133 research outputs found
Resonant and non-resonant Tunneling through a double barrier
An explicit expression is obtained for the phase-time corresponding to
tunneling of a (non-relativistic) particle through two rectangular barriers,
both in the case of resonant and in the case of non-resonant tunneling. It is
shown that the behavior of the transmission coefficient and of the tunneling
phase-time near a resonance is given by expressions with "Breit-Wigner type"
denominators. By contrast, it is shown that, when the tunneling probability is
low (but not negligible), the non-resonant tunneling time depends on the
barrier width and on the distance between the barriers only in a very weak
(exponentially decreasing) way: This can imply in various cases, as well-known,
the highly Superluminal tunneling associated with the so-called "generalized
Hartman Effect"; but we are now able to improve and modify the mathematical
description of such an effect, and to compare more in detail our results with
the experimental data for non-resonant tunneling of photons. Finally, as a
second example, the tunneling phase-time is calculated, and compared with the
available experimental results, in the case of the quantum-mechanical tunneling
of neutrons through two barrier-filters at the resonance energy of the set-up.Comment: replaced with some improvements in the text and in the references:
pdf (11 pages) produced from a source-file in Word; including one Figur
Lorentz Invariant Superluminal Tunneling
It is shown that superluminal optical signalling is possible without
violating Lorentz invariance and causality via tunneling through photonic band
gaps in inhomogeneous dielectrics of a special kind.Comment: 10 pages revtex, no figure, more discussions added, submitted to
Phys. Rev.
Is Excision of Radial Scars Identified on CNB Necessary?
Introduction: Quantifying the risk of upgrade to malignancy with radial scars has been an ongoing challenge, as the published upgrade rate varies widely from 0-40%, making management strategy controversial. The lack of consensus on optimal management highlights the need for further analysis. We sought to identify our institutional upgrade rate of radial scar identified on core needle biopsy (CNB).
Methods: A retrospective review of pathology and radiology databases was performed to identify radial scars found on CNB. We excluded patients with malignancy associated with radial scar and those who did not undergo surgical excision. The upgrade rates to malignancy or other atypia on surgical excision were then evaluated.
Results: We identified 127 patients with radial scar on CNB, of which 75 patients were excluded, leaving 52 patients for analysis. Of these, 4 of 52 (7.7%) patients had an upgrade to malignancy upon excision. Eight patients had additional atypia with radial scar on CNB, two of which upgraded to malignancy on excision. The rate of malignancy upgrade for isolated radial scar was 2 of 44 (4.5%). Of the 44 patients with isolated radial scar, 15 (34%) were found to have additional atypia on excision.
Discussion: Although the upgrade rate to malignancy was only 4.5%, there was a substantial upgrade rate of isolated radial scar to additional atypia which can alter subsequent management. Additionally, 25% of radial scars with atypia upgraded to malignancy. Thus, careful consideration should be given to surgical excision of CNB showing radial scar with and without atypia
Energy spectrum, density of states and optical transitions in strongly biased narrow-gap quantum wells
We study theoretically the effect of an electric field on the electron states
and far-infrared optical properties in narrow-gap lead salt quantum wells. The
electron states are described by a two-band Hamiltonian. An application of a
strong electric field across the well allows the control of the energy gap
between the two-dimensional (2D) states in a wide range. A sufficiently strong
electric field transforms the narrow-gap quantum well to a nearly gapless 2D
system, whose electron energy spectrum is described by linear dispersion
relations \epsilon_{\sigma} (k) ~\pm (k-k_{\sigma}), where k_{\sigma} are the
field-dependent 2D momenta corresponding to the minimum energy gaps for the
states with spin numbers \sigma. Due to the field-induced shift of the 2D
subband extrema away from k=0 the density of states has inverse-square-root
divergencies at the edges. This property may result in a considerable increase
of the magnitude of the optical absorption and in the efficiency of the
electrooptical effect.Comment: Text 18 pages in Latex/Revtex format, 7 Postscript figure
Sub-femtosecond determination of transmission delay times for a dielectric mirror (photonic bandgap) as a function of angle of incidence
Using a two-photon interference technique, we measure the delay for
single-photon wavepackets to be transmitted through a multilayer dielectric
mirror, which functions as a ``photonic bandgap'' medium. By varying the angle
of incidence, we are able to confirm the behavior predicted by the group delay
(stationary phase approximation), including a variation of the delay time from
superluminal to subluminal as the band edge is tuned towards to the wavelength
of our photons. The agreement with theory is better than 0.5 femtoseconds (less
than one quarter of an optical period) except at large angles of incidence. The
source of the remaining discrepancy is not yet fully understood.Comment: 5 pages and 5 figure
Tin telluride: a weakly co-elastic metal
We report resonant ultrasound spectroscopy (RUS),
dilatometry/magnetostriction, magnetotransport, magnetization, specific heat,
and Sn M\"ossbauer spectroscopy measurements on SnTe and
SnCrTe. Hall measurements at K indicate that our
Bridgman-grown single crystals have a -type carrier concentration of cm and that our Cr-doped crystals have an -type
concentration of cm. Although our SnTe crystals are
diamagnetic over the temperature range , the Cr-doped crystals are room temperature ferromagnets with a Curie
temperature of 294 K. For each sample type, three-terminal capacitive
dilatometry measurements detect a subtle 0.5 micron distortion at K. Whereas our RUS measurements on SnTe show elastic hardening near the
structural transition, pointing to co-elastic behavior, similar measurements on
SnCrTe show a pronounced softening, pointing to
ferroelastic behavior. Effective Debye temperature, , values of SnTe
obtained from Sn M\"ossbauer studies show a hardening of phonons in the
range 60--115K ( = 162K) as compared with the 100--300K range
( = 150K). In addition, a precursor softening extending over
approximately 100 K anticipates this collapse at the critical temperature, and
quantitative analysis over three decades of its reduced modulus finds with , a value
indicating a three-dimensional softening of phonon branches at a temperature
K, considerably below . We suggest that the differences in
these two types of elastic behaviors lie in the absence of elastic domain wall
motion in the one case and their nucleation in the other
Negative group delay for Dirac particles traveling through a potential well
The properties of group delay for Dirac particles traveling through a
potential well are investigated. A necessary condition is put forward for the
group delay to be negative. It is shown that this negative group delay is
closely related to its anomalous dependence on the width of the potential well.
In order to demonstrate the validity of stationary-phase approach, numerical
simulations are made for Gaussian-shaped temporal wave packets. A restriction
to the potential-well's width is obtained that is necessary for the wave packet
to remain distortionless in the travelling. Numerical comparison shows that the
relativistic group delay is larger than its corresponding non-relativistic one.Comment: 10 pages, 5 figure
Renormalization-group study of Anderson and Kondo impurities in gapless Fermi systems
Thermodynamic properties are presented for four magnetic impurity models
describing delocalized fermions scattering from a localized orbital at an
energy-dependent rate which vanishes precisely at the Fermi
level, . Specifically, it is assumed that for small ,
with . The cases and
describe dilute magnetic impurities in unconventional superconductors, ``flux
phases'' of the two-dimensional electron gas, and zero-gap semiconductors. For
the nondegenerate Anderson model, the depression of the low-energy scattering
rate suppresses mixed valence in favor of local-moment behavior, and leads to a
marked reduction in the exchange coupling on entry to the local-moment regime,
with a consequent narrowing of the range of parameters within which the
impurity spin becomes Kondo-screened. The relationship between the Anderson
model and the exactly screened Kondo model with power-law exchange is examined.
The intermediate-coupling fixed point identified in the latter model by Withoff
and Fradkin (WF) has clear signatures in the thermodynamic properties and in
the local magnetic response of the impurity. The underscreened,
impurity-spin-one Kondo model and the overscreened, two-channel Kondo model
both exhibit a conditionally stable intermediate-coupling fixed point in
addition to unstable fixed points of the WF type. In all four models, the
presence or absence of particle-hole symmetry plays a crucial role.Comment: 44 two-column REVTex pages, 31 epsf-embedded EPS figures. MINOR
formatting changes. To appear in Phys. Rev.
Conditional probabilities in quantum theory, and the tunneling time controversy
It is argued that there is a sensible way to define conditional probabilities
in quantum mechanics, assuming only Bayes's theorem and standard quantum
theory. These probabilities are equivalent to the ``weak measurement''
predictions due to Aharonov {\it et al.}, and hence describe the outcomes of
real measurements made on subensembles. In particular, this approach is used to
address the question of the history of a particle which has tunnelled across a
barrier. A {\it gedankenexperiment} is presented to demonstrate the physically
testable implications of the results of these calculations, along with graphs
of the time-evolution of the conditional probability distribution for a
tunneling particle and for one undergoing allowed transmission. Numerical
results are also presented for the effects of loss in a bandgap medium on
transmission and on reflection, as a function of the position of the lossy
region; such loss should provide a feasible, though indirect, test of the
present conclusions. It is argued that the effects of loss on the pulse {\it
delay time} are related to the imaginary value of the momentum of a tunneling
particle, and it is suggested that this might help explain a small discrepancy
in an earlier experiment.Comment: 11 pages, latex, 4 postscript figures separate (one w/ 3 parts
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