554 research outputs found
L1551NE - Discovery of a Binary Companion
L1551NE is a very young (class 0 or I) low-mass protostar located close to
the well-studied L1551 IRS5. We present here evidence, from 1.3mm continuum
interferometric observations at ~1'' resolution, for a binary companion to
L1551NE. The companion, whose 1.3mm flux density is ~1/3 that of the primary
component, is located 1.43'' (~230 A.U. at 160pc) to the southeast. The
millimeterwave emission from the primary component may have been just barely
resolved, with deconvolved size ~0.82"x0.70" (~131x112 A.U.). The companion
emission was unresolved (<100 A.U.). The pair is embedded within a flattened
circum-binary envelope of size ~5.4'' x 2.3'' (~860 x 370 A.U.). The masses of
the three components (i.e. from the cicumstellar material of the primary star
and its companion, and the envelope) are approximately 0.044, 0.014 and 0.023
Mo respectively.Comment: 8 pages, 1 figur
Thermal state entanglement in harmonic lattices
We investigate the entanglement properties of thermal states of the harmonic
lattice in one, two and three dimensions. We establish the value of the
critical temperature for entanglement between neighbouring sites and give
physical reasons. Further sites are shown to be entangled only due to boundary
effects. Other forms of entanglement are addressed in the second part of the
paper by using the energy as witness of entanglement. We close with a
comprehensive diagram showing the different phases of entanglement versus
complete separability and propose techniques to swap and tune entanglement
experimentally.Comment: 9 pages, 4 figure
Dense packing on uniform lattices
We study the Hard Core Model on the graphs
obtained from Archimedean tilings i.e. configurations in with the nearest neighbor 1's forbidden. Our
particular aim in choosing these graphs is to obtain insight to the geometry of
the densest packings in a uniform discrete set-up. We establish density bounds,
optimal configurations reaching them in all cases, and introduce a
probabilistic cellular automaton that generates the legal configurations. Its
rule involves a parameter which can be naturally characterized as packing
pressure. It can have a critical value but from packing point of view just as
interesting are the noncritical cases. These phenomena are related to the
exponential size of the set of densest packings and more specifically whether
these packings are maximally symmetric, simple laminated or essentially random
packings.Comment: 18 page
Time Periodic Behavior of Multiband Superlattices in Static Electric Fields
We use an analytic perturbation expansion for the two-band system of tight
binding electrons to discuss Bloch oscillations and Zener tunneling within this
model. We make comparison with recent numerical results and predict
analytically the frequency of radiation expected from Zener tunneling,
including its disappearance, as a function of the system parameters.Comment: 12 pages, no figure include
Wannier-Stark ladders in one-dimensional elastic systems
The optical analogues of Bloch oscillations and their associated
Wannier-Stark ladders have been recently analyzed. In this paper we propose an
elastic realization of these ladders, employing for this purpose the torsional
vibrations of specially designed one-dimensional elastic systems. We have
measured, for the first time, the ladder wave amplitudes, which are not
directly accessible either in the quantum mechanical or optical cases. The wave
amplitudes are spatially localized and coincide rather well with theoretically
predicted amplitudes. The rods we analyze can be used to localize different
frequencies in different parts of the elastic systems and viceversa.Comment: 10 pages, 6 figures, accepted in Phys. Rev. Let
Conductivity of 2D lattice electrons in an incommensurate magnetic field
We consider conductivities of two-dimensional lattice electrons in a magnetic
field. We focus on systems where the flux per plaquette is irrational
(incommensurate flux). To realize the system with the incommensurate flux, we
consider a series of systems with commensurate fluxes which converge to the
irrational value. We have calculated a real part of the longitudinal
conductivity . Using a scaling analysis, we have found
behaves as \,
when and the Fermi energy is near
zero. This behavior is closely related to the known scaling behavior of the
spectrum.Comment: 16 pages, postscript files are available on reques
Wave-packet dynamics in slowly perturbed crystals: Gradient corrections and Berry-phase effects
We present a unified theory for wave-packet dynamics of electrons in crystals
subject to perturbations varying slowly in space and time. We derive the
wave-packet energy up to the first order gradient correction and obtain all
kinds of Berry-phase terms for the semiclassical dynamics and the quantization
rule. For electromagnetic perturbations, we recover the orbital magnetization
energy and the anomalous velocity purely within a single-band picture without
invoking inter-band couplings. For deformations in crystals, besides a
deformation potential, we obtain a Berry-phase term in the Lagrangian due to
lattice tracking, which gives rise to new terms in the expressions for the
wave-packet velocity and the semiclassical force. For multiple-valued
displacement fields surrounding dislocations, this term manifests as a Berry
phase, which we show to be proportional to the Burgers vector around each
dislocation.Comment: 12 pages, RevTe
On the Green's Function of the almost-Mathieu Operator
The square tight-binding model in a magnetic field leads to the
almost-Mathieu operator which, for rational fields, reduces to a
matrix depending on the components , of the wave vector in the
magnetic Brillouinzone. We calculate the corresponding Green's function without
explicit knowledge of eigenvalues and eigenfunctions and obtain analytical
expressions for the diagonal and the first off-diagonal elements; the results
which are consistent with the zero magnetic field case can be used to calculate
several quantities of physical interest (e. g. the density of states over the
entire spectrum, impurity levels in a magnetic field).Comment: 9 pages, 3 figures corrected some minor errors and typo
Off Resonant Pumping for Transition from Continuous to Discrete Spectrum and Quantum Revivals in Systems in Coherent States
We show that in parametrically driven systems and, more generally, in systems
in coherent states, off-resonant pumping can cause a transition from a
continuum energy spectrum of the system to a discrete one, and result in
quantum revivals of the initial state. The mechanism responsible for quantum
revivals in the present case is different from that in the non-linear
wavepacket dynamics of systems such as Rydberg atoms. We interpret the reported
phenomena as an optical analog of Bloch oscillations realized in Fock space and
propose a feasible scheme for inducing Bloch oscillations in trapped ions.Comment: 5 pages, 4 figures, submitted to Jnl. of Optics
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