285 research outputs found
Thermoelectric Modeling of the Non-Ohmic Differential Conductance in a Tunnel Junction containing a Pinhole
To test the quality of a tunnel junction, one sometimes fits the
bias-dependent differential conductance to a theoretical model, such as
Simmons's formula. Recent experimental work by {\AA}kerman and collaborators,
however, has demonstrated that a good fit does not necessarily imply a good
junction. Modeling the electrical and thermal properties of a tunnel junction
containing a pinhole, we extract an effective barrier height and effective
barrier width even when as much as 88% of the current flows through the pinhole
short rather than tunneling. A good fit of differential conductance to a
tunneling form therefore cannot rule out pinhole defects in normal-metal or
magnetic tunnel junctions.Comment: Revtex, 5 figure
Fourier-Space Crystallography as Group Cohomology
We reformulate Fourier-space crystallography in the language of cohomology of
groups. Once the problem is understood as a classification of linear functions
on the lattice, restricted by a particular group relation, and identified by
gauge transformation, the cohomological description becomes natural. We review
Fourier-space crystallography and group cohomology, quote the fact that
cohomology is dual to homology, and exhibit several results, previously
established for special cases or by intricate calculation, that fall
immediately out of the formalism. In particular, we prove that {\it two phase
functions are gauge equivalent if and only if they agree on all their
gauge-invariant integral linear combinations} and show how to find all these
linear combinations systematically.Comment: plain tex, 14 pages (replaced 5/8/01 to include archive preprint
number for reference 22
Photonic quasicrystals for general purpose nonlinear optical frequency conversion
We present a general method for the design of 2-dimensional nonlinear
photonic quasicrystals that can be utilized for the simultaneous phase-matching
of arbitrary optical frequency-conversion processes. The proposed scheme--based
on the generalized dual-grid method that is used for constructing tiling models
of quasicrystals--gives complete design flexibility, removing any constraints
imposed by previous approaches. As an example we demonstrate the design of a
color fan--a nonlinear photonic quasicrystal whose input is a single wave at
frequency and whose output consists of the second, third, and fourth
harmonics of , each in a different spatial direction
Distinguishing cancerous from non-cancerous cells through analysis of electrical noise
Since 1984, electric cell-substrate impedance sensing (ECIS) has been used to
monitor cell behavior in tissue culture and has proven sensitive to cell
morphological changes and cell motility. We have taken ECIS measurements on
several cultures of non-cancerous (HOSE) and cancerous (SKOV) human ovarian
surface epithelial cells. By analyzing the noise in real and imaginary
electrical impedance, we demonstrate that it is possible to distinguish the two
cell types purely from signatures of their electrical noise. Our measures
include power-spectral exponents, Hurst and detrended fluctuation analysis, and
estimates of correlation time; principal-component analysis combines all the
measures. The noise from both cancerous and non-cancerous cultures shows
correlations on many time scales, but these correlations are stronger for the
non-cancerous cells.Comment: 8 pages, 4 figures; submitted to PR
Pinholes May Mimic Tunneling
Interest in magnetic-tunnel junctions has prompted a re-examination of
tunneling measurements through thin insulating films. In any study of
metal-insulator-metal trilayers, one tries to eliminate the possibility of
pinholes (small areas over which the thickness of the insulator goes to zero so
that the upper and lower metals of the trilayer make direct contact). Recently,
we have presented experimental evidence that ferromagnet-insulator-normal
trilayers that appear from current-voltage plots to be pinhole-free may
nonetheless in some cases harbor pinholes. Here, we show how pinholes may arise
in a simple but realistic model of film deposition and that purely classical
conduction through pinholes may mimic one aspect of tunneling, the exponential
decay in current with insulating thickness.Comment: 9 pages, 3 figures, plain TeX; submitted to Journal of Applied
Physic
A Spin Model for Investigating Chirality
Spin chirality has generated great interest recently both from possible
applications to flux phases and intrinsically, as an example of a several-site
magnetic order parameter that can be long-ranged even where simpler order
parameters are not. Previous work (motivated by the flux phases) has focused on
antiferromagnetic chiral order; we construct a model in which the chirality
orders ferromagnetically and investigate the model's behavior as a function of
spin. Enlisting the aid of exact diagonalization, spin-waves, perturbation
theory, and mean fields, we conclude that the model likely has long-ranged
chiral order for spins 1 and greater and no non-trivial chiral order for spin
1/2.Comment: uuencoded gzipped tarred plain tex fil
Extinctions and Correlations for Uniformly Discrete Point Processes with Pure Point Dynamical Spectra
The paper investigates how correlations can completely specify a uniformly
discrete point process. The setting is that of uniformly discrete point sets in
real space for which the corresponding dynamical hull is ergodic. The first
result is that all of the essential physical information in such a system is
derivable from its -point correlations, . If the system is
pure point diffractive an upper bound on the number of correlations required
can be derived from the cycle structure of a graph formed from the dynamical
and Bragg spectra. In particular, if the diffraction has no extinctions, then
the 2 and 3 point correlations contain all the relevant information.Comment: 16 page
Crossover from Poisson to Wigner-Dyson Level Statistics in Spin Chains with Integrability Breaking
We study numerically the evolution of energy-level statistics as an
integrability-breaking term is added to the XXZ Hamiltonian. For finite-length
chains, physical properties exhibit a cross-over from behavior resulting from
the Poisson level statistics characteristic of integrable models to behavior
corresponding to the Wigner-Dyson statistics characteristic of the
random-matrix theory used to describe chaotic systems. Different measures of
the level statistics are observed to follow different crossover patterns. The
range of numerically accessible system sizes is too small to establish with
certainty the scaling with system size, but the evidence suggests that in a
thermodynamically large system an infinitesimal integrability breaking would
lead to Wigner-Dyson behavior.Comment: 8 pages, 8 figures, Revtex
Level Statistics of XXZ Spin Chains with Discrete Symmetries: Analysis through Finite-size Effects
Level statistics is discussed for XXZ spin chains with discrete symmetries
for some values of the next-nearest-neighbor (NNN) coupling parameter. We show
how the level statistics of the finite-size systems depends on the NNN coupling
and the XXZ anisotropy, which should reflect competition among quantum chaos,
integrability and finite-size effects. Here discrete symmetries play a central
role in our analysis. Evaluating the level-spacing distribution, the spectral
rigidity and the number variance, we confirm the correspondence between
non-integrability and Wigner behavior in the spectrum. We also show that
non-Wigner behavior appears due to mixed symmetries and finite-size effects in
some nonintegrable cases.Comment: 19 pages, 6 figure
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