754 research outputs found
Acoustoelectric pumping through a ballistic point contact in the presence of magnetic fields
The acoustoelectric current, J, induced in a ballistic point contact (PC) by
a surface acoustic wave is calculated in the presence of a perpendicular
magnetic field, B. It is found that the dependence of the current on the Fermi
energy in the terminals is strongly correlated with that of the PC conductance:
J is small at the conductance plateaus, and is large at the steps. Like the
conductance, the acoustoelectric current has the same functional behavior as in
the absence of the field, but with renormalized energy scales, which depend on
the strength of the magnetic field, | B|.Comment: 7 page
Hall Coefficient in an Interacting Electron Gas
The Hall conductivity in a weak homogeneous magnetic field, , is calculated. We have shown that to leading order in
the Hall coefficient is not renormalized by the
electron-electron interaction. Our result explains the experimentally observed
stability of the Hall coefficient in a dilute electron gas not too close to the
metal-insulator transition. We avoid the currently used procedure that
introduces an artificial spatial modulation of the magnetic field. The problem
of the Hall effect is reformulated in a way such that the magnetic flux
associated with the scattering process becomes the central element of the
calculation.Comment: 23 pages, 15 figure
Adjustment of the electric current in pulsar magnetospheres and origin of subpulse modulation
The subpulse modulation of pulsar radio emission goes to prove that the
plasma flow in the open field line tube breaks into isolated narrow streams. I
propose a model which attributes formation of streams to the process of the
electric current adjustment in the magnetosphere. A mismatch between the
magnetospheric current distribution and the current injected by the polar cap
accelerator gives rise to reverse plasma flows in the magnetosphere. The
reverse flow shields the electric field in the polar gap and thus shuts up the
plasma production process. I assume that a circulating system of streams is
formed such that the upward streams are produced in narrow gaps separated by
downward streams. The electric drift is small in this model because the
potential drop in narrow gaps is small. The gaps have to drift because by the
time a downward stream reaches the star surface and shields the electric field,
the corresponding gap has to shift. The transverse size of the streams is
determined by the condition that the potential drop in the gaps is sufficient
for the pair production. This yields the radius of the stream roughly 10% of
the polar cap radius, which makes it possible to fit in the observed
morphological features such as the "carousel" with 10-20 subbeams and the
system of the core - two nested cone beams.Comment: 8 pages, 1 figur
How backscattering off a point impurity can enhance the current and make the conductance greater than e^2/h per channel
It is well known that while forward scattering has no effect on the
conductance of one-dimensional systems, backscattering off a static impurity
suppresses the current. We study the effect of a time-dependent point impurity
on the conductance of a one-channel quantum wire. At strong repulsive
interaction (Luttinger liquid parameter g<1/2), backscattering renders the
linear conductance greater than its value e^2/h in the absence of the impurity.
A possible experimental realization of our model is a constricted quantum wire
or a constricted Hall bar at fractional filling factors nu=1/(2n+1) with a
time-dependent voltage at the constriction.Comment: 7 pages, 2 figure
Quantized adiabatic charge pumping and resonant transmission
Adiabatically pumped charge, carried by non-interacting electrons through a
quantum dot in a turnstile geometry, is studied as function of the strength of
the two modulating potentials (related to the conductances of the two
point-contacts to the leads) and of the phase shift between them. It is shown
that the magnitude and sign of the pumped charge are determined by the relative
position and orientation of the closed contour traversed by the system in the
parameter plane, and the transmission peaks (or resonances) in that plane.
Integer values (in units of the electronic charge ) of the pumped charge
(per modulation period) are achieved when a transmission peak falls inside the
pumping contour. The integer value is given by the winding number of the
pumping contour: double winding in the same direction gives a charge of 2,
while winding around two opposite branches of the transmission peaks or winding
in opposite directions can give a charge close to zero.Comment: 7 pages, 12 figure
Theoretical overview on high-energy emission in microquasars
Microquasar (MQ) jets are sites of particle acceleration and synchrotron
emission. Such synchrotron radiation has been detected coming from jet regions
of different spatial scales, which for the instruments at work nowadays appear
as compact radio cores, slightly resolved radio jets, or (very) extended
structures. Because of the presence of relativistic particles and dense photon,
magnetic and matter fields, these outflows are also the best candidates to
generate the very high-energy (VHE) gamma-rays detected coming from two of
these objects, LS 5039 and LS I +61 303, and may be contributing significantly
to the X-rays emitted from the MQ core. In addition, beside electromagnetic
radiation, jets at different scales are producing some amount of leptonic and
hadronic cosmic rays (CR), and evidences of neutrino production in these
objects may be eventually found. In this work, we review on the different
physical processes that may be at work in or related to MQ jets. The jet
regions capable to produce significant amounts of emission at different
wavelengths have been reduced to the jet base, the jet at scales of the order
of the size of the system orbital semi-major axis, the jet middle scales (the
resolved radio jets), and the jet termination point. The surroundings of the
jet could be sites of multiwavelegnth emission as well, deserving also an
insight. We focus on those scenarios, either hadronic or leptonic, in which it
seems more plausible to generate both photons from radio to VHE and high-energy
neutrinos. We briefly comment as well on the relevance of MQ as possible
contributors to the galactic CR in the GeV-PeV range.Comment: Astrophysics & Space Science, in press (invited talk in the
conference: The multimessenger approach to the high-energy gamma-ray
sources", Barcelona/Catalonia, in July 4-7); 10 pages, 6 figures, 2 tables
(one reference corrected
Gravitational radiation from gamma-ray bursts as observational opportunities for LIGO and VIRGO
Gamma-ray bursts are believed to originate in core-collapse of massive stars.
This produces an active nucleus containing a rapidly rotating Kerr black hole
surrounded by a uniformly magnetized torus represented by two counter-oriented
current rings. We quantify black hole spin-interactions with the torus and
charged particles along open magnetic flux-tubes subtended by the event
horizon. A major output of Egw=4e53 erg is radiated in gravitational waves of
frequency fgw=500 Hz by a quadrupole mass-moment in the torus. Consistent with
GRB-SNe, we find (i) Ts=90s (tens of s, Kouveliotou et al. 1993), (ii)
aspherical SNe of kinetic energy Esn=2e51 erg (2e51 erg in SN1998bw, Hoeflich
et al. 1999) and (iii) GRB-energies Egamma=2e50 erg (3e50erg in Frail et al.
2001). GRB-SNe occur perhaps about once a year within D=100Mpc. Correlating
LIGO/Virgo detectors enables searches for nearby events and their spectral
closure density 6e-9 around 250Hz in the stochastic background radiation in
gravitational waves. At current sensitivity, LIGO-Hanford may place an upper
bound around 150MSolar in GRB030329. Detection of Egw thus provides a method
for identifying Kerr black holes by calorimetry.Comment: to appear in PRD, 49
Quantum Pumping and Quantized Magnetoresistance in a Hall Bar
We show how a dc current can be generated in a Hall bar without applying a
bias voltage. The Hall resistance that corresponds to this pumped current
is quantized, just as in the usual integer quantum Hall effect (IQHE). In
contrast with the IQHE, however, the longitudinal resistance does not
vanish on the plateaus, but equals the Hall resistance. We propose an
experimental geometry to measure the pumped current and verify the predicted
behavior of and .Comment: RevTeX, 3 figure
Nearby quasar remnants and ultra-high energy cosmic rays
As recently suggested, nearby quasar remnants are plausible sites of
black-hole based compact dynamos that could be capable of accelerating
ultra-high energy cosmic rays (UHECRs). In such a model, UHECRs would originate
at the nuclei of nearby dead quasars, those in which the putative underlying
supermassive black holes are suitably spun-up. Based on galactic optical
luminosity, morphological type, and redshift, we have compiled a small sample
of nearby objects selected to be highly luminous, bulge-dominated galaxies,
likely quasar remnants. The sky coordinates of these galaxies were then
correlated with the arrival directions of cosmic rays detected at energies EeV. An apparently significant correlation appears in our data. This
correlation appears at closer angular scales than those expected when taking
into account the deflection caused by typically assumed IGM or galactic
magnetic fields over a charged particle trajectory. Possible scenarios
producing this effect are discussed, as is the astrophysics of the quasar
remnant candidates. We suggest that quasar remnants be also taken into account
in the forthcoming detailed search for correlations using data from the Auger
Observatory.Comment: 2 figures, 4 tables, 11 pages. Final version to appear in Physical
Review
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