Melting of two-dimensional solids

Recent theoretical predictions indicate that melting of a two-dimensional solid may be caused by spontaneous creation of dislocations. The theory predicts that melting occurs by a two-step process involving an intermediate phase, called the hexatic phase, in which there is order in the local crystalline axes but not in the positions of atoms. These ideas are being tested by numerical simulations and by experiments on electrons on liquid helium, liquid crystal films, and rare gas layers adsorbed on graphite. Experiments on liquid crystal films indicate that the three-dimensional analog of the hexatic phase exists, and xenon on graphite exhibits a melting transition close to the form predicted.     

Macroscopic separation of dense fluid phase and liquid phase of phosphorus

Structural transformation between a dense molecular fluid and a polymeric liquid of phosphorus that occurred at about 1 gigapascal and 1000 degrees C was investigated by in situ x-ray radiography. When the low-pressure fluid was compressed, dark and round objects appeared in the radiograph. X-ray diffraction measurements confirmed that these objects were the highpressure liquid. The drops grew and eventually filled the sample space. Decompressing caused the reverse process. The macroscopic phase separation supported the existence of a first-order phase transition between two stable disordered phases besides the liquid-gas transition. X-ray absorption measurements revealed that the change in density at the transition corresponds to about 40% of the density of the high-pressure liquid.     

Compression of Ice to 210 Gigapascals: Infrared Evidence for a Symmetric Hydrogen-Bonded Phase

Protonated and deuterated ices (H2O and D2O) compressed to a maximum pressure of 210 gigapascals at 85 to 300 kelvin exhibit a phase transition at 60 gigapascals in H2O ice (70 gigapascals in D2O ice) on the basis of their infrared reflectance spectra determined with synchrotron radiation. The transition is characterized by soft-mode behavior of the nu3 O-H or O-D stretch below the transition, followed by a hardening (positive pressure shift) above it. This behavior is interpreted as the transformation of ice phase VII to a structure with symmetric hydrogen bonds. The spectroscopic features of the phase persisted to the maximum pressures (210 gigapascals) of the measurements, although changes in vibrational mode coupling were observed at 150 to 160 gigapascals.     

Surface tension measurements of surface freezing in liquid normal alkanes

Surface tension measurements reveal surface freezing in liquid n-alkanes. A solid monolayer of molecules is found to exist up to 30 degrees C above the bulk freezing point. This surface phase exists only for carbon numbers 14 n 相似文献   

B1-b2 transition in calcium oxide from shock-wave and diamond-cell experiments

Volume and structural data obtained by shock-wave and diamond-cell techniques demonstrate that calcium oxide transforms from the B1 (sodium chloride type) to the B2 (cesium chloride type) structure at 60 to 70 gigapascals (0.6 to 0.7 megabar) with a volume decrease of 11 percent. The agreement between the shockwave and diamond-cell results independently confirms the ruby-fluorescence pressure scale to about 65 gigapascals. The shock-wave data agree closely with ultrasonic measurements on the B1 phase and also agree satisfactorily with equations of state derived from ab initio calculations. The discovery of this B1-B2 transition is significant in that it allows considerable enrichment of calcium components in the earth's lower mantle, which is consistent with inhomogeneous accretion theories.     

Lambda transition in liquid sulfur

Density measurements on liquid sulfur near the transition at 160 degrees C show a logarithmic singularity and discontinuous density. The transition shows the kinetic behavior of a zero-order reaction. The transition is expected to have a latent heat characteristic of a first-order phase transition but is of the cooperative nature associated with second-order transitions such as the lambda point in liquid helium.     

Divergent nematic susceptibility in an iron arsenide superconductor

Within the Landau paradigm of continuous phase transitions, ordered states of matter are characterized by a broken symmetry. Although the broken symmetry is usually evident, determining the driving force behind the phase transition can be complicated by coupling between distinct order parameters. We show how measurement of the divergent nematic susceptibility of the iron pnictide superconductor Ba(Fe(1-x)Co(x))(2)As(2) distinguishes an electronic nematic phase transition from a simple ferroelastic distortion. These measurements also indicate an electronic nematic quantum phase transition near the composition with optimal superconducting transition temperature.     

Carbon tetrachloride: a new crystalline modification

X-ray and optical studies of single crystals of carbon tetrachloride have established the existence of a previously unreported rhombohedral modification stable between the melting point and the transition temperature. The discovery of this phase emphasizes the need for a careful reinterpretation of available measurements of the thermal properties of solid carbon tetrachloride.     

Gigantic photoresponse in 1/4-filled-band organic salt (EDO-TTF)2PF6

We report that the organic salt (EDO-TTF)2PF6 with 3/4-filled-band (1/4-filled in terms of holes), which forms an organic metal with strong electron and lattice correlation, shows a highly sensitive response to photoexcitation. An ultrafast, photoinduced phase transition from the insulator phase to the metal phase can be induced with very weak excitation intensity at near room temperature. This response makes the material attractive for applications in switching devices with room-temperature operation. The observed photo-induced spectroscopic change shows that this photoinduced phase transition process is caused by the cooperative melting of charge ordering assisted by coherent phonon generation.     

Surface crystallization in a liquid AuSi alloy

X-ray measurements reveal a crystalline monolayer at the surface of the eutectic liquid Au82Si18, at temperatures above the alloy's melting point. Surface-induced atomic layering, the hallmark of liquid metals, is also found below the crystalline monolayer. The layering depth, however, is threefold greater than that of all liquid metals studied to date. The crystallinity of the surface monolayer is notable, considering that AuSi does not form stable bulk crystalline phases at any concentration and temperature and that no crystalline surface phase has been detected thus far in any pure liquid metal or nondilute alloy. These results are discussed in relation to recently suggested models of amorphous alloys.     

Melting of (Mg, Fe)SiO3-Perovskite to 625 Kilobars: Indication of a High Melting Temperature in the Lower Mantle

The melting curves of two compositions of (Mg,Fe) SiO3-perovskite, the likely dominant mineral phase in the lower mantle, have been measured in a C02 laser-heated diamond cell with direct temperature measurements and in situ detection of melting. At 625 kilobars, the melting temperature is 5000 +/- 200 kelvin, independent of composition. Extrapolation to the core-mantle boundary pressure of 1.35 megabar with three different melting relations yields melting temperatures between 7000 and 8500 kelvin. Thus, the temperature at the base of the lower mantle, accepted to lie between 2550 and 2750 kelvin, is only at about one-third of the melting temperature. The large difference between mantle temperature and corresponding melting temperature has several important implications; particularly the temperature sensitivity of the viscosity is reduced thus allowing large lateral temperature variations inferred from seismic tomographic velocity anomalies and systematics found in measured velocity-density functions. Extensive melting of the lower mantle can be ruled out throughout the history of the Earth.     

High-Pressure Iron-Sulfur Compound, Fe3S2, and Melting Relations in the Fe-FeS System

An iron-sulfur compound (Fe3S2) was synthesized at pressures greater than 14 gigapascals in the system Fe-FeS. The formation of Fe3S2 changed the melting relations from a simple binary eutectic system to a binary system with an intermediate compound that melted incongruently. The eutectic temperature in the system at 14 gigapascals was about 400°C lower than that extrapolated from Usselman's data, implying that previous thermal models of Fe-rich planetary cores could overestimate core temperature. If it is found in a meteorite, the Fe3S2 phase could also be used to infer the minimum size of a parent body.     

Atomic-scale visualization of inertial dynamics

The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.     

Experimental Evidence for a New Iron Phase and Implications for Earth's Core

Iron is known to occur in four different crystal structural forms. One of these, the densest form (epsilon phase, hexagonal close-packed) is considered to have formed Earth's core. Theoretical arguments based on available high-temperature and high-pressure iron data indicate the possibility of a fifth less dense iron phase forming the core. Study of iron phase transition conducted between pressures of 20 to 100 gigapascals and 1000 to 2200 Kelvin provides an experimental confirmation of the existence of this new phase. Thee epsilon iron phase transforms to this lower density phase before melting. The new phase may form a large part of Earth's core.     

High-Temperature XAS Study of Fe2SiO4 Liquid: Reduced Coordination of Ferrous Iron

X-ray absorption spectroscopy (XAS) of Fe(2+) in Fe(2)SiO(4) liquid at 1575 kelvin and 10(-4) gigapascal (1 bar) shows that the Fe(2+) -O bond length is 1.98 +/- 0.02 angstroms compared with approximately 2.22 angstroms in crystalline Fe(2)SiO(4) (fayalite) at the melting point (1478 kelvin), which indicates a decrease in average Fe(2+) coordination number from six in fayalite to four in the liquid. Anharmonicity in the liquid was accounted for using a data analysis procedure. This reduction in coordination number is similar to that observed on the melting of certain ionic salts. These results are used to develop a model of the medium-range structural environment of Fe(2+) in olivine-composition melts, which helps explain some of the properties of Fe(2)SiO(4) liquid, including density, viscosity, and the partitioning of iron and nickel between silicate melts and crystalline olivines. Some of the implications of this model for silicate melts in the Earth's crust and mantle are discussed.     

Revealing the superfluid lambda transition in the universal thermodynamics of a unitary Fermi gas

Fermi gases, collections of fermions such as neutrons and electrons, are found throughout nature, from solids to neutron stars. Interacting Fermi gases can form a superfluid or, for charged fermions, a superconductor. We have observed the superfluid phase transition in a strongly interacting Fermi gas by high-precision measurements of the local compressibility, density, and pressure. Our data completely determine the universal thermodynamics of these gases without any fit or external thermometer. The onset of superfluidity is observed in the compressibility, the chemical potential, the entropy, and the heat capacity, which displays a characteristic lambda-like feature at the critical temperature T(c)/T(F) = 0.167(13). The ground-state energy is 3/5ξN E(F) with ξ = 0.376(4). Our measurements provide a benchmark for many-body theories of strongly interacting fermions.     

The pyrite-type high-pressure form of silica

Silica (SiO2) exhibits extensive polymorphism at elevated pressures. X-ray diffraction measurements showed that a high-pressure form with a pyrite-type structure, denser than other known silica phases, is stable above 268 giga-pascals and 1800 kelvin. The silicon coordination number increases from 6 in the alpha-PbO2-type phase to 6+2 in the pyrite-type phase, leading to a large increase in density by about 5% at the phase transition.     

Structural diversity of sodium

Sodium exhibits a pronounced minimum of the melting temperature at approximately 118 gigapascals and 300 kelvin. Using single-crystal high-pressure diffraction techniques, we found that the minimum of the sodium melting curve is associated with a concentration of seven different crystalline phases. Slight changes in pressure and/or temperature induce transitions between numerous structural modifications, several of which are highly complex. The complexity of the phase behavior above 100 gigapascals suggests extraordinary liquid and solid states of sodium at extreme conditions and has implications for other seemingly simple metals.     

Critical-like phenomena associated with liquid-liquid transition in a molecular liquid

Contrary to the conventional wisdom that there is only one unique liquid state for any material, recent evidence suggests that there can be more than two liquid states even for a single-component substance. The transition between these liquid states is called a liquid-liquid phase transition. We report the detailed experimental investigation on the kinetics of the continuous spinodal-decomposition-type transformation of one liquid into another for triphenyl phosphite. From the analysis of the linear regime, we found that the correlation length, xi, of fluctuations of the relevant order parameter diverges as xi = xi(0)[(T(SD) - T)/T(SD)](-nu) (where xi(0) = 60 nm and nu = 0.5) while approaching the spinodal temperature, T(SD). This is an indication of a critical-like anomaly associated with the liquid-liquid transition. We also revealed that the order parameter governing the liquid-liquid transition must be of a nonconserved nature.