186,889 research outputs found
Observation of Fluctuation-Dissipation-Theorem Violations in a Structural Glass
The fluctuation-dissipation theorem (FDT), connecting dielectric
susceptibility and polarization noise was studied in glycerol below its glass
transition temperature Tg. Weak FDT violations were observed after a quench
from just above to just below Tg, for frequencies above the alpha peak.
Violations persisted up to 10^5 times the thermal equilibration time of the
configurational degrees of freedom under study, but comparable to the average
relaxation time of the material. These results suggest that excess energy flows
from slower to faster relaxing modes.Comment: Improved discussion; final version to appear in Phys. Rev. Lett. 4
pages, 5 PS figures, RevTe
A linked cluster expansion for the calculation of the semi-inclusive A(e,e'p)X processes using correlated Glauber wave functions
The distorted one-body mixed density matrix, which is the basic nuclear
quantity appearing in the definition of the cross section for the
semi-inclusive A(e,e'p)X processes, is calculated within a linked-cluster
expansion based upon correlated wave functions and the Glauber multiple
scattering theory to take into account the final state interaction of the
ejected nucleon. The nuclear transparency for 16O and 40Ca is calculated using
realistic central and non-central correlations and the important role played by
the latter is illustrated.Comment: 18 pages, RevTeX, 3 ps figures. Final version, to appear in Phys.
Rev.
In-Situ Evidence for Uranium Immobilization and Remobilization
The in-situ microbial reduction and immobilization of uranium was assessed as a means of preventing the migration of this element in the terrestrial subsurface. Uranium immobilization (putatively identified as reduction) and microbial respiratory activities were evaluated in the presence of exogenous electron donors and acceptors with field push-pull tests using wells installed in an anoxic aquifer contaminated with landfill leachate. Uranium-(VI) amended at 1.5 µM was reduced to less than 1 nM in groundwater in less than 8 d during all field experiments. Amendments of 0.5 mM sulfate or 5 mM nitrate slowed U(VI) immobilization and allowed for the recovery of 10% and 54% of the injected element, respectively, as compared to 4% in the unamended treatment. Laboratory incubations confirmed the field tests and showed that the majority of the U(VI) immobilized was due to microbial reduction. In these tests, nitrate treatment (7.5 mM) inhibited U(VI) reduction, and nitrite was transiently produced. Further push-pull tests were performed in which either 1 or 5 mM nitrate was added with 1.0 µM U(VI) to sediments that already contained immobilized uranium. After an initial loss of the amendments, the concentration of soluble U(VI) increased and eventually exceeded the injected concentration, indicating that previously immobilized uranium was remobilized as nitrate was reduced. Laboratory experiments using heat-inactivated sediment slurries suggested that the intermediates of dissimilatory nitrate reduction (denitrification or dissimilatory nitrate reduction to ammonia), nitrite, nitrous oxide, and nitric oxide were all capable of oxidizing and mobilizing U(IV). These findings indicate that insitu subsurface U(VI) immobilization can be expected to take place under anaerobic conditions, but the permanence of the approach can be impaired by disimilatory nitrate reduction intermediates that can mobilize previously reduced uranium
Cluster Dynamical Mean-field calculations for TiOCl
Based on a combination of cluster dynamical mean field theory (DMFT) and
density functional calculations, we calculated the angle-integrated spectral
density in the layered quantum magnet TiOCl. The agreement with recent
photoemission and oxygen K-edge X-ray absorption spectroscopy experiments is
found to be good. Th e improvement achieved with this calculation with respect
to previous single-site DMFT calculations is an indication of the correlated
nature and low-dimensionality of TiOCl.Comment: 9 pages, 3 figures, improved version as publishe
Quantum Effects in the Mechanical Properties of Suspended Nanomechanical Systems
We explore the quantum aspects of an elastic bar supported at both ends and
subject to compression. If strain rather than stress is held fixed, the system
remains stable beyond the buckling instability, supporting two potential
minima. The classical equilibrium transverse displacement is analogous to a
Ginsburg-Landau order parameter, with strain playing the role of temperature.
We calculate the quantum fluctuations about the classical value as a function
of strain. Excitation energies and quantum fluctuation amplitudes are compared
for silicon beams and carbon nanotubes.Comment: RevTeX4. 5 pages, 3 eps figures. Submitted to Physical Review Letter
First Passage Time Densities in Non-Markovian Models with Subthreshold Oscillations
Motivated by the dynamics of resonant neurons we consider a differentiable,
non-Markovian random process and particularly the time after which it
will reach a certain level . The probability density of this first passage
time is expressed as infinite series of integrals over joint probability
densities of and its velocity . Approximating higher order terms
of this series through the lower order ones leads to closed expressions in the
cases of vanishing and moderate correlations between subsequent crossings of
. For a linear oscillator driven by white or coloured Gaussian noise,
which models a resonant neuron, we show that these approximations reproduce the
complex structures of the first passage time densities characteristic for the
underdamped dynamics, where Markovian approximations (giving monotonous first
passage time distribution) fail
Quasi two-dimensional antiferromagnet on a triangular lattice RbFe(MoO4)2
RbFe(MoO4)2 is a rare example of a nearly two-dimensional Heisenberg
antiferromagnet on a triangular lattice. Magnetic resonance spectra and
magnetization curves reveal that the system has a layered spin structure with
six magnetic sublattices. The sublattices within a layer are arranged in a
triangular manner with the magnetization vectors 120 degree apart. The H-T
phase diagram, containing at least five different magnetic phases is
constructed. In zero field, RbFe(MoO4)2 undergoes a phase transition at T_N=3.8
K into a non-collinear triangular spin structure with all the spins confined in
the basal plane. The application of an in-plane magnetic field induces a
collinear spin state between the fields H_c1=47 kOe and H_c2=71 kOe and
produces a magnetization plateau at one-third of the saturation moment. Both
the ESR and the magnetization measurements also clearly indicate an additional
first-order phase transition in a field of 35 kOe. The exact nature of this
phase transition is uncertain.Comment: 9 pages incl 11 figure
Isolation and primary cultures of human intrahepatic bile ductular epithelium
A technique for the isolation of human intrahepatic bile ductular epithelium, and the establishment of primary cultures using a serum- and growth-factor-supplemented medium combined with a connective tissue substrata is described. Initial cell isolates and monolayer cultures display phenotypic characteristics of biliary epithelial cells (low molecular weight prekeratin positive; albumin, alphafetoprotein, and Factor VIII-related antigen negative). Ultrastructural features of the cultured cells show cell polarization with surface microvilli, numerous interepithelial junctional complexes and cytoplasmic intermediate prekeratin filaments. © 1988 Tissue Culture Association, Inc
Modulational instability in periodic quadratic nonlinear materials
We investigate the modulational instability of plane waves in quadratic
nonlinear materials with linear and nonlinear quasi-phase-matching gratings.
Exact Floquet calculations, confirmed by numerical simulations, show that the
periodicity can drastically alter the gain spectrum but never completely
removes the instability. The low-frequency part of the gain spectrum is
accurately predicted by an averaged theory and disappears for certain gratings.
The high-frequency part is related to the inherent gain of the homogeneous
non-phase-matched material and is a consistent spectral feature.Comment: 4 pages, 7 figures corrected minor misprint
- …