Observation of a Liquid Phase with an Orthorhombic Orientational Order

Abstract

Using surface x-ray diffraction, we have determined the structure of liquid Bi monolayers on Cu(111) for a range of coverages. By combining diffuse scattering data from the liquid with information from the substrate scattering, the ordering properties of Bi have been fully determined. Even though the substrate is hexagonal, we find that the liquid does not show hexatic order but has an orthorhombic orientational order that occurs in three domains. Simultaneously, Bi has partial solidlike properties, even well above its melting point.

Temperature-dependent structure, elasticity, and entropic stability of Bi phases on Cu{111}

“…2, we conclude that the electron reflectivity is not only a sensitive measure for the density of a dense Bi overlayer phase, but that the density of the overlayer phase is approximately constant until 796 K. To reconcile the total amount of Bi that is deposited with the relative fractions of the two visible phases, and produce a coverage of the √ 3 phase of 0.276 ML at 680 K, the Bi coverage of the overlayer phase was used as a free parameter. The value that yields best agreement is a Bi coverage of the overlayer of 0.465 ± 0.003 ML [8] for the temperature interval from 600 to 796 K. This value is consistent with the coverage of the crystalline overlayer phase of 0.5 ML that was determined for a crystalline overlayer phase at room temperature using surface x-ray diffraction (SXRD) [6,12].…”

“…2 is the electron reflectivity of the overlayer phase as a function of temperature. At temperatures exceeding 480 K, the overlayer phase is in a disordered, liquid state [12]. The electron reflectivity of a randomly distributed phase of particles was demonstrated to provide an estimate of the density of that 075431-2 phase [14][15][16].…”

“…It is a commensurate phase that has to remain in registry with the substrate. The overlayer phase is in a fully incommensurate, liquid state and has freedom to expand or contract [12]. As a consequence, the magnitude of the elastic response of both phases is anticipated to be of a different order of magnitude.…”

“…Any additional Bi that is deposited in excess of 1/3 ML is incorporated in an overlayer phase [6,[9][10][11]. The precise structure of the overlayer phase depends on coverage and temperature and has been described in detail elsewhere [12]. In what follows we will refer to the Cu{111}(…”

“…Bi was vapor deposited from a Knudsen cell. An accurate temperature measurement was achieved by comparing to previous SXRD and LEEM data sets that we have for this system and cross correlating the observed phase transitions and other phenomena [6,12].…”