657 research outputs found
Kinetics and mechanism of the reaction between atomic chlorine and dimethyl selenide; comparison with the reaction between atomic chlorine and dimethyl sulfide
Dimethyl selenide is the most abundant gaseous selenium species in marine environments. In this work, the value of the rate coefficient for the gas-phase reaction between dimethyl selenide and Cl atoms has been determined for the first time. The value of the second-order rate coefficient obtained was (5.0±1.4)×10–10 cm3 molecule–1 s–1. The very fast nature of the reaction means that, when estimating the lifetime of dimethyl selenide in the atmosphere, loss due to reaction with Cl atoms should be considered along with loss due to reaction with O3 and with OH and NO3 radicals. Analysis of the available kinetic data suggests that at 760 Torr the dominant reaction pathway for the reaction of Cl atoms with dimethyl selenide will be the addition of Cl to the Se atom forming an adduct of the type CH3Se(Cl)CH3. Theoretical calculations, at the B3LYP/6-311++G(2df,p)//B3LYP/6-311++G(d,p) level of theory, show that at 298 K the value of rH for the formation of the adduct is –111.4 kJ mol–1. This value may be compared to –97.0 kJ mol–1, the value calculated for rH for the formation of the analogous sulfur adduct, CH3S(Cl)CH3, following the reaction between Cl atoms and dimethyl sulfide. Variational RRKM theory was used to predict the thermal decomposition rates of the two adducts back to starting materials. The estimated rate constant for the decomposition of the selenium adduct to the reactants is 5×10–5 s–1, compared to 0.02 s–1 in the case of the sulfur adduct. However, our calculations suggest that the CH3Se(Cl)CH3 adduct, which is initially formed highly excited, will not be stabilised under atmospheric conditions, but rather will decompose to yield CH3SeCl and CH3, a process that is calculated to be exothermic with respect to the initial reactants by 5.8 kJ mol–1. The formation of CH3SCl and CH3 from the sulfur adduct, on the other hand, is endothermic by 20.8 kJ mol–1 with respect to the initial reactants, and is thus not expected to occur
Quantum correlations and least disturbing local measurements
We examine the evaluation of the minimum information loss due to an unread
local measurement in mixed states of bipartite systems, for a general entropic
form. Such quantity provides a measure of quantum correlations, reducing for
pure states to the generalized entanglement entropy, while in the case of mixed
states it vanishes just for classically correlated states with respect to the
measured system, as the quantum discord. General stationary conditions are
provided, together with their explicit form for general two-qubit states.
Closed expressions for the minimum information loss as measured by quadratic
and cubic entropies are also derived for general states of two-qubit systems.
As application, we analyze the case of states with maximally mixed marginals,
where a general evaluation is provided, as well as X states and the mixture of
two aligned states.Comment: 10 pages, 3 figure
Meson-meson interactions -- from static to dynamic valence quarks
A method for the extraction of an effective meson-meson potential from Green
functions, which can be obtained from a lattice simulation, is presented.
Simulations are carried out for compact QED and QCD in four dimensions using
the quenched approximation and the hopping parameter expansion. In a further
study, a heavy-light meson is considered employing a conjugate gradient
algorithm for the light propagators. Due to the Pauli exclusion principle, the
results for QED indicate the existence of a hard core, but for QCD there is
strong attraction at small meson distances.Comment: 4 pages, uuencoded gziped postscript file, contribution to
LATTICE'95, Melbourne, Australia (list of authors completed
Conditional purity and quantum correlation measures in two qubit mixed states
We analyze and show experimental results of the conditional purity, the
quantum discord and other related measures of quantum correlation in mixed
two-qubit states constructed from a pair of photons in identical polarization
states. The considered states are relevant for the description of spin pair
states in interacting spin chains in a transverse magnetic field. We derive
clean analytical expressions for the conditional local purity and other
correlation measures obtained as a result of a remote local projective
measurement, which are fully verified by the experimental results. A simple
exact expression for the quantum discord of these states in terms of the
maximum conditional purity is also derived.Comment: 16 pages, 5 figures, minor changes, to be published in J. Phys.
Generalized entropic measures of quantum correlations
We propose a general measure of non-classical correlations for bipartite
systems based on generalized entropic functions and majorization properties.
Defined as the minimum information loss due to a local measurement, in the case
of pure states it reduces to the generalized entanglement entropy, i.e., the
generalized entropy of the reduced state. However, in the case of mixed states
it can be non-zero in separable states, vanishing just for states diagonal in a
general product basis, like the Quantum Discord. Simple quadratic measures of
quantum correlations arise as a particular case of the present formalism. The
minimum information loss due to a joint local measurement is also discussed.
The evaluation of these measures in a few simple relevant cases is as well
provided, together with comparison with the corresponding entanglement
monotones.Comment: 9 pages, 2 figure
Quantum discord and related measures of quantum correlations in XY chains
We examine the quantum correlations of spin pairs in the ground state of
finite XY chains in a transverse field, by evaluating the quantum discord as
well as other related entropic measures of quantum correlations. A brief review
of the latter, based on generalized entropic forms, is also included. It is
shown that parity effects are of crucial importance for describing the behavior
of these measures below the critical field. It is also shown that these
measures reach full range in the immediate vicinity of the factorizing field,
where they become independent of separation and coupling range. Analytical and
numerical results for the quantum discord, the geometric discord and other
measures in spin chains with nearest neighbor coupling and in fully connected
spin arrays are also provided.Comment: accepted in Int. J. Mod. Phys. B, special issue "Classical Vs Quantum
correlations in composite systems" edited by L. Amico, S. Bose, V. Korepin
and V. Vedra
Momentum-dependent charge correlations in YBaCuO superconductors probed by resonant x-ray scattering: Evidence for three competing phases
We have used resonant x-ray scattering to determine the momentum dependent
charge correlations in YBaCuO samples with highly ordered
chain arrays of oxygen acceptors (ortho-II structure). The results reveal
nearly critical, biaxial charge density wave (CDW) correlations at in-plane
wave vectors (0.315, 0) and (0, 0.325). The corresponding scattering intensity
exhibits a strong uniaxial anisotropy. The CDW amplitude and correlation length
are enhanced as superconductivity is weakened by an external magnetic field.
Analogous experiments were carried out on a YBaCuO crystal with
a dilute concentration of spinless (Zn) impurities, which had earlier been
shown to nucleate incommensurate magnetic order. Compared to pristine crystals
with the same doping level, the CDW amplitude and correlation length were found
to be strongly reduced. These results indicate a three-phase competition
between spin-modulated, charge-modulated, and superconducting states in
underdoped YBaCuO.Comment: 6 pages, 3 figures revised version, to appear in Phys. Rev. Let
Lattice dynamical signature of charge density wave formation in underdoped YBa2Cu3O6+x
We report a detailed Raman scattering study of the lattice dynamics in
detwinned single crystals of the underdoped high temperature superconductor
YBa2Cu3O6+x (x=0.75, 0.6, 0.55 and 0.45). Whereas at room temperature the
phonon spectra of these compounds are similar to that of optimally doped
YBa2Cu3O6.99, additional Raman-active modes appear upon cooling below ~170-200
K in underdoped crystals. The temperature dependence of these new features
indicates that they are associated with the incommensurate charge density wave
state recently discovered using synchrotron x-ray scattering techniques on the
same single crystals. Raman scattering has thus the potential to explore the
evolution of this state under extreme conditions.Comment: 12 pages, 11 figure
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