3,230 research outputs found
Do Tougher Licensing Provisions Limit Occupational Entry? The Case of Dentistry
The effect of licensing as a mechanism to control entry into occupations has been a neglected area of both regulation and labor market research. This study examines the role of occupational licensing for entry into dentistry, an occupation with standards that vary by state. Our research first closely replicates Freeman's previous work on labor market cobwebs by employing national data to examine purely market phenomena in the determination of training for the dental profession. We subsequently approximate the government barrier to practice in the profession by adding a weighted average state examination pass rate to the previous model. Next, we employ pooled cross-section time series analysis to explore market determinants of professional entry with state level data. Finally, these results are supplemented by measures of statutory and pass rate entry restrictiveness. Our most consistent evidence suggests that a higher state licensing failure rate deters entry into dental practice.
Intrinsic Tunneling in Cuprates and Manganites
The most anisotropic high temperature superconductors like Bi2Sr2CaCu2O8, as
well as the recently discovered layered manganite La1.4Sr1.6Mn2O7 are layered
metallic systems where the interlayer current transport occurs via sequential
tunneling of charge carriers. As a consequence, in Bi2Sr2CaCu2O8 adjacent CuO2
double layers form an intrinsic Josephson tunnel junction while in in
La1.4Sr1.6Mn2O7 tunneling of spin polarized charge carriers between adjacent
MnO2 layers leads to an intrinsic spin valve effect. We present and discuss
interlayer transport experiments for both systems. To perform the experiments
small sized mesa structures were patterned on top of single crystals of the
above materials defining stacks of a small number of intrinsic Josephson
junctions and intrinsic spin valves, respectively.Comment: 6 pages, 8 figure
Imaging of Thermal Domains in ultrathin NbN films for Hot Electron Bolometers
We present low-temperature scanning electron microscopy (LTSEM)
investigations of superconducting microbridges made from ultrathin NbN films as
used for hot electron bolometers. LTSEM probes the thermal structure within the
microbridges under various dc current bias conditions, either via
electron-beam-induced generation of an unstable hotspot, or via the
beam-induced growth of a stable hotspot. Such measurements reveal
inhomogeneities on a micron scale, which may be due to spatial variations in
the NbN film or film-interface properties. Comparison with model calculations
for the stable hotspot regime confirm the basic features of common hot spot
models.Comment: 3 pages, 3 figure
Collective Dynamics of Josephson Vortices in Intrinsic Josephson Junctions :Exploration of In-phase Locked Superradiant Vortex Flow States
In order to clarify the ``superradiant'' conditions for the moving Josephson
vortices to excite in-phase AC electromagnetic fields over all junctions, we
perform large scale simulations of realistic dimensions for intrinsic Josephson
junctions under the layer parallel magnetic field. Three clear step-like
structures in the I-V curve are observed above a certain high field (
in the present simulations), at which we find structural transitions in the
moving flux-line lattice. The Josephson vortex flow states are accordingly
classified into four regions (region I IV with increasing current), in
each of which the power spectrum for the electric field oscillations at the
sample edge are measured and typical snapshots for Josephson vortex
configurations are displayed. Among the four regions, especially in the region
III, an in-phase rectangular vortex lattice flow state emerges and the power
spectrum shows remarkably sharp peak structure, i.e., superradiant state.
Comparison of the simulation results with an eigenmode analysis for the
transverse propagating Josephson plasma oscillations reveals that the
resonances between Josephson vortex flow states and some of the eigenmodes are
responsible for the clear flux lattice structural transitions. Furthermore, the
theoretical analysis clarifies that the width of the superradiant state region
in the I-V characteristics enlarges with decreasing both the superconducting
and insulating layer thickness.Comment: 8 pages, Revtex, 7 figures; figure arrangements improved. no changes
in tex
Simulation of I-V Hysteresis Branches in An Intrinsic Stack of Josephson Junctions in High Superconductors
I-V characteristics of the high T superconductor
BiSrCaCO shows a strong hysteresis, producing many
branches. The origin of hysteresis jumps is studied by use of the model of
multi-layered Josephson junctions proposed by one of the authors (T. K.). The
charging effect at superconducting layers produces a coupling between the next
nearest neighbor phase-differences, which determines the structure of
hysteresis branches. It will be shown that a solution of phase motions is
understood as a combination of rotating and oscillating phase-differences, and
that, at points of hysteresis jumps, there occurs a change in the number of
rotating phase-differences. Effects of dissipation are analyzed. The
dissipation in insulating layers works to damp the phase motion itself, while
the dissipation in superconducting layers works to damp relative motions of
phase-differences. Their effects to hysteresis jumps are discussed.Comment: 18 pages, Latex, 8 figures. To be appear in Phys.Rev.B Vol.60(1999
Dynamics of semifluxons in Nb long Josephson 0-pi junctions
We propose, implement and test experimentally long Josephson 0-pi junctions
fabricated using conventional Nb-AlOx-Nb technology. We show that using a pair
of current injectors, one can create an arbitrary discontinuity of the
Josephson phase and in particular a pi-discontinuity, just like in
d-wave/s-wave or in d-wave/d-wave junctions, and study fractional Josephson
vortices which spontaneously appear. Moreover, using such junctions, we can
investigate the \emph{dynamics} of the fractional vortices -- a domain which is
not yet available for natural 0-pi-junctions due to their inherently high
damping. We observe half-integer zero-field steps which appear on the
current-voltage characteristics due to hopping of semifluxons.Comment: Fractional vortices in conventional superconductors ;-
Hot spots and waves in Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks -a study by Low Temperature Scanning Laser Microscopy
Recently, it has been shown that large stacks of intrinsic Josephson
junctions in Bi2Sr2CaCu2O8 emit synchronous THz radiation, the synchronization
presumably triggered by a cavity resonance. To investigate this effect we use
Low Temperature Scanning Laser Microscopy to image electric field
distributions. Apart from verifying the appearance of cavity modes at low bias
we find that, in a high input power regime, standing-wave patterns are created
through interactions with a hot spot, possibly pointing to a new mode of
generating synchronized radiation in intrinsic Josephson junction stacks.Comment: 6 pages, 5 figures, supplementary information include
Coupling between phonons and intrinsic Josephson oscillations in cuprate superconductors
The recently reported subgap structures observed in the current-voltage
characteristic of intrinsic Josephson junctions in the high-T_c superconductors
Tl_2Ba_2Ca_2Cu_3O_{10+\delta} and Bi_2Sr_2CaCu_2O_{8+\delta} are explained by
the coupling between c-axis phonons and Josephson oscillations. A model is
developed where c-axis lattice vibrations between adjacent superconducting
multilayers are excited by the Josephson oscillations in a resistive junction.
The voltages of the lowest structures correspond well to the frequencies of
longitudinal c-axis phonons with large oscillator strength in the two
materials, providing a new measurement technique for this quantity.Comment: 4 pages, 3 figures, revtex, aps, epsf, psfig. submitted to Physical
Review Letters, second version improved in detai
Theory for Photon-Assisted Macroscopic Quantum Tunneling in a Stack of Intrinsic Josephson Junctions
We propose a theory for photon-assisted macroscopic quantum tunneling (MQT)
in a stack of capacitively-coupled intrinsic Josephson junctions in which the
longitudinal Josephson plasma, i.e., longitudinal collective phase oscillation
modes, is excited. The scheme of energy-level quantization in the collective
oscillatory states is clarified in the -junction system. When the MQT occurs
from the single-plasmon states excited by microwave irradiation in the
multi-photon process to the uniform voltage state, our theory predicts that the
escape rate is proportional to . This result is consistent with the recent
observation in Bi-2212 intrinsic Josephson junctions.Comment: 5 pages, 2 figure
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