879 research outputs found
The electronic and transport properties of a molecular junction studied by an integrated piecewise thermal equilibrium approach
An integrated piecewise thermal equilibrium approach based on the
first-principles calculation method has been developed to calculate bias
dependent electronic structures and current- and differential
conductance-voltage characteristics of the gold-benzene-1,4-dithiol-gold
molecular junction. The calculated currents and differential conductance have
the same order of magnitude as experimental ones. An electron transfer was
found between the two electrodes when a bias is applied, which renders the two
electrodes to have different local electronic structures. It was also found
that when Au 5d electrons were treated as core electrons the calculated
currents were overestimated, which can be understood as an underestimate of the
Au-S covalent bonding and consequently the contact potential barrier and the
replacement of delocalized Au 5d carriers by more itinerant delocalized Au 6sp
carriers in the electrodes
Extended Dynamical Mean Field Theory Study of the Periodic Anderson Model
We investigate the competition of the Kondo and the RKKY interactions in
heavy fermion systems. We solve a periodic Anderson model using Extended
Dynamical Mean Field Theory (EDMFT) with QMC. We monitor simultaneously the
evolution of the electronic and magnetic properties. As the RKKY coupling
increases the heavy fermion quasiparticle unbinds and a local moment forms. At
a critical RKKY coupling there is an onset of magnetic order. Within EDMFT the
two transitions occur at different points and the disapparence of the magnetism
is not described by a local quantum critical point.Comment: 4 pages, 4 figure
Vortex State of TlBaCuO via Tl NMR at 2 Tesla
We report a Tl NMR study of vortex state for an aligned
polycrystalline sample of an overdoped high- superconductor
TlBaCuO (85 K) with magnetic field 2 T along
the c axis. We observed an imperfect vortex lattice, so-called Bragg glass at
=5 K, coexistence of vortex solid with liquid between 10 and 60 K, and
vortex melting between 65 and 85 K. No evidence for local antiferromagnetic
ordering at vortex cores was found for our sample.Comment: 4 pages with 5 figure
Antiferromagnetic order driven by the molecular orbital order of C in --tetra----(dimethylamino)--ethylene--C
We have studied the ground state of a fullerene--based magnet, the
--phase tetra----(dimethylamino)--ethylene--C
(--TDAE--C), by electron spin resonance (ESR) and magnetic
torque measurements. Below T K, non--paramagnetic field dependent
resonances with a finite excitation gap (1.7 GHz) are observed along the
--axis. Strong enhancement in their intensity as temperature is decreased is
inconsistent with excitation from a singlet state, which had been proposed for
the --phase ground state. Below T, non--quadratic field dependence
of magnetic torque signal is also observed in contrast to quadratic field
dependence in the paramagnetic phase. The angle--dependent torque signals below
T indicate the existence of an anisotropy of the bulk magnetization. From
both experiments, we propose an antiferromagnetic ground state driven by the
cooperative orientational ordering of C in the
--TDAE--C.Comment: 4 pages, 3 figure
Exoplanets or Dynamic Atmospheres? The Radial Velocity and Line Shape Variations of 51 Pegasi and Tau Bootis
Because of our relatively low spectral resolution, we compare our
observations with Gray's line bisector data by fitting observed line profiles
to an expansion in terms of orthogonal (Hermite) functions. To obtain an
accurate comparison, we model the emergent line profiles from rotating and
pulsating stars, taking the instrumental point spread function into account. We
describe this modeling process in detail.
We find no evidence for line profile or strength variations at the radial
velocity period in either 51 Peg or in Tau Boo. For 51 Peg, our upper limit for
line shape variations with 4.23-day periodicity is small enough to exclude with
10 sigma confidence the bisector curvature signal reported by Gray & Hatzes;
the bisector span and relative line depth signals reported by Gray (1997) are
also not seen, but in this case with marginal (2 sigma) confidence. We cannot,
however, exclude pulsations as the source of 51 Peg's radial velocity
variation, because our models imply that line shape variations associated with
pulsations should be much smaller than those computed by Gray & Hatzes; these
smaller signals are below the detection limits both for Gray & Hatzes' data and
for our own.
Tau Boo's large radial velocity amplitude and v*sin(i) make it easier to test
for pulsations in this star. Again we find no evidence for periodic line-shape
changes, at a level that rules out pulsations as the source of the radial
velocity variability. We conclude that the planet hypothesis remains the most
likely explanation for the existing data.Comment: 44 pages, 19 figures, plain TeX, accepted to ApJS (companion to
letter astro-ph/9712279
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