7,466 research outputs found
Tight-binding study of bilayer graphene Josephson junctions
Using highly efficient simulations of the tight-binding Bogoliubov-de Gennes
model we solved self-consistently for the pair correlation and the Josephson
current in a Superconducting-Bilayer graphene-Superconducting Josephson
junction. Different doping levels for the non-superconducting link are
considered in the short and long junction regime. Self-consistent results for
the pair correlation and superconducting current resemble those reported
previously for single layer graphene except in the Dirac point where remarkable
differences in the proximity effect are found as well as a suppression of the
superconducting current in long junction regime. Inversion symmetry is broken
by considering a potential difference between the layers and we found that the
supercurrent can be switched if junction length is larger than the Fermi
length
Integration of Langevin Equations with Multiplicative Noise and Viability of Field Theories for Absorbing Phase Transitions
Efficient and accurate integration of stochastic (partial) differential
equations with multiplicative noise can be obtained through a split-step
scheme, which separates the integration of the deterministic part from that of
the stochastic part, the latter being performed by sampling exactly the
solution of the associated Fokker-Planck equation. We demonstrate the
computational power of this method by applying it to most absorbing phase
transitions for which Langevin equations have been proposed. This provides
precise estimates of the associated scaling exponents, clarifying the
classification of these nonequilibrium problems, and confirms or refutes some
existing theories.Comment: 4 pages. 4 figures. RevTex. Slightly changed versio
Limb imaging of the Venus O2 visible nightglow with the Venus Monitoring Camera
We investigated the Venus O2 visible nightglow with imagery from the Venus
Monitoring Camera on Venus Express. Drawing from data collected between April
2007 and January 2011, we study the global distribution of this emission,
discovered in the late 70s by the Venera 9 and 10 missions. The inferred
limb-viewing intensities are on the order of 150 kiloRayleighs at the lower
latitudes and seem to drop somewhat towards the poles. The emission is
generally stable, although there are episodes when the intensities rise up to
500 kR. We compare a set of Venus Monitoring Camera observations with
coincident measurements of the O2 nightglow at 1.27 {\mu}m made with the
Visible and Infrared Thermal Imaging Spectrometer, also on Venus Express. From
the evidence gathered in this and past works, we suggest a direct correlation
between the instantaneous emissions from the two O2 nightglow systems. Possible
implications regarding the uncertain origin of the atomic oxygen green line at
557.7 nm are noted.Comment: 7 pages, 3 figure
Nonequilibrium wetting of finite samples
As a canonical model for wetting far from thermal equilibrium we study a
Kardar-Parisi-Zhang interface growing on top of a hard-core substrate.
Depending on the average growth velocity the model exhibits a non-equilibrium
wetting transition which is characterized by an additional surface critical
exponent theta. Simulating the single-step model in one spatial dimension we
provide accurate numerical estimates for theta and investigate the distribution
of contact points between the substrate and the interface as a function of
time. Moreover, we study the influence of finite-size effects, in particular
the time needed until a finite substrate is completely covered by the wetting
layer for the first time.Comment: 17 pages, 8 figures, revisio
Hall-effect and resistivity measurements in CdTe and ZnTe at high pressure: Electronic structure of impurities in the zincblende phase and the semi-metallic or metallic character of the high-pressure phases
We carried out high-pressure resistivity and Hall-effect measurements in
single crystals of CdTe and ZnTe up to 12 GPa. Slight changes of transport
parameters in the zincblende phase of CdTe are consitent with the shallow
character of donor impurities. Drastic changes in all the transport parameters
of CdTe were found around 4 GPa, i.e. close to the onset of the cinnabar to
rock-salt transition. In particular, the carrier concentration increases by
more than five orders of magnitude. Additionally, an abrupt decrease of the
resistivity was detected around 10 GPa. These results are discussed in
comparison with optical, thermoelectric, and x-ray diffraction experiments. The
metallic character of the Cmcm phase of CdTe is confirmed and a semi-metallic
character is determined for the rock-salt phase. In zincblende ZnTe, the
increase of the hole concentration by more than two orders of magnitude is
proposed to be due to a deep-to-shallow transformation of the acceptor levels.
Between 9 and 11 GPa, transport parameters are consistent with the
semiconducting character of cinnabar ZnTe. A two orders of magnitude decrease
of the resistivity and a carrier-type inversion occurs at 11 GPa, in agreement
with the onset of the transition to the Cmcm phase of ZnTe. A metallic
character for this phase is deduced.Comment: 20 pages, 4 figure
Recycling of quantum information: Multiple observations of quantum systems
Given a finite number of copies of an unknown qubit state that have already
been measured optimally, can one still extract any information about the
original unknown state? We give a positive answer to this question and quantify
the information obtainable by a given observer as a function of the number of
copies in the ensemble, and of the number of independent observers that, one
after the other, have independently measured the same ensemble of qubits before
him. The optimality of the protocol is proven and extensions to other states
and encodings are also studied. According to the general lore, the state after
a measurement has no information about the state before the measurement. Our
results manifestly show that this statement has to be taken with a grain of
salt, specially in situations where the quantum states encode confidential
information.Comment: 4 page
Entangled networks, synchronization, and optimal network topology
A new family of graphs, {\it entangled networks}, with optimal properties in
many respects, is introduced. By definition, their topology is such that
optimizes synchronizability for many dynamical processes. These networks are
shown to have an extremely homogeneous structure: degree, node-distance,
betweenness, and loop distributions are all very narrow. Also, they are
characterized by a very interwoven (entangled) structure with short average
distances, large loops, and no well-defined community-structure. This family of
nets exhibits an excellent performance with respect to other flow properties
such as robustness against errors and attacks, minimal first-passage time of
random walks, efficient communication, etc. These remarkable features convert
entangled networks in a useful concept, optimal or almost-optimal in many
senses, and with plenty of potential applications computer science or
neuroscience.Comment: Slightly modified version, as accepted in Phys. Rev. Let
Mean-field limit of systems with multiplicative noise
A detailed study of the mean-field solution of Langevin equations with
multiplicative noise is presented. Three different regimes depending on
noise-intensity (weak, intermediate, and strong-noise) are identified by
performing a self-consistent calculation on a fully connected lattice. The most
interesting, strong-noise, regime is shown to be intrinsically unstable with
respect to the inclusion of fluctuations, as a Ginzburg criterion shows. On the
other hand, the self-consistent approach is shown to be valid only in the
thermodynamic limit, while for finite systems the critical behavior is found to
be different. In this last case, the self-consistent field itself is broadly
distributed rather than taking a well defined mean value; its fluctuations,
described by an effective zero-dimensional multiplicative noise equation,
govern the critical properties. These findings are obtained analytically for a
fully connected graph, and verified numerically both on fully connected graphs
and on random regular networks. The results presented here shed some doubt on
what is the validity and meaning of a standard mean-field approach in systems
with multiplicative noise in finite dimensions, where each site does not see an
infinite number of neighbors, but a finite one. The implications of all this on
the existence of a finite upper critical dimension for multiplicative noise and
Kardar-Parisi-Zhang problems are briefly discussed.Comment: 9 Pages, 8 Figure
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