During the past five years, major progress has been made in the experimental study of solid hydrogen at ultrahigh pressures as a result of developments in diamond-cell technology. Pressures at which ...metallization has been predicted to occur have been reached (250--300 Gigapascals). Detailed studies of the dynamic, structural, and electronic properties of dense hydrogen reveal a system unexpectedly rich in physical phenomena, exhibiting a variety of transitions at ultrahigh pressures. This colloquium explores the study of dense hydrogen as an archetypal problem in condensed-matter physics.
Stochastic resonance on a circle Wiesenfeld, K; Pierson, D; Pantazelou, E ...
Physical review letters,
04/1994, Letnik:
72, Številka:
14
Journal Article
Recenzirano
We describe a new realization of stochastic resonance, applicable to a broad class of systems, based on an underlying excitable dynamics with deterministic reinjection. A simple but general theory of ...such single-trigger'' systems is compared with analog simulations of the Fitzhugh-Nagumo model, as well as experimental data obtained from stimulated sensory neurons in the crayfish.
The atmospheric chemistry of the 2- to 4-ring polycyclic aromatic hydrocarbons (PAH), which exist mainly in the gas phase in the atmosphere, is discussed. The dominant loss process for the gas-phase ...PAH is by reaction with the hydroxyl radical, resulting in calculated lifetimes in the atmosphere of generally less than one day. The hydroxyl (OH) radical-initiated reactions and nitrate ( NO3) radical-initiated reactions often lead to the formation of mutagenic nitro-PAH and other nitropolycyclic aromatic compounds, including nitrodibenzopyranones. These atmospheric reactions have a significant effect on ambient mutagenic activity, indicating that health risk assessments of combustion emissions should include atmospheric transformation products.
The conjecture is verified that the optimum, energy minimizing, magnetic flux for a half-filled band of electrons hopping on a planar, bipartite graph is pi per square plaquette. We require ital only ...that the graph has periodicity in one direction and the result includes the hexagonal lattice (with flux 0 per hexagon) as a special case. The theorem goes beyond previous conjectures in several ways: (1) It does not assume, ital a priori, that all plaquettes have the same flux (as in Hofstadter's model). (2) A Hubbard-type on-site interaction of any sign, as well as certain longer range interactions, can be included. (3) The conclusion holds for positive temperature as well as the ground state. (4) The results hold in ital Dge2 dimensions if there is periodicity in ital Dminus1 directions (e.g., the cubic lattice has the lowest energy if there is flux pi in each square face).
( e , 2 e ) spectroscopy Coplan, M. A.; Moore, J. H.; Doering, J. P.
Reviews of modern physics,
07/1994, Letnik:
66, Številka:
3
Journal Article
Recenzirano
Most of our knowledge of the electronic structure of atoms and molecules is derived from excitation energies and transition probabilities. These observable quantities are related to the electronic ...wave functions by integrals over unmeasured variables. Another observable more directly related to the wave function than energy or transition probability is the single-electron momentum density, the probability that an electron in a well-defined orbital has a given value of momentum. Over the last twenty years a technique has been developed for measuring momentum densities in atoms and molecules. The technique, (ital e,2ital e) spectroscopy, is based on electron-impact ionization with complete determination of the momenta of both incoming and outgoing electrons. The conditions necessary to extract momentum-density information from the ionization experiments are examined and related to general theories of electron scattering. Different experimental arrangements are reviewed and momentum-density results from selected examples are discussed.
X-ray absorption measurements from H-passivated porous Si and from oxidized Si nanocrystals, combined with electron microscopy, ir absorption, alpha recoil, and luminescence emission data, provide a ...consistent structural picture of the species responsible for the visible luminescence observed in these samples. The mass-weighted average structures in por-Si are particles, not wires, with dimensions significantly smaller than previously reported or proposed.
New tests of the universality of free fall Su, Y; Heckel, BR; Adelberger, EG ...
Physical review. D, Particles and fields,
09/1994, Letnik:
50, Številka:
6
Journal Article
Recenzirano
A fivefold tighter limit follows if composition-dependent interactions are restricted to vector forces. Our galactic-source results test the UFF for ordinary matter attracted toward dark matter, ...yielding etasup DM(Be,Cu)=(minus1.3plus minus0.9)times10sup minus3, etasup DM(Be,Al)=(+1.8plus minus1.4)times10sup minus3, and etasup DM(Si/Al,Cu)=(+0.7 plus minus1.0)times10sup minus3. This provides laboratory confirmation of the usual assumption that gravity is the dominant long-range interaction between dark and luminous matter. We also test Weber's claim that solar neutrinos scatter coherently from single crystals with cross sections similar to10sup 23 times larger than the generally accepted value and rule out the existence of such cross sections.
Theoretical models for the action spectrum and the current-voltage characteristics of microporous (colloidal) semiconductor films in photoelectrochemical cells have been derived. The derivation is ...based on the assumptions that the charge carrier transport in the semiconductor occurs via diffusion, and that the diffusion length is constant. Theoretically fitted action spectra and I-V characteristics agree well with experimental results. 16 refs., 3 figs., 1 tab.
This article describes the advances that have been made over the past ten years on the problem of fracton excitations in fractal structures. The relevant systems to this subject are so numerous that ...focus is limited to a specific structure, the percolating network. Recent progress has followed three directions: scaling, numerical simulations, and experiment. In a happy coincidence, large-scale computations, especially those involving array processors, have become possible in recent years. Experimental techniques such as light- and neutron-scattering experiments have also been developed. Together, they form the basis for a review article useful as a guide to understanding these developments and for charting future research directions. In addition, new numerical simulation results for the dynamical properties of diluted antiferromagnets are presented and interpreted in terms of scaling arguments. The authors hope this article will bring the major advances and future issues facing this field into clearer focus, and will stimulate further research on the dynamical properties of random systems.
It is shown that low ital n, pressure-driven, external modes in tokamaks can be fully stabilized by resistive walls when the plasma rotates at some fraction of the sound speed. The stabilization ...depends on toroidal coupling to sound waves and is affected by ion Landau damping. Two-dimensional stability calculations are presented to show the resulting gains in the beta limit.
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