2,726 research outputs found
Glassy behavior of electrons near metal-insulator transitions
The emergence of glassy behavior of electrons is investigated for systems
close to the disorder and/or interaction-driven metal-insulator transitions.
Our results indicate that Anderson localization effects strongly stabilize such
glassy behavior, while Mott localization tends to suppress it. We predict the
emergence of an intermediate metallic glassy phase separating the insulator
from the normal metal. This effect is expected to be most pronounced for
sufficiently disordered systems, in agreement with recent experimental
observations.Comment: Final version as published in Physical Review Letter
Universal Crossover between Efros-Shklovskii and Mott Variable-Range-Hopping Regimes
A universal scaling function, describing the crossover between the Mott and
the Efros-Shklovskii hopping regimes, is derived, using the percolation picture
of transport in strongly localized systems. This function is agrees very well
with experimental data. Quantitative comparison with experiment allows for the
possible determination of the role played by polarons in the transport.Comment: 7 pages + 1 figure, Revte
Electronic correlation effects and the Coulomb gap at finite temperature
We have investigated the effect of the long-range Coulomb interaction on the
one-particle excitation spectrum of n-type Germanium, using tunneling
spectroscopy on mechanically controllable break junctions. The tunnel
conductance was measured as a function of energy and temperature. At low
temperatures, the spectra reveal a minimum at zero bias voltage due to the
Coulomb gap. In the temperature range above 1 K the Coulomb gap is filled by
thermal excitations. This behavior is reflected in the temperature dependence
of the variable-range hopping resitivity measured on the same samples: Up to a
few degrees Kelvin the Efros-Shkovskii ln law is obeyed,
whereas at higher temperatures deviations from this law are observed,
indicating a cross-over to Mott's ln law. The mechanism of
this cross-over is different from that considered previously in the literature.Comment: 3 pages, 3 figure
Geometrical Models of the Phase Space Structures Governing Reaction Dynamics
Hamiltonian dynamical systems possessing equilibria of stability type display \emph{reaction-type
dynamics} for energies close to the energy of such equilibria; entrance and
exit from certain regions of the phase space is only possible via narrow
\emph{bottlenecks} created by the influence of the equilibrium points. In this
paper we provide a thorough pedagogical description of the phase space
structures that are responsible for controlling transport in these problems. Of
central importance is the existence of a \emph{Normally Hyperbolic Invariant
Manifold (NHIM)}, whose \emph{stable and unstable manifolds} have sufficient
dimensionality to act as separatrices, partitioning energy surfaces into
regions of qualitatively distinct behavior. This NHIM forms the natural
(dynamical) equator of a (spherical) \emph{dividing surface} which locally
divides an energy surface into two components (`reactants' and `products'), one
on either side of the bottleneck. This dividing surface has all the desired
properties sought for in \emph{transition state theory} where reaction rates
are computed from the flux through a dividing surface. In fact, the dividing
surface that we construct is crossed exactly once by reactive trajectories, and
not crossed by nonreactive trajectories, and related to these properties,
minimizes the flux upon variation of the dividing surface.
We discuss three presentations of the energy surface and the phase space
structures contained in it for 2-degree-of-freedom (DoF) systems in the
threedimensional space , and two schematic models which capture many of
the essential features of the dynamics for -DoF systems. In addition, we
elucidate the structure of the NHIM.Comment: 44 pages, 38 figures, PDFLaTe
Non-ergodic effects in the Coulomb glass: specific heat
We present a numerical method for the investigation of non-ergodic effects in
the Coulomb glass. For that, an almost complete set of low-energy many-particle
states is obtained by a new algorithm. The dynamics of the sample is mapped to
the graph formed by the relevant transitions between these states, that means
by transitions with rates larger than the inverse of the duration of the
measurement. The formation of isolated clusters in the graph indicates
non-ergodicity. We analyze the connectivity of this graph in dependence on
temperature, duration of measurement, degree of disorder, and dimensionality,
studying how non-ergodicity is reflected in the specific heat.Comment: Submited Phys. Rev.
Effect of electronic patient record use on mortality in End Stage Renal Disease, a model chronic disease: retrospective analysis of 9 years of prospectively collected data
<p>Abstract</p> <p>Background</p> <p>In chronic disease, health information technology promises but has yet to demonstrate improved outcomes and decreased costs. The main aim of the study was to determine the effects on mortality and cost of an electronic patient record used in daily patient care in a model chronic disease, End Stage Renal Disease, treated by chronic maintenance hemodialysis. Dialysis treatment is highly regulated, and near uniform in treatment modalities and drugs used.</p> <p>Methods</p> <p>The particular electronic patient record, patient-centered and extensively coded, was used first in patient care in 3 dialysis units in New York, NY in 1998, 1999, and 2000. All data were stored "live"; none were archived. By December 31, 2006, the patients had been treated by maintenance hemodialysis for a total of 3924 years. A retrospective analysis was made using query tools embedded in the software. The United States Renal Data System dialysis population served as controls. In all there were 1790 patients, with many underlying primary diseases and multiple comorbid conditions affecting many organ systems. Year by year mortality, hospital admissions, and staffing were analyzed, and the data were compared with national data compiled by the United States Renal Data System.</p> <p>Results</p> <p>Analyzed by calendar year after electronic patient record implementation, mortality decreased strikingly. In years 3–9 mortality was lower than in years 1–2 by 23%, 48%, and 34% in the 3 units, and was 37%, 37%, and 35% less than that reported by the United States Renal Data System. Clinical staffing was 25% fewer per 100 patients than the national average, thereby lowering costs.</p> <p>Conclusion</p> <p>To our knowledge, this is the first demonstration that an electronic patient record, albeit of particular design, can have a favorable effect on outcomes and cost in chronic disease. That the population studied has many underlying diseases affecting all organ systems suggests that the electronic patient record design may enable application to many fields of medical practice.</p
Non-Markovian Configurational Diffusion and Reaction Coordinates for Protein Folding
The non-Markovian nature of polymer motions is accounted for in folding
kinetics, using frequency-dependent friction. Folding, like many other problems
in the physics of disordered systems, involves barrier crossing on a correlated
energy landscape. A variational transition state theory (VTST) that reduces to
the usual Bryngelson-Wolynes Kramers approach when the non-Markovian aspects
are neglected is used to obtain the rate, without making any assumptions
regarding the size of the barrier, or the memory time of the friction. The
transformation to collective variables dependent on the dynamics of the system
allows the theory to address the controversial issue of what are ``good''
reaction coordinates for folding.Comment: 9 pages RevTeX, 3 eps-figures included, submitted to PR
Impact of early seed quality selection on maize inbreds and hybrids
Seed composition, genetic background, and environment influence seed quality. Plant breeders selecting for improved seed composition seldom select their inbreds for improved seed quality traits. The standard germination test evaluates seed viability, but it often overestimates field performance. Therefore, seed vigor tests are used to predict seed germination under stressful environments. There is little information on the possible genetic improvement of seed selected for both, improved seed composition and vigor. The objectives of this study were 1) to evaluate the seed quality attributes of a group of maize (Zea mays L.) inbreds selected for high protein content; 2) to assess whether early selection improves the seed quality and decreases the phenotypic variability of seed vigor in a group of inbreds; and 3) to calculate the breeding parameters of general (GCA) and specific combining ability (SCA). During summer 2002 and 2003, related sets of inbred lines were grown in replicated nurseries near Clinton, IL, and Ames, IA. Seed from each inbred was produced by self pollination. Some of the inbred lines grown in 2002 and all grown in 2003 were high-protein white lines that also had been selected for germination cold tolerance and high post-accelerated aging field emergence. In 2002, the mean percentage of standard germination test, saturated cold test, accelerated aging test, soak test, and fast green test for the group of selected high-protein white inbreds were significantly (P ≤ 0.05) higher than the corresponding average values of the yellow inbred checks. There was genetic variability for seed quality in these sets of high protein white inbreds even after a very intense selection process for improved seed quality traits. GCA effects for seed quality were more important than the SCA effects, indicating that the additive effect of the inbreds was more important than the dominant effect to the final seed quality of the hybrids. Selecting inbreds for high seed quality early in the breeding program is beneficial and important for improving germination and field performance
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