Ab initio molecular dynamics for molecules with variable numbers of electrons

The ab initio molecular dynamics method is extended to treat exchange of electrons between molecules and a reservoir at fixed chemical potential. The method is based on a rigorously grand-canonical density functional approach using separate potential energy surfaces for each oxidation state. It is shown that the resulting discontinuous dependency of excess charge on chemical potential is consistent with the statistical thermodynamics of equilibrium gas-phase reactions. The method is illustrated by an application to the adiabatic redox dynamics of an aniline molecule.     

Chemical specificity in three-dimensional imaging with multiplex coherent anti-Stokes Raman scattering microscopy

We demonstrate the three-dimensional (3D) imaging capabilities and chemical specificity of multiplex coherent anti-Stokes Raman scattering microscopy. The simultaneous acquisition of a significant part of the vibrational spectrum at each specimen position permits straightforward differentiation among chemical species. 3D imaging is illustrated with a lipid multilamellar vesicle, and lateral and axial resolutions are determined.     

Nonuniqueness of solutions of a degenerate parabolic equation

Summary We give some results about nonuniqueness of the solutions of the Cauchy problem for a class of nonlinear degenerate parabolic equations arising in several applications in biology and physics. This phenomenon is a truly nonlinear one and occurs because of the degeneracy of the equation at the points where u=0. For a given set of values of the parameter involved, we prove that there exists a one parameter family of weak solutions; moreover, restricting the parameter set, nonuniqueness appears even in the class of classical solutions.     

First synthesis of a branched beta-C-tetrasaccharide using a triple ring closing metathesis cyclization

The first synthesis of a branched beta-C-tetrasaccharide has been carried out through the use of an esterification-ring closing metathesis (RCM) strategy. The precursor triacid 2a was readily prepared via standard chemical methods from a known starting material, and dehydrative coupling with an excess of olefin alcohol 1a gave triester 3a in excellent yield. Methylenation of the triester 3a and subsequent triple RCM reaction was followed by an in situ hydroboration-oxidation to furnish the branched beta-C-tetrasaccharide 6a in good overall yield.     

Photon statistics from coupled quantum dots

We present an optical study of two closely stacked self-assembled InAs/GaAs quantum dots. The energy spectrum and correlations between photons subsequently emitted from a single pair provide not only clear evidence of coupling between the quantum dots but also insight into the coupling mechanism. Our results are in agreement with recent theories predicting that tunneling is largely suppressed between nonidentical quantum dots and that the interaction is instead dominated by dipole-dipole coupling and phonon-assisted energy transfer processes.     

On a Spector Ultrapower for the Solovay Model

We prove that a Spector-like ultrapower extension ?? of a countable Solovay model ?? (where all sets of reals are Lebesgue measurable) is equal to the set of all sets constructible from reals in a generic extension ??[a], where a is a random real over ??. The proof involves the Solovay almost everywhere uniformization technique.     

Spectral distribution of steady-state photoconductivity in amorphous As2Se3

The spectral distribution of steady-state photoconductivity has been examined for a series of bulk and evaporated samples of amorphous As₂Se₃. All bulk samples, irrespective of preparation technique or sample treatment, show a distinct shoulder in the photocurrent spectral distribution at energies near 1.4 eV. This feature is interpreted as spectroscopic evidence for the existence of a well-defined defect level in the gap of amorphous As₂Se₃. Evaporated As₂Se₃ films do not show any structure in the spectral response.     

Excitations in a nonequilibrium Bose-Einstein condensate of exciton polaritons

We develop a mean-field theory of the dynamics of a nonequilibrium Bose-Einstein condensate of exciton polaritons in a semiconductor microcavity. The spectrum of elementary excitations around the stationary state is analytically studied by means of a generalized Gross-Pitaevskii equation. A diffusive behavior of the Goldstone mode is found in the spatially homogeneous case and new features are predicted for the Josephson effect in a two-well geometry.