6,417 research outputs found
On the propagation speed of evanescent modes
The group-velocity of evanescent waves (in undersized waveguides, for
instance) was theoretically predicted, and has been experimentally verified, to
be Superluminal (v_g > c). By contrast, it is known that the precursor speed in
vacuum cannot be larger than c. In this paper, by computer simulations based on
Maxwell equations only, we show the existence of both phenomena. In other
words, we verify the actual possibility of Superluminal group velocities,
without violating the so-called (naive) Einstein causality. (Subject classes:
General physics, Classical physics, Optics, Special Relativity; PACS nos.:
73.40Gk, 03.80+z, 03.65Bz; Keywords: evanescent waves; tunnelling photons;
Hartman effect; group velocity; Superluminal waves; precursors; transient
waves; front velocity; Maxwell equations; electromagnetic waves; computer
simulations; Special Relativity; Extended Relativity).Comment: plain LaTeX file (14 pages), plus 15 figures in .jp
Nuclear State Preparation via Landau-Zener-Stueckelberg transitions in Double Quantum Dots
We theoretically model a nuclear-state preparation scheme that increases the
coherence time of a two-spin qubit in a double quantum dot. The two-electron
system is tuned repeatedly across a singlet-triplet level-anticrossing with
alternating slow and rapid sweeps of an external bias voltage. Using a
Landau-Zener-Stueckelberg model, we find that in addition to a small nuclear
polarization that weakly affects the electron spin coherence, the slow sweeps
are only partially adiabatic and lead to a weak nuclear spin measurement and a
nuclear-state narrowing which prolongs the electron spin coherence. This
resolves some open problems brought up by a recent experiment [D. J. Reilly et
al., Science 321, 817 (2008).]. Based on our description of the weak
measurement, we simulate a system with up to n=200 nuclear spins per dot.
Scaling in n indicates a stronger effect for larger n.Comment: 4.1 pages, 2 figure
3-D Photoionization Structure and Distances of Planetary Nebulae III. NGC 6781
Continuing our series of papers on the three-dimensional (3-D) structures of
and accurate distances to Planetary Nebulae (PNe), we present our study of the
planetary nebula NGC6781. For this object we construct a 3-D photoionization
model and, using the constraints provided by observational data from the
literature we determine the detailed 3-D structure of the nebula, the physical
parameters of the ionizing source and the first precise distance. The procedure
consists in simultaneously fitting all the observed emission line morphologies,
integrated intensities and the 2-D density map from the [SII] line ratios to
the parameters generated by the model, and in an iterative way obtain the best
fit for the central star parameters and the distance to NGC6781, obtaining
values of 950+-143pc and 385 Lsun for the distance and luminosity of the
central star respectively. Using theoretical evolutionary tracks of
intermediate and low mass stars, we derive the mass of the central star of
NGC6781 and its progenitor to be 0.60+-0.03 Msun and 1.5+-0.5 Msun
respectively.Comment: 16 pp, 6 figues, 2 tables, submitted to the Ap
Extending electron orbital precession to the molecular case: Can orbital alignment be used to observe wavepacket dynamics?
The complexity of ultrafast molecular photoionization presents an obstacle to
the modelling of pump-probe experiments. Here, a simple optimized model of
atomic rubidium is combined with a molecular dynamics model to predict
quantitatively the results of a pump-probe experiment in which long range
rubidium dimers are first excited, then ionized after a variable delay. The
method is illustrated by the outline of two proposed feasible experiments and
the calculation of their outcomes. Both of these proposals use Feshbach 87Rb2
molecules. We show that long-range molecular pump-probe experiments should
observe spin-orbit precession given a suitable pump-pulse, and that the
associated high-frequency beat signal in the ionization probability decays
after a few tens of picoseconds. If the molecule was to be excited to only a
single fine structure state state, then a low-frequency oscillation in the
internuclear separation would be detectable through the timedependent
ionization cross section, giving a mechanism that would enable observation of
coherent vibrational motion in this molecule.Comment: 9 pages, 10 figures, PRA submissio
The Dynamical Mean Field Theory phase space extension and critical properties of the finite temperature Mott transition
We consider the finite temperature metal-insulator transition in the half
filled paramagnetic Hubbard model on the infinite dimensional Bethe lattice. A
new method for calculating the Dynamical Mean Field Theory fixpoint surface in
the phase diagram is presented and shown to be free from the convergence
problems of standard forward recursion. The fixpoint equation is then analyzed
using dynamical systems methods. On the fixpoint surface the eigenspectra of
its Jacobian is used to characterize the hysteresis boundaries of the first
order transition line and its second order critical end point. The critical
point is shown to be a cusp catastrophe in the parameter space, opening a
pitchfork bifurcation along the first order transition line, while the
hysteresis boundaries are shown to be saddle-node bifurcations of two merging
fixpoints. Using Landau theory the properties of the critical end point is
determined and related to the critical eigenmode of the Jacobian. Our findings
provide new insights into basic properties of this intensively studied
transition.Comment: 11 pages, 12 figures, 1 tabl
Kinematic and morphological modeling of the bipolar nebula Sa2-237
We present [OIII]500.7nm and Halpha+[NII] images and long-slit, high
resolution echelle spectra in the same spectral regions of Sa2--237, a possible
bipolar planetary nebula. The image shows a bipolar nebula of about 34" extent,
with a narrow waist, and showing strong point symmetry about the central
object, indicating it's likely binary nature. The long slit spectra were taken
over the long axis of the nebula, and show a distinct ``eight'' shaped pattern
in the velocity--space plot, and a maximum projected outflow velocity of
V=106km/s, both typical of expanding bipolar planetary nebulae. By model
fitting the shape and spectrum of the nebula simultaneously, we derive the
inclination of the long axis to be 70 degrees, and the maximum space velocity
of expansion to be 308 km/s. Due to asymmetries in the velocities we adopt a
new value for the system's heliocentric radial velocity of -30km/s. We use the
IRAS and 21cm radio fluxes, the energy distribution, and the projected size of
Sa2-237 to estimate it's distance to be 2.1+-0.37kpc. At this distance Sa2-237
has a luminosity of 340 Lsun, a size of 0.37pc, and -- assuming constant
expansion velocity -- a nebular age of 624 years. The above radial velocity and
distance place Sa2--237 in the disk of the Galaxy at z=255pc, albeit with
somewhat peculiar kinematics.Comment: 10pp, 4 fig
A pump-probe study of the formation of rubidium molecules by ultrafast photoassociation of ultracold atoms
An experimental pump-probe study of the photoassociative creation of
translationally ultracold rubidium molecules is presented together with
numerical simulations of the process. The formation of loosely bound
excited-state dimers is observed as a first step towards a fully coherent
pump-dump approach to the stabilization of Rb into its lowest ground
vibrational states. The population that contributes to the pump-probe process
is characterized and found to be distinct from a background population of
pre-associated molecules.Comment: Accepted for publication in Phys. Rev. A (10 pages, 9 figures
Coherent Adiabatic Spin Control in the Presence of Charge Noise Using Tailored Pulses
We study finite-time Landau-Zener transitions at a singlet-triplet level
crossing in a GaAs double quantum dot, both experimentally and theoretically.
Sweeps across the anticrossing in the high driving speed limit result in
oscillations with a small visibility. Here we demonstrate how to increase the
oscillation visibility while keeping sweep times shorter than T2* using a
tailored pulse with a detuning dependent level velocity. Our results show an
improvement of a factor ~2.9 for the oscillation visibility. In particular, we
were able to obtain a visibility of ~0.5 for St\"uckelberg oscillations, which
demonstrates the creation of an equally weighted superposition of the qubit
states.Comment: Related papers at http://pettagroup.princeton.ed
A Survey of Laboratory Programs for First Year Medical Students
Basic science courses offered to freshmen medical students have been traditionally taught by didactic presentations and laboratory work. Various factors have prompted many departments to either markedly reduce the time allotted for the traditional, but more vulnerable, laboratory phase of these courses or drastically alter the content of the laboratory programs. Information concerning the desirability for such changes is incomplete and no evaluation has been developed to determine the effectiveness of change. Before beginning any further alterations in the laboratory programs at the Medical College of Virginia, it seemed desirable to determine anew what we wish to teach in the laboratory. This problem was approached in part by surveying the attitudes of other anatomy, biochemistry, and physiology departments
Counting Arithmetical Structures on Paths and Cycles
Let G be a finite, connected graph. An arithmetical structure on G is a pair of positive integer vectors d, r such that (diag (d) - A) r=0 , where A is the adjacency matrix of G. We investigate the combinatorics of arithmetical structures on path and cycle graphs, as well as the associated critical groups (the torsion part of the cokernels of the matrices (diag (d) - A)). For paths, we prove that arithmetical structures are enumerated by the Catalan numbers, and we obtain refined enumeration results related to ballot sequences. For cycles, we prove that arithmetical structures are enumerated by the binomial coefficients ((2n-1)/(n-1)) , and we obtain refined enumeration results related to multisets. In addition, we determine the critical groups for all arithmetical structures on paths and cycles
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