10,385 research outputs found
Coherent shuttle of electron-spin states
We demonstrate a coherent spin shuttle through a GaAs/AlGaAs
quadruple-quantum-dot array. Starting with two electrons in a spin-singlet
state in the first dot, we shuttle one electron over to either the second,
third or fourth dot. We observe that the separated spin-singlet evolves
periodically into the spin-triplet and back before it dephases due to
nuclear spin noise. We attribute the time evolution to differences in the local
Zeeman splitting between the respective dots. With the help of numerical
simulations, we analyse and discuss the visibility of the singlet-triplet
oscillations and connect it to the requirements for coherent spin shuttling in
terms of the inter-dot tunnel coupling strength and rise time of the pulses.
The distribution of entangled spin pairs through tunnel coupled structures may
be of great utility for connecting distant qubit registers on a chip.Comment: 21 pages, 10 figure
Finite size corrections in massive Thirring model
We calculate for the first time the finite size corrections in the massive
Thirring model. This is done by numerically solving the equations of periodic
boundary conditions of the Bethe ansatz solution. It is found that the
corresponding central charge extracted from the term is around 0.4 for
the coupling constant of and decreases down to zero when
. This is quite different from the predicted central
charge of the sine-Gordon model.Comment: 8 pages, Latex, 2 figure
A Brownian Model for Recurrent Volcanic Eruptions: an Application to Miyakejima Volcano (Japan)
The definition of probabilistic models as mathematical structures to describe the
response of a volcanic system is a plausible approach to characterize the temporal behavior
of volcanic eruptions, and constitutes a tool for long-term eruption forecasting. This kind
of approach is motivated by the fact that volcanoes are complex systems in which a com-
pletely deterministic description of the processes preceding eruptions is practically impos-
sible. To describe recurrent eruptive activity we apply a physically-motivated probabilistic
model based on the characteristics of the Brownian passage-time (BPT) distribution; the
physical process defining this model can be described by the steady rise of a state variable
from a ground state to a failure threshold; adding Brownian perturbations to the steady load-
ing produces a stochastic load-state process (a Brownian relaxation oscillator) in which an
eruption relaxes the load state to begin a new eruptive cycle. The Brownian relaxation os-
cillator and Brownian passage-time distribution connect together physical notions of unob-
servable loading and failure processes of a point process with observable response statistics.
The Brownian passage-time model is parameterized by the mean rate of event occurrence,
μ , and the aperiodicity about the mean, α . We apply this model to analyze the eruptive his-
tory of Miyakejima volcano, Japan, finding a value of 44.2(±6.5 years) for the μ parameter
and 0.51(±0.01) for the (dimensionless) α parameter. The comparison with other models
often used in volcanological literature shows that this pysically-motivated model may be a
good descriptor of volcanic systems that produce eruptions with a characteristic size. BPT
is clearly superior to the exponential distribution and the fit to the data is comparable to
other two-parameters models. Nonetheless, being a physically-motivated model, it provides
an insight into the macro-mechanical processes driving the system
Uniform current in graphene strip with zigzag edges
Graphene exhibits zero-gap massless-Dirac fermion and zero density of states
at E = 0. These particles form localized states called edge states on finite
width strip with zigzag edges at E = 0. Naively thinking, one may expect that
current is also concentrated at the edge, but Zarbo and Nikolic numerically
obtained a result that the current density shows maximum at the center of the
strip. We derive a rigorous relation for the current density, and clarify the
reason why the current density of edge state has a maximum at the center.Comment: 5 pages, 3 figures; added references and corrected typos, to be
published in J. Phys. Soc. Jpn. Vol.78 No.
The Baryonic and Dark Matter Distributions in Abell 401
We combine spatially resolved ASCA temperature data with ROSAT imaging data
to constrain the total mass distribution in the cluster A401, assuming that the
cluster is in hydrostatic equilibrium. We obtain a total mass within the X-ray
core (290/h_50 kpc) of 1.2[+0.1,-0.5] 10^14 /h_50 Msun at the 90% confidence
level, 1.3 times larger than the isothermal estimate. The total mass within
r_500 (1.7/h_50 Mpc) is M_500 = 0.9[+0.3,-0.2] 10^15/ h_50 Msun at 90%
confidence, in agreement with the optical virial mass estimate, and 1.2 times
smaller than the isothermal estimate. Our M_500 value is 1.7 times smaller than
that estimated using the mass-temperature scaling law predicted by simulations.
The best fit dark matter density profile scales as r^{-3.1} at large radii,
which is consistent with the Navarro, Frenk & White (NFW) ``universal profile''
as well as the King profile of the galaxy density in A401. From the imaging
data, the gas density profile is shallower than the dark matter profile,
scaling as r^{-2.1} at large radii, leading to a monotonically increasing gas
mass fraction with radius. Within r_500 the gas mass fraction reaches a value
of f_gas = 0.21[+0.06,-0.05] h_50^{-3/2} (90% confidence errors). Assuming that
f_gas (plus an estimate of the stellar mass) is the universal value of the
baryon fraction, we estimate the 90% confidence upper limit of the cosmological
matter density to be Omega_m < 0.31.Comment: 17 pages, 6 figures, accepted by Ap
Dimerization structures on the metallic and semiconducting fullerene tubules with half-filled electrons
Possible dimerization patterns and electronic structures in fullerene tubules
as the one-dimensional pi-conjugated systems are studied with the extended
Su-Schrieffer-Heeger model. We assume various lattice geometries, including
helical and nonhelical tubules. The model is solved for the half-filling case
of -electrons. (1) When the undimerized systems do not have a gap, the
Kekule structures prone to occur. The energy gap is of the order of the room
temperatures at most and metallic properties would be expected. (2) If the
undimerized systems have a large gap (about 1eV), the most stable structures
are the chain-like distortions where the direction of the arranged
trans-polyacetylene chains is along almost the tubular axis. The electronic
structures are ofsemiconductors due to the large gap.Comment: submitted to Phys. Rev. B, pages 15, figures 1
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