15,182 research outputs found
Flavor Asymmetry of the Polarized Light Sea: Models vs. Data
The flavor asymmetry of the polarized light sea, , discriminates between different model calculations of helicity densities.
We show that the chiral chromodielectric model, differently from models based
on a expansion, predicts a small value for this asymmetry, what seems
in agreement with preliminary HERMES data.Comment: 10 pages, 3 figure
Quantum Computational Gates with Radiation Free Couplings
We examine a generic three state mechanism which realizes all fundamental
single and double qubit quantum logic gates operating under the effect of
adiabatically controllable static (radiation free) bias couplings between the
states. At the instant of time that the gate operations are defined the third
level is unoccupied which, in a certain sense, derives analogy with the
recently suggested dissipation free qubit subspaces. The physical
implementation of the mechanism is tentatively suggested in a form of the
Aharonov-Bohm persistent current loop in crossed electric and magnetic fields,
with the output of the loop read out by a (quantum) Hall effect aided
mechanism.Comment: 21 pages including 7 figures, revte
Polarization dependence of the third-harmonic generation in multiband superconductors
In a superconductor the third-harmonic generation (THG) of a strong THz pulse
is enhanced below Tc by the resonant excitation of lattice-modulated charge
fluctuations (LCF), which modulate the response according to the polarization
of the field. Here we compute the THG within a multiband model for the
prototype NbN superconductor. We show that the non-resonant contribution coming
from the instantaneous electronic response and the finite width of the pulse
significantly suppress the polarization dependence of the signal, challenging
its observation in real systems.Comment: Final version, as publishe
Investigation of low temperature quantum crossover in Josephson junctions
The evidence for macroscopic quantum tunneling (MQT) in Josephson junctions
at low temperatures has been reassessed. Swept bias escape distributions have
been modeled with an algorithm-based simulation and the results compared with
data from representative published experiments. Signatures expected of a
crossover to MQT are not found in the analyzed data.Comment: 7 pages, 6 figure
Voltage-induced Shapiro steps in a superconducting multi-terminal structure
When a superconducting tunnel junction at a finite voltage is irradiated with
microwaves, the interplay between the alternating Josephson current and the ac
field gives rise to steps in the dc current known as Shapiro steps. In this
work we predict that in a mesoscopic structure connected to several
superconducting terminals one can induce Shapiro-like steps in the absence of
any external radiation simply by tuning the voltages of the leads. To
illustrate this effect we make quantitative predictions for a three-terminal
structure which comprises a diffusive superconductor-normal
metal-superconductor junction and a tunneling probe, a set-up which can be
realized experimentally.Comment: revtex4, 5 pages, 5 figures, to appear in Phys. Rev.
Low-temperature transport through a quantum dot between two superconductor leads
We consider a quantum dot coupled to two BCS superconductors with same gap
energies . The transport properties are investigated by means of
infinite- noncrossing approximation. In equilibrium density of states, Kondo
effect shows up as two sharp peaks around the gap bounds. Application of a
finite voltage bias leads these peaks to split, leaving suppressed peaks near
the edges of energy gap of each lead. The clearest signatures of the Kondo
effect in transport are three peaks in the nonlinear differential conductance:
one around zero bias, another two at biases . This result is
consistent with recent experiment. We also predict that with decreasing
temperature, the differential conductances at biases anomalously
increase, while the linear conductance descends.Comment: replaced with revised versio
Supercurrent and Andreev bound state dynamics in superconducting quantum point contacts under microwave irradiation
We present here an extensive theoretical analysis of the supercurrent of a
superconducting point contact of arbitrary transparency in the presence of a
microwave field. Our study is mainly based on two different approaches: a
two-level model that describes the dynamics of the Andreev bound states in
these systems and a fully microscopic method based on the Keldysh-Green
function technique. This combination provides both a deep insight into the
physics of irradiated Josephson junctions and quantitative predictions for
arbitrary range of parameters. The main predictions of our analysis are: (i)
for weak fields and low temperatures, the microwaves can induce transitions
between the Andreev states leading to a large suppression of the supercurrent
at certain values of the phase, (ii) at strong fields, the current-phase
relation is strongly distorted and the corresponding critical current does not
follow a simple Bessel-function-like behavior, and (iii) at finite temperature,
the microwave field can enhance the critical current by means of transitions
connecting the continuum of states outside the gap region and the Andreev
states inside the gap. Our study is of relevance for a large variety of
superconducting weak links as well as for the proposals of using the Andreev
bound states of a point contact for quantum computing applications.Comment: 16 pages, 11 figures, submitted to Phys. Rev.
A spectroscopically confirmed z=1.327 galaxy-scale deflector magnifying a z~8 Lyman-Break galaxy in the Brightest of Reionizing Galaxies survey
We present a detailed analysis of an individual case of gravitational lensing
of a Lyman-Break galaxy (LBG) in a blank field, identified in Hubble
Space Telescope imaging obtained as part of the Brightest of Reionizing
Galaxies survey. To investigate the close proximity of the bright
() -dropout to a small group of foreground galaxies, we
obtained deep spectroscopy of the dropout and two foreground galaxies using
VLT/X-Shooter. We detect H-, H-, [OIII] and [OII] emission in
the brightest two foreground galaxies (unresolved at the natural seeing of
arcsec), placing the pair at . We can rule out emission lines
contributing all of the observed broadband flux in band at
, allowing us to exclude the candidate as a low redshift
interloper with broadband photometry dominated by strong emission lines. The
foreground galaxy pair lies at the peak of the luminosity, redshift and
separation distributions for deflectors of strongly lensed objects,
and we make a marginal detection of a demagnified secondary image in the
deepest () filter. We show that the configuration can be accurately
modelled by a singular isothermal ellipsoidal deflector and a S\'{e}rsic source
magnified by a factor of . The reconstructed source in the
best-fitting model is consistent with luminosities and morphologies of
LBGs in the literature. The lens model yields a group mass of
and a stellar mass-to-light ratio for the
brightest deflector galaxy of within its effective radius. The foreground galaxies'
redshifts would make this one of the few strong lensing deflectors discovered
at .Comment: Accepted for publication in MNRAS. 16 pages, 11 figures, 3 table
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