times\else\texttimes\fi{}1)[\frac{1}{2}]$ 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 $({T}_{c}=25.68\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, the order-disorder transition at the tricritical point $({T}_{TC}=17.88\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, and the commensurate-incommensurate (C-IC) transition at ${\mathrm{D}}_{2}$ fillings above $1.2\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ monolayers. All these measurements gave convincing evidence that the system ${\mathrm{D}}_{2}∕\mathrm{Kr}∕\text{graphite}$ 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 $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is the lowest temperature to which a liquidlike phase of ${\mathrm{D}}_{2}$ has ever been found. This observation confirms the Kosterlitz-Thouless criterion and the theory of the C-IC transition.