Properties Of Dense Hydrogen Research Articles

Aromaticity, Closed-Shell Effects, and Metallization of Hydrogen

CONSPECTUS: Recent theoretical and experimental studies reveal that compressed molecular hydrogen at 200-350 GPa transforms to layered structures consisting of distorted graphene sheets. The discovery of chemical bonding motifs in these phases that are far from close-packed contrasts with the long-held view that hydrogen should form simple, symmetric, ambient alkali-metal-like structures at these pressures. Chemical bonding considerations indicate that the realization of such unexpected structures can be explained by consideration of simple low-dimensional model systems based on H6 rings and graphene-like monolayers. Both molecular quantum chemistry and solid-state physics approaches show that these model systems exhibit a special stability, associated with the completely filled set of bonding orbitals or valence bands. This closed-shell effect persists in the experimentally observed layered structures where it prevents the energy gap from closing, thus delaying the pressure-induced metallization. Metallization occurs upon further compression by destroying the closed shell electronic structure, which is mainly determined by the 1s electrons via lowering of the bonding bands stemming from the unoccupied atomic 2s and 2p orbitals. Because enhanced diamagnetic susceptibility is a fingerprint of aromaticity, magnetic measurements provide a potentially important tool for further characterization of compressed hydrogen. The results indicate that the properties of dense hydrogen are controlled by chemical bonding forces over a much broader range of conditions than previously considered.

The influence of Pauli blocking effects on the properties of dense hydrogen

We investigate the effects of Pauli blocking on the properties of hydrogen at high pressures, where recent experiments have shown a transition from insulating behavior to metal-like conductivity. Since the Pauli principle prevents multiple occupation of electron states (Pauli blocking), atomic states disintegrate subsequently at high densities (Mott effect). We calculate the energy shifts due to Pauli blocking and discuss the Mott effect solving an effective Schrödinger equation for strongly correlated systems. The ionization equilibrium is treated on the basis of a chemical approach. Results for the ionization equilibrium and the pressure in the region 4000 K <T < 20 000 K are presented. We show that the transition to a highly conducting state is softer than found in earlier work. A first-order phase transition is observed at T < 6450 K, but a diffuse transition appears still up to 20 000 K.

Ab initio study of deuterium in the dissociating regime: sound speed and transport properties.

The sound speed and the transport properties of dense hydrogen (deuterium) are computed from local spin-density approximation molecular-dynamics simulations in the dissociating regime. The sound speed c(s) is evaluated from the thermodynamical differentiation of the equation of state in the molecular phase and is in very good agreement with recent experiments. The diffusion constant D and the viscosity eta are extracted from simulations performed at V=6, 4, and 2.7 cm(3)/mole, corresponding, respectively, for deuterium at rho=0.672, 1.0, and 1.5 g/cm(3) in a range of temperatures 1000 K<T<50,000 K. In the dissociated regime, the diffusion coefficient is well predicted by one-component plasma formulas using a renormalized coupling parameter recently proposed by Murillo [M. S. Murillo, Phys. Rev. B 62, 4115 (2000)]. The behavior of the shear viscosity in the dissociated regime is more complex and exhibits a crossover between atomic and screened plasma formulation. A comparison with recent molecular-dynamics simulations of Yukawas systems shows that the inverse of the screening length must lie between 1 and 2, in nearest-neighbor radius units, as suggested by the results on the diffusion.

Inertial mode oscillations of Jupiter

The properties of the inertial modes of Jupiter are investigated using several different models for that planet. The inertial modes are rotationally induced oscillation modes for which the kinetic energy generally dominates the potential energy of oscillation. The frequency spectrum of the inertial modes mainly depends on the density stratification of Jupiter and sensitively reflects the existence or nonexistence of density discontinuities in the interior. A particularly interesting consequence of the calculations is that observations of the inertial oscillations of Jupiter may enable us to determine whether or not the dense hydrogen in the envelope of this planet undergoes the plasma phase transition (PPT). This may provide an astrophysical test of current theories of the thermodynamic properties of dense hydrogen. It is also found that the discontinuity modes associated with the PPT have observable amplitdues at the surface. These modes may provide an independent test of the interior structure of Jupiter.