5,913 research outputs found
Dynamic treatment of vibrational energy relaxation in a heterogeneous and fluctuating environment
A computational approach to describe the energy relaxation of a
high-frequency vibrational mode in a fluctuating heterogeneous environment is
outlined. Extending previous work [H. Fujisaki, Y. Zhang, and J.E. Straub, J.
Chem. Phys. {\bf 124}, 144910 (2006)], second-order time-dependent perturbation
theory is employed which includes the fluctuations of the parameters in the
Hamiltonian within the vibrational adiabatic approximation. This means that the
time-dependent vibrational frequencies along an MD trajectory are obtained via
a partial geometry optimization of the solute with fixed solvent and a
subsequent normal mode calculation. Adopting the amide I mode of
N-methylacetamide in heavy water as a test problem, it is shown that the
inclusion of dynamic fluctuations may significantly change the vibrational
energy relaxation. In particular, it is found that relaxation occurs in two
phases, because for short times ( 200 fs) the spectral density
appears continuous due to the frequency-time uncertainty relation, while at
longer times the discrete nature of the bath becomes apparent. Considering the
excellent agreement between theory and experiment, it is speculated if this
behavior can explain the experimentally obtained biphasic relaxation the amide
I mode of N-methylacetamide.Comment: 24 pages, 7 figures, submitted to J. Chem. Phy
Interference of an array of independent Bose-Einstein condensates
We have observed high-contrast matter wave interference between 30
Bose-Einstein condensates with uncorrelated phases. Interference patterns were
observed after independent condensates were released from a one-dimensional
optical lattice and allowed to expand and overlap. This initially surprising
phenomenon is explained with a simple theoretical model which generalizes the
analysis of the interference of two independent condensates
Seeing zeros of random polynomials: quantized vortices in the ideal Bose gas
We propose a physical system allowing one to experimentally observe the
distribution of the complex zeros of a random polynomial. We consider a
degenerate, rotating, quasi-ideal atomic Bose gas prepared in the lowest Landau
level. Thermal fluctuations provide the randomness of the bosonic field and of
the locations of the vortex cores. These vortices can be mapped to zeros of
random polynomials, and observed in the density profile of the gas.Comment: 4 page
Stochastic Trends and Economic Fluctuations
Recent developments in macroeconomic theory emphasize that transient economic fluctuations can arise as responses to changes in long run factors -- in particular, technological improvements -- rather than short run factors. This contrasts with the view that short run fluctuations and shifts in long run trends are largely unrelated. We examine empirically the effect of shifts in stochastic trends that are common to several macroeconomic series. Using a linear time series model related to a VAR, we consider first a system with GNP, consumption and investment with a single common stochastic trend; we then examine this system augmented by money and prices and an additional stochastic trend. Our results suggest that movements in the "real" stochastic trend account for one-half to two-thirds of the variation in postwar U.S. GNP.
Excitations in the quantum paramagnetic phase of the quasi-one-dimensional Ising magnet CoNbO in a transverse field: Geometric frustration and quantum renormalization effects
The quasi-one-dimensional (1D) Ising ferromagnet CoNbO has recently
been driven via applied transverse magnetic fields through a continuous quantum
phase transition from spontaneous magnetic order to a quantum paramagnet, and
dramatic changes were observed in the spin dynamics, characteristic of weakly
perturbed 1D Ising quantum criticality. We report here extensive single-crystal
inelastic neutron scattering measurements of the magnetic excitations
throughout the three-dimensional (3D) Brillouin zone in the quantum
paramagnetic phase just above the critical field to characterize the effects of
the finite interchain couplings. In this phase, we observe that excitations
have a sharp, resolution-limited line shape at low energies and over most of
the dispersion bandwidth, as expected for spin-flip quasiparticles. We map the
full bandwidth along the strongly dispersive chain direction and resolve clear
modulations of the dispersions in the plane normal to the chains,
characteristic of frustrated interchain couplings in an antiferromagnetic
isosceles triangular lattice. The dispersions can be well parametrized using a
linear spin-wave model that includes interchain couplings and further neighbor
exchanges. The observed dispersion bandwidth along the chain direction is
smaller than that predicted by a linear spin-wave model using exchange values
determined at zero field, and this effect is attributed to quantum
renormalization of the dispersion beyond the spin-wave approximation in fields
slightly above the critical field, where quantum fluctuations are still
significant.Comment: 11 pages, 6 figures. Updated references. Minor changes to text and
figure
From soft harmonic phonons to fast relaxational dynamics in CHNHPbBr
The lead-halide perovskites, including CHNHPbBr, are
components in cost effective, highly efficient photovoltaics, where the
interactions of the molecular cations with the inorganic framework are
suggested to influence the electronic and ferroelectric properties.
CHNHPbBr undergoes a series of structural transitions
associated with orientational order of the CHNH (MA) molecular
cation and tilting of the PbBr host framework. We apply high-resolution
neutron scattering to study the soft harmonic phonons associated with these
transitions, and find a strong coupling between the PbBr framework and
the quasistatic CHNH dynamics at low energy transfers. At higher
energy transfers, we observe a PbBr octahedra soft mode driving a
transition at 150 K from bound molecular excitations at low temperatures to
relatively fast relaxational excitations that extend up to 50-100 meV.
We suggest that these temporally overdamped dynamics enables possible indirect
band gap processes in these materials that are related to the enhanced
photovoltaic properties.Comment: (main text - 5 pages, 4 figures; supplementary information - 3 pages,
3 figures
Different Ways of Reading, or Just Making the Right Noises?
What does reading look like? Can learning to read be reduced to the acquisition of a set of isolable skills, or proficiency in reading be equated with the independence of the solitary, silent reader of prose fiction? These conceptions of reading and reading development, which figure strongly in educational policy, may appear to be simple common sense. But both ethnographic data and evidence from literary texts suggest that such paradigms offer, at most, a partial and ahistorical picture of reading. An important dimension, neglected in the dominant paradigms, is the irreducibly social quality of reading practices
Neutron scattering study of a quasi-2D spin-1/2 dimer system Piperazinium Hexachlorodicuprate under hydrostatic pressure
We report inelastic neutron scattering study of a quasi-two-dimensional S=1/2
dimer system Piperazinium Hexachlorodicuprate under hydrostatic pressure. The
spin gap {\Delta} becomes softened with the increase of the hydrostatic
pressure up to P= 9.0 kbar. The observed threefold degenerate triplet
excitation at P= 6.0 kbar is consistent with the theoretical prediction and the
bandwidth of the dispersion relation is unaffected within the experimental
uncertainty. At P= 9.0 kbar the spin gap is reduced to 0.55 meV from 1.0 meV at
ambient pressure.Comment: 4 pages, 5 figure
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