1,221 research outputs found

### Universal conductance reduction in a quantum wire

Even a single point defect in a quantum wire causes a conductance reduction.
In this paper we prove (without any approximations) that for any point impurity
this conductance reduction in all the sub-bands is exactly 2e^2/h. Moreover, it
is shown that in the case of a surface defect, not only is the conductance
minimum independent of the defect characteristics, but the transmission matrix
also converges to universal (defect-independent) values. We also discuss
particle confinement between two arbitrarily weak point defects.Comment: 4 pages, 4 figures (Revtex

### Potential Barrier Classification by Short-Time Measurement

We investigate the short-time dynamics of a delta-function potential barrier
on an initially confined wave-packet. There are mainly two conclusions: A) At
short times the probability density of the first particles that passed through
the barrier is unaffected by it. B) When the barrier is absorptive (i.e., its
potential is imaginary) it affects the transmitted wave function at shorter
times than a real potential barrier. Therefore, it is possible to distinguish
between an imaginary and a real potential barrier by measuring its effect at
short times only on the transmitting wavefunction.Comment: 6 pages, 5 figure

### Emergence of a confined state in a weakly bent wire

In this paper we use a simple straightforward technique to investigate the
emergence of a bound state in a weakly bent wire. We show that the bend behaves
like an infinitely shallow potential well, and in the limit of small bending
angle and low energy the bend can be presented by a simple 1D delta function
potential.Comment: 4 pages, 3 Postscript figures (uses Revtex); added references and
rewritte

### Scatterer that leaves "footprints" but no "fingerprints"

We calculate the exact transmission coefficient of a quantum wire in the
presence of a single point defect at the wire's cut-off frequencies. We show
that while the conductance pattern (i.e., the scattering) is strongly affected
by the presence of the defect, the pattern is totally independent of the
defect's characteristics (i.e., the defect that caused the scattering cannot be
identified from that pattern).Comment: 4 pages, 3 figure

### Afterglow Observations Shed New Light on the Nature of X-ray Flashes

X-ray flashes (XRFs) and X-ray rich gamma-ray bursts (XRGRBs) share many
observational characteristics with long duration GRBs, but the reason for which
their prompt emission peaks at lower photon energies, $E_p$, is still under
debate. Although many different models have been invoked in order to explain
the lower $E_p$ values, their implications for the afterglow emission were not
considered in most cases, mainly because observations of XRF afterglows have
become available only recently. Here we examine the predictions of the various
XRF models for the afterglow emission, and test them against the observations
of XRF 030723 and XRGRB 041006, the events with the best monitored afterglow
light curves in their respective class. We show that most existing XRF models
are hard to reconcile with the observed afterglow light curves, which are very
flat at early times. Such light curves are, however, naturally produced by a
roughly uniform jet with relatively sharp edges that is viewed off-axis (i.e.
from outside of the jet aperture). This type of model self consistently
accommodates both the observed prompt emission and the afterglow light curves
of XRGRB 041006 and XRF 030723, implying viewing angles $\theta_{obs}$ from the
jet axis of $(\theta_{obs}-\theta_0)\sim 0.15\theta_0$ and $\sim \theta_0$,
respectively, where $\theta_0\sim 3$ deg is the jet half-opening angle. This
suggests that GRBs, XRGRBs and XRFs are intrinsically similar relativistic jets
viewed from different angles, corresponding to $\gamma(\theta_{obs}-\theta_0)$
of less than 1, between 1 and a few, and more than a few, respectively, where
$\gamma$ is the Lorentz factor. Future observations with Swift could help test
this unification scheme in which GRBs, XRGRBs and XRFs share the same basic
physics and differ only by their orientation relative to our line of sight.Comment: some references added, small typos corrected, and the important role
of HETE II emphasize

### The Prompt Gamma-Ray and Afterglow Energies of Short-Duration Gamma-Ray Bursts

I present an analysis of the gamma-ray and afterglow energies of the complete
sample of 17 short duration GRBs with prompt X-ray follow-up. I find that 80%
of the bursts exhibit a linear correlation between their gamma-ray fluence and
the afterglow X-ray flux normalized to t=1 d, a proxy for the kinetic energy of
the blast wave ($F_{X,1}~F_{gamma}^1.01). An even tighter correlation is
evident between E_{gamma,iso} and L_{X,1} for the subset of 13 bursts with
measured or constrained redshifts. The remaining 20% of the bursts have values
of F_{X,1}/F_{gamma} that are suppressed by about three orders of magnitude,
likely because of low circumburst densities (Nakar 2007). These results have
several important implications: (i) The X-ray luminosity is generally a robust
proxy for the blast wave kinetic energy, indicating nu_X>nu_c and hence a
circumburst density n>0.05 cm^{-3}; (ii) most short GRBs have a narrow range of
gamma-ray efficiency, with ~0.85 and a spread of 0.14 dex; and
(iii) the isotropic-equivalent energies span 10^{48}-10^{52} erg. Furthermore,
I find tentative evidence for jet collimation in the two bursts with the
highest E_{gamma,iso}, perhaps indicative of the same inverse correlation that
leads to a narrow distribution of true energies in long GRBs. I find no clear
evidence for a relation between the overall energy release and host galaxy
type, but a positive correlation with duration may be present, albeit with a
large scatter. Finally, I note that the outlier fraction of 20% is similar to
the proposed fraction of short GRBs from dynamically-formed neutron star
binaries in globular clusters. This scenario may naturally explain the
bimodality of the F_{X,1}/F_{gamma} distribution and the low circumburst
densities without invoking speculative kick velocities of several hundred km/s.Comment: Submitted to ApJ; 9 pages, 2 figures, 1 tabl

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