236,119 research outputs found
Symplectic reduction and topology for applications in classical molecular dynamics
This paper aims to introduce readers with backgrounds in classical molecular dynamics to some ideas in geometric mechanics that may be useful. This is done through some simple but specific examples: (i) the separation of the rotational and internal energies in an arbitrarily floppy N-body system and (ii) the reduction of the phase space accompanying the change from the laboratory coordinate system to the center of mass coordinate system relevant to molecular collision dynamics. For the case of two-body molecular systems constrained to a plane, symplectic reduction is employed to demonstrate explicitly the separation of translational, rotational, and internal energies and the corresponding reductions of the phase space describing the dynamics for Hamiltonian systems with symmetry. Further, by examining the topology of the energy-momentum map, a unified treatment is presented of the reduction results for the description of (i) the classical dynamics of rotating and vibrating diatomic molecules, which correspond to bound trajectories and (ii) the classical dynamics of atom–atom collisions, which correspond to scattering trajectories. This provides a framework for the treatment of the dynamics of larger N-body systems, including the dynamics of larger rotating and vibrating polyatomic molecular systems and the dynamics of molecule–molecule collisions
Pricing under innovation
We study pricing when firms introduce process and product innovations over
time. We set up a model of endogenous productivity and markup under imperfect
competition and dynamic pricing. We estimate it using output price indices
reported by an unbalanced panel of 2,300 Spanish manufacturing firms during
1990-2006. Markups turn out to be procyclical and change with the introduction
of innovations. Firms use innovation to increase margins, but product innovators
are careful to raise prices on new or improved goods. Process innovations tend to
leave prices unchanged, product innovations tend to raise prices and firms that
introduce both tend to decrease them
Observation of strong electron dephasing in disordered CuGeAu thin films
We report the observation of strong electron dephasing in a series of
disordered CuGeAu thin films. A very short electron dephasing
time possessing very weak temperature dependence around 6 K, followed by an
upturn with further decrease in temperature below 4 K, is found. The upturn is
progressively more pronounced in more disordered samples. Moreover, a ln
dependent, but high-magnetic-field-insensitive, resistance rise persisting from
above 10 K down to 30 mK is observed in the films. These results suggest a
nonmagnetic dephasing process which is stronger than any known mechanism and
may originate from the coupling of conduction electrons to dynamic defects.Comment: to appear in Phys. Rev. Let
Telomere length as a predictor of response to Pioglitazone in patients with unremitted depression: a preliminary study.
We studied peripheral leukocyte telomere length (LTL) as a predictor of antidepressant response to PPAR-γ agonist in patients with unremitted depression. In addition we examined correlation between LTL and the insulin resistance (IR) status in these subjects. Forty-two medically stable men and women ages 23-71 with non-remitted depression participated in double-blind placebo-controlled add-on of Pioglitazone to treatment-as-usual. Oral glucose tolerance tests were administered at baseline and at 12 weeks. Diagnostic evaluation of psychiatric disorders was performed at baseline and mood severity was followed weekly throughout the duration of the trial. At baseline, no differences in LTL were detected by depression severity, duration or chronicity. LTL was also not significantly different between insulin-resistant and insulin-sensitive subjects at baseline. Subjects with longer telomeres exhibited greater declines in depression severity in the active arm, but not in a placebo arm, P=0.005, r=-0.63, 95% confidence interval (95% CI)=(-0.84,-0.21). In addition, LTL predicted improvement in insulin sensitivity in the group overall and did not differ between intervention arms, P=0.036, r=-0.44, 95% CI=(-0.74,0.02) for the active arm, and P=0.026, r=-0.50, 95% CI=(-0.78,-0.03) for the placebo arm. LTL may emerge as a viable predictor of antidepressant response. An association between insulin sensitization and LTL regardless of the baseline IR status points to potential role of LTL as a non-specific moderator of metabolic improvement in these patients
Neutrino scattering in supernovae and spin correlations of a unitary gas
Core collapse supernova simulations can be sensitive to neutrino interactions
near the neutrinosphere. This is the surface of last scattering. We model the
neutrinosphere region as a warm unitary gas of neutrons. A unitary gas is a low
density system of particles with large scattering lengths. We calculate
modifications to neutrino scattering cross sections because of the universal
spin and density correlations of a unitary gas. These correlations can be
studied in laboratory cold atom experiments. We find significant reductions in
cross sections, compared to free space interactions, even at relatively low
densities. These reductions could reduce the delay time from core bounce to
successful explosion in multidimensional supernova simulations.Comment: 5 pages, 2 figures, minor corrections in response to referee, Phys.
Rev. C in pres
The metal insulator transition in cluster dynamical mean field theory: intersite correlation, cluster size, interaction strength, and the location of the transition line
To gain insight into the physics of the metal insulator transition and the
effectiveness of cluster dynamical mean field theory (DMFT) we have used one,
two and four site dynamical mean field theory to solve a polaron model of
electrons coupled to a classical phonon field. The cluster size dependence of
the metal to polaronic insulator phase boundary is determined along with
electron spectral functions and cluster correlation functions. Pronounced
cluster size effects start to occur in the intermediate coupling region in
which the cluster calculation leads to a gap and the single-site approximation
does not. Differences (in particular a sharper band edge) persist in the strong
coupling regime. A partial density of states is defined encoding a generalized
nesting property of the band structure; variations in this density of states
account for differences between the dynamical cluster approximation and the
cellular-DMFT implementations of cluster DMFT, and for differences in behavior
between the single band models appropriate for cuprates and the multiband
models appropriate for manganites. A pole or strong resonance in the self
energy is associated with insulating states; the momentum dependence of the
pole is found to distinguish between Slater-like and Mott-like mechanisms for
metal insulator transition. Implications for the theoretical treatment of doped
manganites are discussed.Comment: 28 pages (single column, double space) 15 figure
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