10,152 research outputs found
Sub-barrier fusion of the oxygen isotopes: A more complete picture
We have measured the sub-barrier fusion cross sections for 16,17,18O on 16O and we present the data and the experimental method in detail. The data were analyzed with a one-dimensional potential barrier inversion model, a two-dimensional incoming wave boundary condition model, and a two-dimensional Wentzel-Kramers-Brillouin model. We find that a multidimensional model is necessary to describe the enhanced fusion cross sections in oxygen and the two-dimensional incoming wave boundary condition model suggests that a complete description of the fusion, inelastic, and elastic cross sections is possible
Interpreting forest and grassland biome productivity utilizing nested scales of image resolution and biogeographical analysis
Several hardware, software, and data collection problems encountered were conquered. The Geographic Information System (GIS) data from other systems were converted to ERDAS format for incorporation with the image data. Statistical analysis of the relationship between spectral values and productivity is being pursued. Several project sites, including Jackson, Pope, Boulder, Smokies, and Huntington Forest are evolving as the most intensively studied areas, primarily due to availability of data and time. Progress with data acquisition and quality checking, more details on experimental sites, and brief summarizations of research results and future plans are discussed. Material on personnel, collaborators, facilities, site background, and meetings and publications of the investigators are included
Effects of point defects on the phase diagram of vortex states in high-Tc superconductors in B//c axis
The phase diagram for the vortex states of high- superconductors
with point defects in axis is drawn by large-scale Monte
Carlo simulations. The vortex slush (VS) phase is found between the vortex
glass (VG) and vortex liquid (VL) phases. The first-order transition between
this novel normal phase and the VL phase is characterized by a sharp jump of
the density of dislocations. The first-order transition between the Bragg glass
(BG) and VG or VS phases is also clarified. These two transitions are compared
with the melting transition between the BG and VL phases.Comment: 4 pages, 9 eps figures (included in text), uses revtex.sty, overall
changes with several additional data points, though conclusion is unchange
Proposal for an Experiment to Test a Theory of High Temperature Superconductors
A theory for the phenomena observed in Copper-Oxide based high temperature
superconducting materials derives an elusive time-reversal and rotational
symmetry breaking order parameter for the observed pseudogap phase ending at a
quantum-critical point near the composition for the highest . An
experiment is proposed to observe such a symmetry breaking. It is shown that
Angle-resolved Photoemission yields a current density which is different for
left and right circularly polarized photons. The magnitude of the effect and
its momentum dependence is estimated. Barring the presence of domains of the
predicted phase an asymmetry of about 0.1 is predicted at low temperatures in
moderately underdoped samples.Comment: latex, 2 figure
Moving Wigner Glasses and Smectics: Dynamics of Disordered Wigner Crystals
We examine the dynamics of driven classical Wigner solids interacting with
quenched disorder from charged impurities. For strong disorder, the initial
motion is plastic -- in the form of crossing winding channels. For increasing
drive, the disordered Wigner glass can reorder to a moving Wigner smectic --
with the electrons moving in non-crossing 1D channels. These different dynamic
phases can be related to the conduction noise and I(V) curves. For strong
disorder, we show criticality in the voltage onset just above depinning. We
also obtain the dynamic phase diagram for driven Wigner solids and prove that
there is a finite threshold for transverse sliding, recently found
experimentally.Comment: 4 pages, 4 postscript figure
Validity of the rigid band picture for the t-J model
We present an exact diagonalization study of the doping dependence of the
single particle Green's function in 16, 18 and 20 site clusters of t-J model.
We find evidence for rigid-band behaviour starting from the half-filled case:
upon doping, the topmost states of the quasiparticle band observed in the
photoemisson spectrum at half-filling cross the chemical potential and reappear
as the lowermost states of the inverse photoemission spectrum. Features in the
inverse photoemission spectra which are inconsistent with rigid-band behaviour
are shown to originate from the nontrivial point group symmetry of the ground
state with two holes, which enforces different selection rules than at
half-filling. Deviations from rigid band behaviour which lead to the formation
of the `large Fermi surface' in the momentum distribution occur only at
energies far from the chemical potential. A Luttinger Fermi surface and a
nearest neighbor hopping band do not exist.Comment: Remarks: Revtex file + 7 figures attached as compressed postscript
files Figures can also be obtained by ordinary mail on reques
Dynamic transition in driven vortices across the peak effect in superconductors
We study the zero-temperature dynamic transition from the disordered flow to
an ordered flow state in driven vortices in type-II superconductors. The
transition current is marked by a sharp kink in the
characteristic with a concomitant large increase in the defect concentration.
On increasing magnetic field , the follows the behaviour of the
critical current . Specifically, in the peak effect regime
increases rapidly along with . We also discuss the effect of varying
disorder strength on .Comment: 4 pages, 4 figure
Peak Effect in Superconductors: Absence of Phase Transition and Possibility of Jamming in Vortex Matter
The magnetic field dependence of the critical current for the
vortex phase of a disordered superconductor is studied numerically at zero
temperature. The increases rapidly near the upper critical field
similar to the peak effect (PE) phenomenon observed in many
superconductors. The real space configuration across the PE changes
continuously from a partially ordered domain (polycrystalline) state into an
amorphous state. The topological defect density
with for . There is no evidence of a phase transition in
the vicinity of the PE suggesting that an order-disorder transition is not
essential for the occurrence of the PE phenomenon. An alternative view is
presented wherein the vortex system with high dislocation density undergoes
jamming at the onset of the PE.Comment: 8 pages and 5 figure
Structure and Melting of Two-Species Charged Clusters in a Parabolic Trap
We consider a system of charged particles interacting with an unscreened
Coulomb repulsion in a two-dimensional parabolic confining trap. The static
charge on a portion of the particles is twice as large as the charge on the
remaining particles. The particles separate into a shell structure with those
of greater charge situated farther from the center of the trap. As we vary the
ratio of the number of particles of the two species, we find that for certain
configurations, the symmetry of the arrangement of the inner cluster of
singly-charged particles matches the symmetry of the outer ring of
doubly-charged particles. These matching configurations have a higher melting
temperature and a higher thermal threshold for intershell rotation between the
species than the nonmatching configurations.Comment: 4 pages, 6 postscript figure
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