113 research outputs found
Influence of electronic correlations on the frequency-dependent hopping transport in Si:P
At low energy scales charge transport in the insulating Si:P is dominated by
activated hopping between the localized donor electron states. Thus,
theoretical models for a disordered system with electron-electron interaction
are appropriate to interpret the electric conductivity spectra. With a newly
developed technique we have measured the complex broadband microwave
conductivity of Si:P from 100 MHz to 5 GHz in a broad range of phosphorus
concentration from 0.56 to 0.95 relative to the critical value
cm corresponding to the metal-insulator
transition driven by doping. At our base temperature of K the samples
are in the zero-phonon regime where they show a super-linear frequency
dependence of the conductivity indicating the influence of the Coulomb gap in
the density of the impurity states. At higher doping , an abrupt drop
in the conductivity power law \sig(\omega)\sim\omega^\alpha is observed. The
dielectric function \eps increases upon doping following a power law in
(). Dynamic response at elevated temperatures has also been
investigated.Comment: 5 pages, 7 figures, conference "Transport in Interacting Disordered
Systems" Marburg, August 6 - 10, 200
Measurements of the Complex Conductivity of NbxSi1-x Alloys on the Insulating Side of the Metal-Insulator Transition
We have conducted temperature and frequency dependent transport measurements
in amorphous Nb_x Si_{1-x} samples in the insulating regime. We find a
temperature dependent dc conductivity consistent with variable range hopping in
a Coulomb glass. The frequency dependent response in the millimeter-wave
frequency range can be described by the expression with the exponent somewhat smaller than one. Our ac
results are not consistent with extant theories for the hopping transport.Comment: 4 pages with 3 figures; published version has a different title from
original (was: "Electrodynamics in a Coulomb glass"
Crystal structure of an epoxysterol: 9α,11α-epoxy-5α-cholest-7-ene-3β,5,6α-triol 3,6-diacetate
CENTORI: a global toroidal electromagnetic two-fluid plasma turbulence code
A new global two-fluid electromagnetic turbulence code, CENTORI, has been
developed for the purpose of studying magnetically-confined fusion plasmas on
energy confinement timescales. This code is used to evolve the combined system
of electron and ion fluid equations and Maxwell equations in toroidal
configurations with axisymmetric equilibria. Uniquely, the equilibrium is
co-evolved with the turbulence, and is thus modified by it. CENTORI is
applicable to tokamaks of arbitrary aspect ratio and high plasma beta. A
predictor-corrector, semi-implicit finite difference scheme is used to compute
the time evolution of fluid quantities and fields. Vector operations and the
evaluation of flux surface averages are speeded up by choosing the Jacobian of
the transformation from laboratory to plasma coordinates to be a function of
the equilibrium poloidal magnetic flux. A subroutine, GRASS, is used to
co-evolve the plasma equilibrium by computing the steady-state solutions of a
diffusion equation with a pseudo-time derivative. The code is written in
Fortran 95 and is efficiently parallelized using Message Passing Interface
(MPI). Illustrative examples of output from simulations of a tearing mode in a
large aspect ratio tokamak plasma and of turbulence in an elongated
conventional aspect ratio tokamak plasma are provided.Comment: 9 figure
No evidence for the localized heating of solar wind protons at intense velocity shear zones
Using measurements from the Wind spacecraft at 1 AU, the heating of protons in the solar wind at locations of intense velocity shear is examined. The 4321 sites of intense shear in fast coronal hole origin plasma are analyzed. The proton temperature, the proton specific entropy, and the proton number density at the locations of the shears are compared with the same quantities in the plasmas adjacent to the shears. A very slight but statistically significant enhancement of the proton temperature is seen at the sites of the shears, but it is accompanied by a larger enhancement of the proton number density at the sites of the shears. Consequently, there is no enhancement of the proton specific entropy at the shear sites, indicating no production of entropy; hence, no evidence for plasma heating is found at the sites of the velocity shears. Since the shearing velocities have appreciable Mach numbers, the authors suggest that there can be a slight adiabatic compression of the plasma at the shear zones. Key Points No proton heating is observed at the sites of intense velocity shear Temperature‐density signatures are consistent with adiabatic compressions The compressions could be associated with the large Mach numbers of the shearsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106821/1/jgra50896.pd
Plasma Dynamics
Contains table of contents for Section 2 and reports on two research projects.Princeton University/National Spherical Torus Experiment Grant S04020G PPPLU.S. Department of Energy Grant DE-FGO2-91-ER-54109National Science Foundation Grant ECS 94-24282Los Alamos National Laboratory Grant No. E29060017
Plasma Dynamics
Contains reports on five research projects.U.S. Air Force - Office of Scientifc Research (Contract AFOSR 84-0026)National Science Foundation (Grant ECS 85-14517)Lawrence Livermore National Laboratory (Subcontract 6264005)National Science Foundation (Grant ECS 85-15032)U.S. Department of Energy (Contract DE-ACO2-78-ET-51013)U.S. Department of Energy (Contract DE-ACO2-ET-51013
Plasma Dynamics
Contains table of contents for Section 2 and reports on three research projects.U.S. Navy - Office of Naval Research Grant N00014-90-J-4130National Science Foundation Contract ATM 94-24282U.S. Department of Energy Contract DE-FG02-91-ER-54109U.S. Department of Energy Tokamak Fusion Test Reactor Contract DE-AC02-78-ET-5101
Plasma Dynamics
Contains table of contents for Section 2 and reports on four research projects.Lawrence Livermore National Laboratory Subcontract 6264005National Science Foundation Grant ECS 84-13173National Science Foundation Grant ECS 85-14517U.S. Air Force - Office of Scientific Research Contract AFOSR 89-0082-AU.S. Army - Harry Diamond Laboratories Contract DAAL02-86-C-0050U.S. Navy - Office of Naval Research Contract N00014-87-K-2001Lawrence Livermore National Laboratory Subcontract B108472National Science Foundation Grant ECS 88-22475U.S. Department of Energy Contract DE-FG02-91-ER-54109National Aeronautics and Space Administration Grant NAGW-2048U.S. Department of Energy Contract DE-AC02-ET-51013U.S. Department of Energy Contract DE-AC02-78-ET-5101
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