156 research outputs found
A Development of Readiness for First Grade
This paper is an attempt to develop a readiness program for grade one. In order to delimit this study, visual perception was chosen because it is of primary importance in the development of reading. It is, however, only one of the factors involved in the development of the reading readiness program
Effect of thermal phase fluctuations on the superfluid density of two-dimensional superconducting films
High precision measurements of the complex sheet conductivity of
superconducting Mo77Ge23 thin films have been made from 0.4 K through Tc. A
sharp drop in the inverse sheet inductance, 1/L(T), is observed at a
temperature, Tc, which lies below the mean-field transition temperature, Tco.
Just below Tc, the suppression of 1/L(T) below its mean-field value indicates
that longitudinal phase fluctuations have nearly their full classical
amplitude, but they disappear rapidly as T decreases. We argue that there is a
quantum crossover at about 0.94 Tco, below which classical phase fluctuations
are suppressed.Comment: 14 pages, 3 figures. Subm. to PR
Dynamic Impedance of Two-Dimensional Superconducting Films Near the Superconducting Transition
The sheet impedances, Z(w,T), of several superconducting a-Mo77Ge23 films and
one In/InOx film have been measured in zero field using a two-coil mutual
inductance technique at frequencies from 100 Hz to 100 kHz. Z(w,T) is found to
have three contributions: the inductive superfluid, renormalized by nonvortex
phase fluctuations; conventional vortex-antivortex pairs, whose contribution
turns on very rapidly just below the usual Kosterlitz-Thouless-Berezinskii
unbinding temperature; and an anomalous contribution. The latter is
predominantly resistive, persists well below the KTB temperature, and is weakly
dependent on frequency down to remarkably low frequencies, at least 100 Hz. It
increases with T as e-U'(T)/kT, where the activation energy, U'(T), is about
half the energy to create a vortex-antivortex pair, indicating that the
frequency dependence is that of individual excitations, rather than critical
behavior.Comment: 10 pages, 10 figs; subm PR
Effect of the pseudogap on the mean-field magnetic penetration depth of YBCO thin films
We report measurements of the magnetic penetration depth in YBCO films at
various oxygen concentrations. At optimal doping, critical fluctuation effects
are absent, and the penetration depth from 4 K to 0.99 Tc is well described by
d-wave, BCS, strong-coupling theory with a gap, Delta0/ kT ~ 3.3. This implies
that the T-dependence of the penetration depth comes largely from
single-particle excitations. As in crystals, underdoping reduces the zero
temperature superfluid density without affecting the low-T slope or curvature
of the penetration depth. We show that these results, as well as heat capacity
measurements, are well described by an ad hoc model in which superfluid is lost
from regions of the Fermi surface occupied by the pseudogap while the low lying
excitations near the nodes remain unaffected.Comment: 4 pages, 4 eps figures, RevTex; Submitted to PRL July 199
Simulations of the inelastic response of silicon to shock compression
Recent experiments employing nanosecond white-light X-ray diffraction have demonstrated a complex response of pure, single crystal silicon to shock compression on ultra-fast timescales. We present here details of a Lagrangian code which tracks both longitudinal and transverse strains, and successfully reproduces the experimental response by incorporating a model of the shock-induced, yet kinetically inhibited, phase transition. This model is also shown to reproduce results of classical molecular dynamics simulations of shock compressed silicon
Experimental Study of the Inductance of Pinned Vortices in Superconducting YBa2Cu3O7-d Films
Using a two-coil mutual inductance method, we have measured the complex
resistivity, rho_v(T,Be), of pinned vortices in c-axis pulsed laser deposited
YBa2Cu3O7-d films with magnetic field Be applied perpendicular to the film. At
low frequencies, (<100 kHz), rho_v is inductive and is inversely proportional
to the Labusch parameter, the average vortex pinning force constant, kappa_exp.
The observed weakening of kappa_exp with Be is consistent with a simple model
based on linear pinning defects. Adding classical thermal fluctuations to the
model in a simple way describes the observed linear T dependence of rho_v,
below ~15 K and provides reasonable values for the effective radius (.3 nm to
>.8 nm) of the defects and the depth of the pinning potential. The success of
this model implies that thermal supercurrent (phase) fluctuations have their
full classical amplitude down to 5 K for frequencies below the characteristic
depinning frequency. To date, no sufficient theory exists to explain the data
between ~15 K and the vortex glass melting temperature.Comment: 31 pages, 8 figures. Subm. to PR
Inelastic response of silicon to shock compression
The elastic and inelastic response of [001] oriented silicon to laser compression has been a topic of considerable discussion for well over a decade, yet there has been little progress in understanding the basic behaviour of this apparently simple material. We present experimental x-ray diffraction data showing complex elastic strain profiles in laser compressed samples on nanosecond timescales. We also present molecular dynamics and elasticity code modelling which suggests that a pressure induced phase transition is the cause of the previously reported 'anomalous' elastic waves. Moreover, this interpretation allows for measurement of the kinetic timescales for transition. This model is also discussed in the wider context of reported deformation of silicon to rapid compression in the literature
Questioning the Equivalence Principle
The Equivalence Principle (EP) is not one of the ``universal'' principles of
physics (like the Action Principle). It is a heuristic hypothesis which was
introduced by Einstein in 1907, and used by him to construct his theory of
General Relativity. In modern language, the (Einsteinian) EP consists in
assuming that the only long-range field with gravitational-strength couplings
to matter is a massless spin-2 field. Modern unification theories, and notably
String Theory, suggest the existence of new fields (in particular, scalar
fields: ``dilaton'' and ``moduli'') with gravitational-strength couplings. In
most cases the couplings of these new fields ``violate'' the EP. If the field
is long-ranged, these EP violations lead to many observable consequences
(variation of ``constants'', non-universality of free fall, relative drift of
atomic clocks,...). The best experimental probe of a possible violation of the
EP is to compare the free-fall acceleration of different materials.Comment: 14 pages, contribution to the ONERA workshop on "Missions spatiales
en physique fondamentale" (Chatillon, 18-19 Jan 2001), to appear in a special
issue of the Comptes Rendus de l'Academie des Sciences (Paris), edited by C.
Borde and P. Toubou
Evidence for a Nodeless Gap from the Superfluid Density of Optimally Doped Pr_{1.855}Ce_{0.145}CuO_{4-y} Films
We present measurements of the ab-plane magnetic penetration depth,
\lambda(T), in five optimally doped Pr_{1.855}Ce_{0.145}CuO_{4-y} films for 1.6
K \leq T \leq T_c \sim 24 K. Low resistivities, high superfluid densities
n_s(T)\propto \lambda^{-2}(T), high T_c's, and small transition widths are
reproducible and indicative of excellent film quality. For all five films,
\lambda^{-2}(T)/\lambda^{-2}(0) at low T is well fitted by an exponential
temperature dependence with a gap, \Delta_{min}, of 0.85 k_B T_c. This behavior
is consistent with a nodeless gap and is incompatible with d-wave
superconductivity.Comment: 5 pages, 4 figures, reorganized for clarit
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