75,571 research outputs found
Microwave Nanotube Transistor Operation at High Bias
We measure the small signal, 1 GHz source-drain dynamical conductance of a
back-gated single-walled carbon nanotube field effect transistor at both low
and high dc bias voltages. At all bias voltages, the intrinsic device dynamical
conductance at 1 GHz is identical to the low frequency dynamical conductance,
consistent with the prediction of a cutoff frequency much higher than 1 GHz.
This work represents a significant step towards a full characterization of a
nanotube transistor for RF and microwave amplifiers.Comment: 3 pages, 4 figure
Transport Properties in the "Strange Metal Phase" of High Tc Cuprates: Spin-Charge Gauge Theory Versus Experiments
The SU(2)xU(1) Chern-Simons spin-charge gauge approach developed earlier to
describe the transport properties of the cuprate superconductors in the
``pseudogap'' regime, in particular, the metal-insulator crossover of the
in-plane resistivity, is generalized to the ``strange metal'' phase at higher
temperature/doping. The short-range antiferromagnetic order and the gauge field
fluctuations, which were the key ingredients in the theory for the pseudogap
phase, also play an important role in the present case. The main difference
between these two phases is caused by the existence of an underlying
statistical -flux lattice for charge carriers in the former case, whereas
the background flux is absent in the latter case. The Fermi surface then
changes from small ``arcs'' in the pseudogap to a rather large closed line in
the strange metal phase. As a consequence the celebrated linear in T dependence
of the in-plane and out-of-plane resistivity is shown explicitly to recover.
The doping concentration and temperature dependence of theoretically calculated
in-plane and out-of-plane resistivity, spin-relaxation rate and AC conductivity
are compared with experimental data, showing good agreement.Comment: 14 pages, 5 .eps figures, submitted to Phys. Rev. B, revised version
submitted on 24 Oc
Enhanced collimated GeV monoenergetic ion acceleration from a shaped foil target irradiated by a circularly polarized laser pulse
Using multi-dimensional particle-in-cell (PIC) simulations we study ion
acceleration from a foil irradiated by a circularly polarized laser pulse at
1022W/cm^2 intensity. When the foil is shaped initially in the transverse
direction to match the laser intensity profile, the center part of the target
can be uniformly accelerated for a longer time compared to a usual flat target.
Target deformation and undesirable plasma heating are effectively suppressed.
The final energy spectrum of the accelerated ion beam is improved dramatically.
Collimated GeV quasi-mono-energetic ion beams carrying as much as 18% of the
laser energy are observed in multi-dimensional simulations. Radiation damping
effects are also checked in the simulations.Comment: 4 pages, 4 figure
Fractional exclusion and braid statistics in one dimension: a study via dimensional reduction of Chern-Simons theory
The relation between braid and exclusion statistics is examined in
one-dimensional systems, within the framework of Chern-Simons statistical
transmutation in gauge invariant form with an appropriate dimensional
reduction. If the matter action is anomalous, as for chiral fermions, a
relation between braid and exclusion statistics can be established explicitly
for both mutual and nonmutual cases. However, if it is not anomalous, the
exclusion statistics of emergent low energy excitations is not necessarily
connected to the braid statistics of the physical charged fields of the system.
Finally, we also discuss the bosonization of one-dimensional anyonic systems
through T-duality.Comment: 19 pages, fix typo
On the Relation of Hard X-ray Peak Flux and Outburst Waiting Time in the Black Hole Transient GX 339-4
Aims. In this work we re-investigated the empirical relation between the hard
X-ray peak flux and the outburst waiting time found previously in the black
hole transient GX 339-4. We tested the relation using the observed hard X-ray
peak flux of the 2007 outburst of GX 339-4, clarified issues about faint
flares, and estimated the lower limit of hard X-ray peak flux for the next
outburst. Methods. We included Swift/BAT data obtained in the past four years.
Together with the CGRO/BATSE and RXTE/HEXTE light curves, the observations used
in this work cover a period of 18 years. Results. The observation of the 2007
outburst confirms the empirical relation discovered before. This strengthens
the apparent link between the mass in the accretion disk and the peak
luminosity of the brightest hard state that the black hole transient can reach.
We also show that faint flares with peak fluxes smaller than about 0.12 crab do
not affect the empirical relation. We predict that the hard X-ray peak flux of
the next outburst should be larger than 0.65 crab, which will make it at least
the second brightest in the hard X-ray since 1991.Comment: 4 pages, 3 figures, accepted by A&
Neutrino emission from a GRB afterglow shock during an inner supernova shock breakout
The observations of a nearby low-luminosity gamma-ray burst (GRB) 060218
associated with supernova SN 2006aj may imply an interesting astronomical
picture where a supernova shock breakout locates behind a relativistic GRB jet.
Based on this picture, we study neutrino emission for early afterglows of GRB
060218-like GRBs, where neutrinos are expected to be produced from photopion
interactions in a GRB blast wave that propagates into a dense wind.
Relativistic protons for the interactions are accelerated by an external shock,
while target photons are basically provided by the incoming thermal emission
from the shock breakout and its inverse-Compton scattered component. Because of
a high estimated event rate of low-luminosity GRBs, we would have more
opportunities to detect afterglow neutrinos from a single nearby GRB event of
this type by IceCube. Such a possible detection could provide evidence for the
picture described above.Comment: 6 pages, 2 figures, accepted for publication in MNRA
Electroproduction of Charmonia off Protons and Nuclei
Elastic virtual photoproduction of charmonia on nucleons is calculated in a
parameter free way with the light-cone dipole formalism and the same input:
factorization in impact parameters, light-cone wave functions for the photons
and the charmonia, and the universal phenomenological dipole cross section
which is fitted to other data. The charmonium wave functions are calculated
with four known realistic potentials, and two models for the dipole cross
section are tested. Very good agreement with data for the cross section of
charmonium electroproduction is found in a wide range of and . Using
the ingredients from those calculations we calculate also exclusive
electroproduction of charmonia off nuclei. Here new effects become important,
(i) color filtering of the pair on its trajectory through nuclear
matter, (ii) dependence on the finite lifetime of the fluctuation
(coherence length) and (iii) gluon shadowing in a nucleus compared to the one
in a nucleon. Total coherent and incoherent cross sections for C, Cu and Pb as
functions of are presented. The results can be tested with future
electron-nucleus colliders or in the peripheral collisions of relativistic
heavy ions.Comment: Talk at 2-nd International Workshop on Hadron Physics, 25-29
September 2002, Coimbra, Portugal. To appear in the Workshop Proceedings
(will be published by the American Institute of Physics
Systematic study of the two band/two gap superconductivity in carbon-substituted MgB2 by point-contact spectroscopy
Point-contact measurements on the carbon-substituted Mg(BC)
filament/powder samples directly reveal a retention of the two superconducting
energy gaps in the whole doping range from to . The
large gap on the -band is decreased in an essentially linear fashion
with increasing the carbon concentrations. The changes in the the small gap
up to 3.8 % C are proportionally smaller and are more difficult
to detect but for the heavily doped sample with and
K both gaps are still present, and significantly reduced, consistent with a
strong essentially linear, reduction of each gap with the transition
temperature.Comment: 5 eps figure
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