147 research outputs found
Characteristics of partially filled Helmholtz resonators
In this work we have developed a technique for the measurement of the
resonance curve of Helmholtz resonators as a function of filling with beads and
sands of different sizes, and water as the reference. Our measurements allowed
us to observe very different behaviors of resonance frequencies and resonance
half-widths as a function of the size of the sand grains and the beads. By
comparing results for beads and water we were able to prove that the sound
penetrates the interstitial space between the beads. This was confirmed by
measurements of the resonance properties in resonators filled with spherical
beads whose experimentally determined filling factor is close to the random
filling factor of about 0.54. The similar behavior of the frequency and
half-width of resonance of sand-filled resonators of three different sizes
allowed us to suggest that sound penetrates the sand in the same way as it does
the beads.Comment: 10 pages, one column, 9 figures, regular pape
Supercooling of the disordered vortex lattice in Bi_2Sr_2CaCu_2O_8+d
Time-resolved local induction measurements near to the vortex lattice
order-disorder transition in optimally doped
BiSrCaCuO single crystals shows that the
high-field, disordered phase can be quenched to fields as low as half the
transition field. Over an important range of fields, the electrodynamical
behavior of the vortex system is governed by the co-existence of the two phases
in the sample. We interpret the results in terms of supercooling of the
high-field phase and the possible first order nature of the order-disorder
transition at the ``second peak''.Comment: 4 pages, 3 figures. Submitted to Nature, July 10th, 1999; Rejected
August 8th for lack of broad interest Submitted to Physical Review Letters
September 10th, 199
Understanding the origins of the intrinsic dead-layer effect in nanocapacitors
Thin films of high-permittivity dielectrics are considered ideal candidates
for realizing high charge density nanosized capacitors for use in next
generation energy storage and nanoelectronic applications. The experimentally
observed capacitance of such film nanocapacitors is, however, an order of
magnitude lower than expected. This dramatic drop in capacitance is attributed
to the so called dead layer - a low-permittivity layer at the metal-dielectric
interface in series with the high-permittivity dielectric. The exact nature of
the dead layer and the reasons for its origin still remain somewhat unclear.
Based on insights gained from recently published ab initio work on
SrRuO3/SrTiO3/SrRuO3 and our first principle simulations on Au/MgO/Au and
Pt/MgO/Pt nanocapacitors, we construct an analytical model that isolates the
contributions of various physical mechanisms to the intrinsic dead layer. In
particular we argue that strain-gradients automatically arise in very thin
films even in absence of external strain inducers and, due to flexoelectric
coupling, are dominant contributors to the dead layer effect. Our theoretical
results compare well with existing as well as our own ab initio calculations
and suggest that inclusion of flexoelectricity is necessary for qualitative
reconciliation of atomistic results. Our results also hint at some novel
remedies for mitigating the dead layer effect.Comment: 17 pages, 6 figure
Hydrodynamic Instability of the Flux-antiflux Interface in Type-II Superconductors
The macroturbulence instability observed in fluxline systems during
remagnetization of superconductors is explained. It is shown that when a region
with flux is invaded by antiflux the interface can become unstable if there is
a relative tangential flux motion. This condition occurs at the interface when
the viscosity is anisotropic, e.g., due to flux guiding by twin boundaries in
crystals. The phenomenon is similar to the instability of the tangential
discontinuity in classical hydrodynamics. The obtained results are supported by
magneto-optical observations of flux distribution on the surface of a YBCO
single crystal with twins.Comment: 12 pages, 3 figures, submitted to Physical Review Letter
Superconductor strip with transport current: Magneto-optical study of current distribution and its relaxation
The dynamics of magnetic flux distributions across a YBaCuO strip carrying
transport current is measured using magneto-optical imaging at 20 K. The
current is applied in pulses of 40-5000 ms duration and magnitude close to the
critical one, 5.5 A. During the pulse some extra flux usually penetrates the
strip, so the local field increases in magnitude. When the strip is initially
penetrated by flux, the local field either increases or decreases depending
both on the spatial coordinate and the current magnitude. Meanwhile, the
current density always tends to redistribute more uniformly. Despite the
relaxation, all distributions remain qualitatively similar to the Bean model
predictions.Comment: RevTeX, 9 pages, 9 figures, submitted to Supercond. Sci. Technol.
Revision: MO image and more refs are adde
Raman spectra of MgB2 at high pressure and topological electronic transition
Raman spectra of the MgB2 ceramic samples were measured as a function of
pressure up to 32 GPa at room temperature. The spectrum at normal conditions
contains a very broad peak at ~590 cm-1 related to the E2g phonon mode. The
frequency of this mode exhibits a strong linear dependence in the pressure
region from 5 to 18 GPa, whereas beyond this region the slope of the
pressure-induced frequency shift is reduced by about a factor of two. The
pressure dependence of the phonon mode up to ~ 5GPa exhibits a change in the
slope as well as a "hysteresis" effect in the frequency vs. pressure behavior.
These singularities in the E2g mode behavior under pressure support the
suggestion that MgB2 may undergo a pressure-induced topological electronic
transition.Comment: 2 figure
Iterated Moire Maps and Braiding of Chiral Polymer Crystals
In the hexagonal columnar phase of chiral polymers a bias towards cholesteric
twist competes with braiding along an average direction. When the chirality is
strong, screw dislocations proliferate, leading to either a tilt grain boundary
phase or a new "moire state" with twisted bond order. Polymer trajectories in
the plane perpendicular to their average direction are described by iterated
moire maps of remarkable complexity.Comment: 10 pages (plain tex) 3 figures uufiled and appende
Flux penetration and expulsion in thin superconducting disks
Using an expansion of the order parameter over the eigenfunctions of the
linearized first Ginzburg-Landau (GL) equation, we obtain numerically the
saddle points of the free energy separating the stable states with different
number of vortices. In contrast to known surface and geometrical barrier
models, we find that in a wide range of magnetic fields below the penetration
field, the saddle point state for flux penetration into a disk does not
correspond to a vortex located nearby the sample boundary, but to a region of
suppressed superconductivity at the disk edge with no winding of the current,
and which is {\it a nucleus} for the following vortex creation. The height of
this {\it nucleation barrier}, which determines the time of flux penetration,
is calculated for different disk radii and magnetic fields.Comment: Accepted for publication in Physical Review Letter
Large Predicted Self-Field Critical Current Enhancements In Superconducting Strips Using Magnetic Screens
A transport current distribution over a wide superconducting sheet is shown
to strongly change in a presence of bulk magnetic screens of a soft magnet with
a high permeability. Depending on the geometry, the effect may drastically
suppress or protect the Meissner state of the sheet through the enhancement or
suppression of the edge barrier critical current. The total transport current
in the magnetically screened Meissner state is expected to compete with the
critical current of the flux-filled sheet only for samples whose critical
current is initially essentially controlled by the edge barrier effect.Comment: 6 figure
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