9,138 research outputs found
Scaling Theory for Steady State Plastic Flows in Amorphous Solids
Strongly correlated amorphous solids are a class of glass-formers whose
inter-particle potential admits an approximate inverse power-law form in a
relevant range of inter-particle distances. We study the steady-state plastic
flow of such systems, firstly in the athermal, quasi-static limit, and secondly
at finite temperatures and strain rates. In all cases we demonstrate the
usefulness of scaling concepts to reduce the data to universal scaling
functions where the scaling exponents are determined a-priori from the
inter-particle potential. In particular we show that the steady plastic flow at
finite temperatures with efficient heat extraction is uniquely characterized by
two scaled variables; equivalently, the steady state displays an equation of
state that relates one scaled variable to the other two. We discuss the range
of applicability of the scaling theory, and the connection to density scaling
in supercooled liquid dynamics. We explain that the description of transient
states calls for additional state variables whose identity is still far from
obvious.Comment: 9 pages, 9 figure
Analysis of microwave radiometric measurements from Skylab
There are no author-identified significant results in this report
Pseudo diamagnetism of four component exciton condensates
We analyze the spin structure of the ground state of four-component exciton
condensates in coupled quantum wells as a function of spin-dependent
interactions and applied magnetic field. The four components correspond to the
degenerate exciton states characterized by and spin projections
to the axis of the structure. We show that in a wide range of parameters, the
chemical potential of the system increases as a function of magnetic field,
which manifests a pseudo-diamagnetism of the system. The transitions to
polarized two- and one-component condensates can be of the first-order in this
case. The predicted effects are caused by energy conserving mixing of
and excitons.Comment: 4 pages, 2 figure
Statistical Physics of Elasto-Plastic Steady States in Amorphous Solids: Finite Temperatures and Strain Rates
The effect of finite temperature and finite strain rate on
the statistical physics of plastic deformations in amorphous solids made of
particles is investigated. We recognize three regimes of temperature where the
statistics are qualitatively different. In the first regime the temperature is
very low, , and the strain is quasi-static. In this regime
the elasto-plastic steady state exhibits highly correlated plastic events whose
statistics are characterized by anomalous exponents. In the second regime
the system-size dependence of the
stress fluctuations becomes normal, but the variance depends on the strain
rate. The physical mechanism of the cross-over is different for increasing
temperature and increasing strain rate, since the plastic events are still
dominated by the mechanical instabilities (seen as an eigenvalue of the Hessian
matrix going to zero), and the effect of temperature is only to facilitate the
transition. A third regime occurs above the second cross-over temperature
where stress fluctuations become dominated by thermal
noise. Throughout the paper we demonstrate that scaling concepts are highly
relevant for the problem at hand, and finally we present a scaling theory that
is able to collapse the data for all the values of temperatures and strain
rates, providing us with a high degree of predictability.Comment: 12 pages, 13 figure
Cosine and Sine Operators Related with Orthogonal Polynomial Sets on the Intervall [-1,1]
The quantization of phase is still an open problem. In the approach of
Susskind and Glogower so called cosine and sine operators play a fundamental
role. Their eigenstates in the Fock representation are related with the
Chebyshev polynomials of the second kind. Here we introduce more general cosine
and sine operators whose eigenfunctions in the Fock basis are related in a
similar way with arbitrary orthogonal polynomial sets on the intervall [-1,1].
To each polynomial set defined in terms of a weight function there corresponds
a pair of cosine and sine operators. Depending on the symmetry of the weight
function we distinguish generalized or extended operators. Their eigenstates
are used to define cosine and sine representations and probability
distributions. We consider also the inverse arccosine and arcsine operators and
use their eigenstates to define cosine-phase and sine-phase distributions,
respectively. Specific, numerical and graphical results are given for the
classical orthogonal polynomials and for particular Fock and coherent states.Comment: 1 tex-file (24 pages), 11 figure
Crossover from diffusive to strongly localized regime in two-dimensional systems
We have studied the conductance distribution function of two-dimensional
disordered noninteracting systems in the crossover regime between the diffusive
and the localized phases. The distribution is entirely determined by the mean
conductance, g, in agreement with the strong version of the single-parameter
scaling hypothesis. The distribution seems to change drastically at a critical
value very close to one. For conductances larger than this critical value, the
distribution is roughly Gaussian while for smaller values it resembles a
log-normal distribution. The two distributions match at the critical point with
an often appreciable change in behavior. This matching implies a jump in the
first derivative of the distribution which does not seem to disappear as system
size increases. We have also studied 1/g corrections to the skewness to
quantify the deviation of the distribution from a Gaussian function in the
diffusive regime.Comment: 4 pages, 4 figure
Gene identification for the cblD defect of vitamin B12 metabolism
Background Vitamin B12 (cobalamin) is an essential cofactor in several metabolic pathways. Intracellular conversion of cobalamin to its two coenzymes, adenosylcobalamin in mitochondria and methylcobalamin in the cytoplasm, is necessary for the homeostasis of methylmalonic acid and homocysteine. Nine defects of intracellular cobalamin metabolism have been defined by means of somatic complementation analysis. One of these defects, the cblD defect, can cause isolated methylmalonic aciduria, isolated homocystinuria, or both. Affected persons present with multisystem clinical abnormalities, including developmental, hematologic, neurologic, and metabolic findings. The gene responsible for the cblD defect has not been identified.
Methods We studied seven patients with the cblD defect, and skin fibroblasts from each were investigated in cell culture. Microcell-mediated chromosome transfer and refined genetic mapping were used to localize the responsible gene. This gene was transfected into cblD fibroblasts to test for the rescue of adenosylcobalamin and methylcobalamin synthesis.
Results The cblD gene was localized to human chromosome 2q23.2, and a candidate gene, designated MMADHC (methylmalonic aciduria, cblD type, and homocystinuria), was identified in this region. Transfection of wild-type MMADHC rescued the cellular phenotype, and the functional importance of mutant alleles was shown by means of transfection with mutant constructs. The predicted MMADHC protein has sequence homology with a bacterial ATP-binding cassette transporter and contains a putative cobalamin binding motif and a putative mitochondrial targeting sequence.
Conclusions Mutations in a gene we designated MMADHC are responsible for the cblD defect in vitamin B12 metabolism. Various mutations are associated with each of the three biochemical phenotypes of the disorder
Toward a microscopic description of flow near the jamming threshold
We study the relationship between microscopic structure and viscosity in
non-Brownian suspensions. We argue that the formation and opening of contacts
between particles in flow effectively leads to a negative selection of the
contacts carrying weak forces. We show that an analytically tractable model
capturing this negative selection correctly reproduces scaling properties of
flows near the jamming transition. In particular, we predict that (i) the
viscosity {\eta} diverges with the coordination z as {\eta} ~
(z_c-z)^{-(3+{\theta})/(1+{\theta})}, (ii) the operator that governs flow
displays a low-frequency mode that controls the divergence of viscosity, at a
frequency {\omega}_min\sim(z_c-z)^{(3+{\theta})/(2+2{\theta})}, and (iii) the
distribution of forces displays a scale f* that vanishes near jamming as
f*/\sim(z_c-z)^{1/(1+{\theta})} where {\theta} characterizes the
distribution of contact forces P(f)\simf^{\theta} at jamming, and where z_c is
the Maxwell threshold for rigidity.Comment: 6 pages, 4 figure
One-dimensional transport of bosons between weakly linked reservoirs
We study a flow of ultracold bosonic atoms through a one-dimensional channel that connects two macroscopic three-dimensional reservoirs of Bose-condensed atoms via weak links implemented as potential barriers between each of the reservoirs and the channel. We consider reservoirs at equal chemical potentials so that a superflow of the quasicondensate through the channel is driven purely by a phase difference 2Φ imprinted between the reservoirs. We find that the superflow never has the standard Josephson form ∼ sin 2Φ. Instead, the superflow discontinuously flips direction at 2Φ ¼ _π and has metastable branches.We show that these features are robust and not smeared by fluctuations or phase slips. We describe a possible experimental setup for observing these phenomen
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