22,692 research outputs found
Two-dimensional Valence Bond Solid (AKLT) states from electrons
Two-dimensional AKLT model on a honeycomb lattice has been shown to be a
universal resource for quantum computation. In this valence bond solid,
however, the spin interactions involve higher powers of the Heisenberg coupling
, making these states seemingly unrealistic on
bipartite lattices, where one expects a simple antiferromagnetic order. We show
that those interactions can be generated by orbital physics in multiorbital
Mott insulators. We focus on electrons on the honeycomb lattice and
propose a physical realization of the spin- AKLT state. We find a phase
transition from the AKLT to the Neel state on increasing Hund's rule coupling,
which is confirmed by density matrix renormalization group (DMRG) simulations.
An experimental signature of the AKLT state consists of protected, free
spins-1/2 on lattice vacancies, which may be detected in the spin
susceptibility
Effects of charge-dependent vibrational frequencies and anharmonicities in transport through molecules
As a step towards a more realistic modeling of vibrations in single-molecule
devices, we investigate the effects of charge-dependent vibrational frequencies
and anharmonic potentials on electronic transport. For weak phonon relaxation,
we find that in both cases vibrational steps split into a multitude of
substeps. This effectively leads to a bias-dependent broadening of vibrational
features in current-voltage and conductance characteristics, which provides a
fingerprint of nonequilibrium vibrations whenever other broadening mechanisms
are secondary. In the case of an asymmetric molecule-lead coupling, we observe
that frequency differences can also cause negative differential conductance.Comment: 4+ pages, 3 figures; accepted for publication in Phys. Rev. B. (Brief
Reports
A spectroscopic binary in the Hercules dwarf spheroidal galaxy
We present the radial velocity curve of a single-lined spectroscopic binary
in the faint Hercules dwarf spheroidal (dSph) galaxy, based on 34 individual
spectra covering more than two years of observations. This is the first time
that orbital elements could be derived for a binary in a dSph. The system
consists of a metal-poor red giant and a low-mass companion, possibly a white
dwarf, with a 135-days period in a moderately eccentric () orbit. Its
period and eccentricity are fully consistent with metal-poor binaries in the
Galactic halo, while the projected semimajor axis is small, at sin =
38 R. In fact, a very close orbit could inhibit the production of
heavier elements through -process nucleosynthesis, leading to the very low
abundances of neutron-capture elements that are found in this star. We discuss
the further implications for the chemical enrichment history of the Hercules
dSph, but find no compelling binary scenario that could reasonably explain the
full, peculiar abundance pattern of the Hercules dSph galaxy.Comment: 7 pages, 3 figures, accepted for publication in the Astrophysical
Journa
Nearest Neighbour Decoding and Pilot-Aided Channel Estimation in Stationary Gaussian Flat-Fading Channels
We study the information rates of non-coherent, stationary, Gaussian,
multiple-input multiple-output (MIMO) flat-fading channels that are achievable
with nearest neighbour decoding and pilot-aided channel estimation. In
particular, we analyse the behaviour of these achievable rates in the limit as
the signal-to-noise ratio (SNR) tends to infinity. We demonstrate that nearest
neighbour decoding and pilot-aided channel estimation achieves the capacity
pre-log - which is defined as the limiting ratio of the capacity to the
logarithm of SNR as the SNR tends to infinity - of non-coherent multiple-input
single-output (MISO) flat-fading channels, and it achieves the best so far
known lower bound on the capacity pre-log of non-coherent MIMO flat-fading
channels.Comment: 5 pages, 1 figure. To be presented at the IEEE International
Symposium on Information Theory (ISIT), St. Petersburg, Russia, 2011.
Replaced with version that will appear in the proceeding
Non-equilibrium dynamics in the dual-wavelength operation of Vertical external-cavity surface-emitting lasers
Microscopic many-body theory coupled to Maxwell's equation is used to
investigate dual-wavelength operation in vertical external-cavity
surface-emitting lasers. The intrinsically dynamic nature of coexisting
emission wavelengths in semiconductor lasers is associated with characteristic
non-equilibrium carrier dynamics which causes significant deformations of the
quasi-equilibrium gain and carrier inversion. Extended numerical simulations
are employed to efficiently investigate the parameter space to identify the
regime for two-wavelength operation. Using a frequency selective intracavity
etalon, two families of modes are stabilized with dynamical interchange of the
strongest emission peaks. For this operation mode, anti-correlated intensity
noise is observed in agreement with the experiment. A method using effective
frequency selective filtering is suggested for stabilization genuine
dual-wavelength output.Comment: 15 pages, 7 figure
Direct Investigation of Superparamagnetism in Co Nanoparticle Films
A direct probe of superparamagnetism was used to determine the complete
anisotropy energy distribution of Co nanoparticle films. The films were
composed of self-assembled lattices of uniform Co nanoparticles 3 nm or 5 nm in
diameter, and a variable temperature scanning-SQUID microscope was used to
measure temperature-induced spontaneous magnetic noise in the samples. Accurate
measurements of anisotropy energy distributions of small volume samples will be
critical to magnetic optimization of nanoparticle devices and media.Comment: 4 pages, 4 figures. Submitted to Physical Review Letter
On the mechanism for orbital-ordering in KCuF3
The Mott insulating perovskite KCuF3 is considered the archetype of an
orbitally-ordered system. By using the LDA+dynamical mean-field theory (DMFT)
method, we investigate the mechanism for orbital-ordering (OO) in this
material. We show that the purely electronic Kugel-Khomskii super-exchange
mechanism (KK) alone leads to a remarkably large transition temperature of T_KK
about 350 K. However, orbital-order is experimentally believed to persist to at
least 800 K. Thus Jahn-Teller distortions are essential for stabilizing
orbital-order at such high temperatures.Comment: 4 pages, 5 figure
Cherenkov Radiation from Jets in Heavy-ion Collisions
The possibility of Cherenkov-like gluon bremsstrahlung in dense matter is
studied. We point out that the occurrence of Cherenkov radiation in dense
matter is sensitive to the presence of partonic bound states. This is
illustrated by a calculation of the dispersion relation of a massless particle
in a simple model in which it couples to two different massive resonance
states. We further argue that detailed spectroscopy of jet correlations can
directly probe the index of refraction of this matter, which in turn will
provide information about the mass scale of these partonic bound states.Comment: 4 pages, 5 figures, revte
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