53 research outputs found
Recommended from our members
The Use of a Solid Hydrocarbon as a Graphite Substitute for Astaloy CrM Sintered Steel
YesAbstract
Höganäs Astaloy CrM powder was used to prepare mixtures with 0.3-1.6 % carbon contents, both with and without 1 wt.% manganese additions. The carbon was added in three ways: as a graphite powder, as a solid CnHm hydrocarbon powder, and as a mixture of both. Green compacts were pressed at 300 - 800 MPa and sintered isothermally at temperatures in the range 1170 - 1295°C under flowing high purity nitrogen or nitrogen/hydrogen (9:1) atmosphere. Compressibility of the powder mixtures was investigated. Carbon loss occurring during sintering was carefully monitored. Sintering behaviour of numerous combinations of carbon content was investigated by dilatometry. For high carbon contents and high sintering temperatures, densification resulted from controlled generation of a liquid phase. Advantages of using solid hydrocarbon as a carbon donor and of Mn addition in powder metallurgy processing of steels are indicated
Statistical Signatures of Photon Localization
The realization that electron localization in disordered systems (Anderson
localization) is ultimately a wave phenomenon has led to the suggestion that
photons could be similarly localized by disorder. This conjecture attracted
wide interest because the differences between photons and electrons - in their
interactions, spin statistics, and methods of injection and detection - may
open a new realm of optical and microwave phenomena, and allow a detailed study
of the Anderson localization transition undisturbed by the Coulomb interaction.
To date, claims of three-dimensional photon localization have been based on
observations of the exponential decay of the electromagnetic wave as it
propagates through the disordered medium. But these reports have come under
close scrutiny because of the possibility that the decay observed may be due to
residual absorption, and because absorption itself may suppress localization.
Here we show that the extent of photon localization can be determined by a
different approach - measurement of the relative size of fluctuations of
certain transmission quantities. The variance of relative fluctuations
accurately reflects the extent of localization, even in the presence of
absorption. Using this approach, we demonstrate photon localization in both
weakly and strongly scattering quasi-one-dimensional dielectric samples and in
periodic metallic wire meshes containing metallic scatterers, while ruling it
out in three-dimensional mixtures of aluminum spheres.Comment: 5 pages, including 4 figure
A New Type of Intensity Correlation in Random Media
A monochromatic point source, embedded in a three-dimensional disordered
medium, is considered. The resulting intensity pattern exhibits a new type of
long-range correlations. The range of these correlations is infinite and their
magnitude, normalized to the average intensity, is of order , where
and are the wave number and the mean free path respectively.Comment: RevTeX, 8 pages, 3 figures, Accepted to Phys. Rev. Let
Dynamic Correlation in Wave Propagation in Random Media
We report time-resolved measurements of the statistics of pulsed transmission
through quasi-one-dimensional dielectric media with static disorder. The
normalized intensity correlation function with displacement and polarization
rotation for an incident pulse of linewidth at delay time t is a
function only of the field correlation function, which is identical to that
found for steady-state excitation, and of , the residual
degree of intensity correlation at points at which the field correlation
function vanishes. The dynamic probability distribution of normalized intensity
depends only upon . Steady-state statistics are recovered
in the limit ->0, in which is the steady-state
degree of correlation.Comment: 4 RevTex pages, 4 figure
Measurement of the Probability Distribution of Total Transmission in Random Waveguides
Measurements have been made of the probability distribution of total
transmission of microwave radiation in waveguides filled with randomly
positioned scatterers which would have values of the dimensionless conductance
g near unity. The distributions are markedly non-Gaussian and have exponential
tails. The measured distributions are accurately described by diagrammatic and
random matrix calculations carried out for nonabsorbing samples in the limit g
>> 1 when g is expressed in terms of the variance of the distribution, which
equals the degree of long-range intensity correlation across the output face of
the sample.Comment: 5 pages, 5 post script figures, RevTe
High-frequency dynamics of wave localisation
We study the effect of localisation on the propagation of a pulse through a
multi-mode disordered waveguide. The correlator of the
transmitted wave amplitude u at two frequencies differing by delta_omega has
for large delta_omega the stretched exponential tail ~exp(-sqrt{tau_D
delta_omega/2}). The time constant tau_D=L^2/D is given by the diffusion
coefficient D, even if the length L of the waveguide is much greater than the
localisation length xi. Localisation has the effect of multiplying the
correlator by a frequency-independent factor exp(-L/2xi), which disappears upon
breaking time-reversal symmetry.Comment: 3 pages, 1 figur
Photonic excess noise and wave localization
This is a theory for the effect of localization on the super-Poissonian noise
of radiation propagating through an absorbing disordered waveguide.
Localization suppresses both the mean photon current I and the noise power P,
but the Fano factor P/I is found to remain unaffected. For strong absorption
the Fano factor has the universal value 1+3f/2 (with f the Bose-Einstein
function), regardless of whether the waveguide is long or short compared to the
localization length.Comment: 3 pages including 3 figure
Field and intensity correlations in random media
Measurements of the microwave field transmitted through a random medium
allows direct access to the field correlation function, whose complex square is
the short range or C1 contribution to the intensity correlation function C. The
frequency and spatial correlation function are compared to their Fourier pairs,
the time of flight distribution and the specific intensity, respectively. The
longer range contribution to intensity correlation is obtained directly by
subtracting C1 from C and is in good agreement with theory.Comment: 9 pages, 5 figures, submitted to Phys.Rev.
Wave Scattering through Classically Chaotic Cavities in the Presence of Absorption: An Information-Theoretic Model
We propose an information-theoretic model for the transport of waves through
a chaotic cavity in the presence of absorption. The entropy of the S-matrix
statistical distribution is maximized, with the constraint : n is the dimensionality of S, and meaning complete (no) absorption. For strong absorption our result
agrees with a number of analytical calculations already given in the
literature. In that limit, the distribution of the individual (angular)
transmission and reflection coefficients becomes exponential -Rayleigh
statistics- even for n=1. For Rayleigh statistics is attained even
with no absorption; here we extend the study to . The model is
compared with random-matrix-theory numerical simulations: it describes the
problem very well for strong absorption, but fails for moderate and weak
absorptions. Thus, in the latter regime, some important physical constraint is
missing in the construction of the model.Comment: 4 pages, latex, 3 ps figure
Photon Localization in Resonant Media
We report measurements of microwave transmission over the first five Mie
resonances of alumina spheres randomly positioned in a waveguide. Though
precipitous drops in transmission and sharp peaks in the photon transit time
are found near all resonances, measurements of transmission fluctuations show
that localization occurs only in a narrow frequency window above the first
resonance. There the drop in the photon density of states is found to be more
pronounced than the fall in the photon transit time, leading to a minimum in
the Thouless number.Comment: To appear in PRL; 5 pages, including 5 figure
- …