12 research outputs found
A note on the metallization of compressed liquid hydrogen
We examine the molecular-atomic transition in liquid hydrogen as it relates
to metallization. Pair potentials are obtained from first principles molecular
dynamics and compared with potentials derived from quadratic response. The
results provide insight into the nature of covalent bonding under extreme
conditions. Based on this analysis, we construct a schematic
dissociation-metallization phase diagram and suggest experimental approaches
that should significantly reduce the pressures necessary for the realization of
the elusive metallic phase of hydrogen.Comment: 11 pages, 4 figure
Structure and phase boundaries of compressed liquid hydrogen
We have mapped the molecular-atomic transition in liquid hydrogen using first
principles molecular dynamics. We predict that a molecular phase with
short-range orientational order exists at pressures above 100 GPa. The presence
of this ordering and the structure emerging near the dissociation transition
provide an explanation for the sharpness of the molecular-atomic crossover and
the concurrent pressure drop at high pressures. Our findings have non-trivial
implications for simulations of hydrogen; previous equation of state data for
the molecular liquid may require revision. Arguments for the possibility of a
order liquid-liquid transition are discussed
Tetrahedral clustering in molten lithium under pressure
A series of electronic and structural transitions are predicted in molten
lithium from first principles. A new phase with tetrahedral local order
characteristic of bonded materials and poor electrical conductivity is
found at pressures above 150 GPa and temperatures as high as 1000 K. Despite
the lack of covalent bonding, weakly bound tetrahedral clusters with finite
lifetimes are predicted to exist. The stabilization of this phase in lithium
involves a unique mechanism of strong electron localization in interstitial
regions and interactions among core electrons. The calculations provide
evidence for anomalous melting above 20 GPa, with a melting temperature
decreasing below 300 K, and point towards the existence of novel low-symmetry
crystalline phases.Comment: 5 pages, 5 figure
Lattice dynamics of dense lithium.
We report low-frequency high-resolution Raman spectroscopy and ab-initio calculations on dense lithium from 40 to 200 GPa at low temperatures. Our experimental results reveal rich first-order Raman activity in the metallic and semiconducting phases of lithium. The computed Raman frequencies are in excellent agreement with the measurements. Free energy calculations provide a quantitative description and physical explanation of the experimental phase diagram only when vibrational effect are correctly treated. The study underlines the importance of zero-point energy in determining the phase stability of compressed lithium
Electronic energy level alignment at metal-molecule interfaces with a GW approach
Using density functional theory and many-body perturbation theory within a GW
approximation, we calculate the electronic structure of a metal-molecule
interface consisting of benzene diamine (BDA) adsorbed on Au(111). Through
direct comparison with photoemission data, we show that a conventional
GW approach can underestimate the energy of the adsorbed molecular
resonance relative to the Au Fermi level by up to 0.8 eV. The source of this
discrepancy is twofold: a 0.7 eV underestimate of the gas phase ionization
energy (IE), and a 0.2 eV overestimate of the Au work function. Refinements to
self-energy calculations within the GW framework that account for deviations in
both the Au work function and BDA gas-phase IE can result in an interfacial
electronic level alignment in quantitative agreement with experiment
A quantum fluid of metallic hydrogen suggested by first-principles calculations
It is generally assumed that solid hydrogen will transform into a metallic
alkali-like crystal at sufficiently high pressure. However, some theoretical
models have also suggested that compressed hydrogen may form an unusual
two-component (protons and electrons) metallic fluid at low temperature, or
possibly even a zero-temperature liquid ground state. The existence of these
new states of matter is conditional on the presence of a maximum in the melting
temperature versus pressure curve (the 'melt line'). Previous measurements of
the hydrogen melt line up to pressures of 44 GPa have led to controversial
conclusions regarding the existence of this maximum. Here we report ab initio
calculations that establish the melt line up to 200 GPa. We predict that subtle
changes in the intermolecular interactions lead to a decline of the melt line
above 90 GPa. The implication is that as solid molecular hydrogen is
compressed, it transforms into a low-temperature quantum fluid before becoming
a monatomic crystal. The emerging low-temperature phase diagram of hydrogen and
its isotopes bears analogies with the familiar phases of 3He and 4He, the only
known zero-temperature liquids, but the long-range Coulombic interactions and
the large component mass ratio present in hydrogen would ensure dramatically
different propertiesComment: See related paper: cond-mat/041040
Reactivity of Xenon with Ice at Planetary Conditions
International audienceWe report results from high pressure and temperature experiments that provide evidence for the reactivity of xenon with water ice at pressures above 50 GPa and a temperature of 1500 K--conditions that are found in the interiors of Uranus and Neptune. The x-ray data are sufficient to determine a hexagonal lattice with four Xe atoms per unit cell and several possible distributions of O atoms. The measurements are supplemented with ab initio calculations, on the basis of which a crystallographic structure with a Xe4O12H12 primitive cell is proposed. The newly discovered compound is formed in the stability fields of superionic ice and η-O2, and has the same oxygen subnetwork as the latter. Furthermore, it has a weakly metallic character and likely undergoes sublattice melting of the H subsystem. Our findings indicate that Xe is expected to be depleted in the atmospheres of the giant planets as a result of sequestration at depth
Electronic and structural transitions in dense liquid sodium
At ambient conditions, the light alkali metals are free-electron-like crystals with a highly symmetric structure. However, they were found recently to exhibit unexpected complexity under pressure 1-6. It was predicted from theory 1.2 - and later confirmed by experiment 3-5 - that lithium and sodium undergo a sequence of symmetry-breaking transitions, driven by a Peierls mechanism, at high pressures. Measurements of the sodium melting curve 6 have subsequently revealed an unprecedented (and still unexplained) pressure-induced drop in melting temperature from 1,000 K at 30 GPa down to room temperature at 120 GPa. Here we report results from ab initio calculations that explain the unusual melting behaviour in dense sodium. We show that molten sodium undergoes a series of pressure-induced structural and electronic transitions, analogous to those observed in solid sodium but commencing at much lower pressure in the presence of liquid disorder. As pressure is increased, liquid sodium initially evolves by assuming a more compact local structure. However, a transition to a lower-coordinated liquid takes place at a pressure of around 65 GPa, accompanied by a threefold drop in electrical conductivity. This transition is driven by the opening of a pseudogap, at the Fermi level, in the electronic density of states - an effect that has not hitherto been observed in a liquid metal. The lower-coordinated liquid emerges at high temperatures and above the stability region of a close-packed free-electron-like metal. We predict that similar exotic behaviour is possible in other materials as well