1,306 research outputs found
Geometrical phase driven predissociation: Lifetimes of 2^2 A' levels of H_3
We discuss the role of the geometrical phase in predissociation dynamics of
vibrational states near a conical intersection of two electronic potential
surfaces of a D_{3h} molecule. For quantitative description of the
predissociation driven by the coupling near a conical intersection, we
developed a method for calculating lifetimes and positions of vibrational
predissociated states (Feshbach resonances) for X_3 molecule. The method takes
into account the two coupled three-body potential energy surfaces, which are
degenerate at the intersection. As an example, we apply the method to obtain
lifetimes and positions of resonances of predissociated vibrational levels of
the 2^2 A' electronic state of the H_3 molecule. The three-body recombination
rate coefficient for the H+H+H -> H_2+H process is estimated.Comment: 4 pages, 4 figure
Empirically testing <i>Tonnetz</i>, voice-leading, and spectral models of perceived triadic distance
We compare three contrasting models of the perceived distance between root-position major and minor chords and test them against new empirical data. The models include a recent psychoacoustic model called spectral pitch class distance, and two well-established music theoretical models – Tonnetz distance and voice-leading distance. To allow a principled challenge, in the context of these data, of the assumptions behind each of the models, we compare them with a simple “benchmark” model that simply counts the number of common tones between chords. Spectral pitch class and Tonnetz have the highest correlations with the experimental data and each other, and perform significantly better than the benchmark. The voice-leading model performs worse than the benchmark. We suggest that spectral pitch class distance provides a psychoacoustic explanation for perceived harmonic distance and its music theory representation, the Tonnetz. Scores and MIDI files of the stimuli, the experimental data, and the computational models are available in the online supplement
Theory of dissociative recombination of highly-symmetric polyatomic ions
A general first-principles theory of dissociative recombination is developed
for highly-symmetric molecular ions and applied to HO and CH,
which play an important role in astrophysical, combustion, and laboratory
plasma environments. The theoretical cross-sections obtained for the
dissociative recombination of the two ions are in good agreement with existing
experimental data from storage ring experiments
Optimal Topological Test for Degeneracies of Real Hamiltonians
We consider adiabatic transport of eigenstates of real Hamiltonians around
loops in parameter space. It is demonstrated that loops that map to nontrivial
loops in the space of eigenbases must encircle degeneracies. Examples from
Jahn-Teller theory are presented to illustrate the test. We show furthermore
that the proposed test is optimal.Comment: Minor corrections, accepted in Phys. Rev. Let
A simplified picture for Pi electrons in conjugated polymers : from PPP Hamiltonian to an effective molecular crystal approach
An excitonic method proper to study conjugated oligomers and polymers is
described and its applicability tested on the ground state and first excited
states of trans-polyacetylene, taken as a model. From the Pariser-Parr-Pople
Hamiltonian, we derive an effective Hamiltonian based on a local description of
the polymer in term of monomers; the relevant electronic configurations are
build on a small number of pertinent local excitations. The intuitive and
simple microscopic physical picture given by our model supplement recent
results, such as the Rice and Garstein ones. Depending of the parameters, the
linear absorption appears dominated by an intense excitonic peak.Comment: 41 Pages, 6 postscript figure
Peierls transition in the quantum spin-Peierls model
We use the density matrix renormalization group method to investigate the
role of longitudinal quantized phonons on the Peierls transition in the
spin-Peierls model. For both the XY and Heisenberg spin-Peierls model we show
that the staggered phonon order parameter scales as (and the
dimerized bond order scales as ) as (where
is the electron-phonon interaction). This result is true for both linear and
cyclic chains. Thus, we conclude that the Peierls transition occurs at
in these models. Moreover, for the XY spin-Peierls model we show
that the quantum predictions for the bond order follow the classical prediction
as a function of inverse chain size for small . We therefore conclude
that the zero phase transition is of the mean-field type
Definite Streptococcus bovis endocarditis: characteristics in 20 patients
ObjectiveTo determine the specific characteristics of Streptococcus bovis infective endocarditis (IE) by reviewing our own experience of S. bovis IE.MethodsTwenty episodes of definite S. bovis IE were reviewed in 20 patients hospitalized from 1980 to 1996.ResultsThe mean age was 62 ± 14 years, and 14 (70%) patients had no known predisposing cardiac condition. The principal antimicrobials used were penicillin G (N = 10) and amoxycillin (N = 8). Surgery was required in four (20%) patients. Neurologic complications occurred in eight (40%) patients, after initiation of therapy in six (75%) (mean time: 14 days). An unfavorable outcome was observed in four of 20 patients and tended to be more frequent in patients who had had neurologic complications (P – 0.10). Colonic tumors were present in 11 of 16 (69%) patients.ConclusionsAdvanced age, occurrence of IE on presumably normal valves, high rate of neurologic complications, associated gastrointestinal diseases and low mortality rate during initial follow-up are characteristic features of S. bovis IE observed in this study
Dynamical Jahn-Teller Effect and Berry Phase in Positively Charged Fullerene I. Basic Considerations
We study the Jahn-Teller effect of positive fullerene ions C
and C. The aim is to discover if this case, in analogy with the
negative ion, possesses a Berry phase or not, and what are the consequences on
dynamical Jahn-Teller quantization. Working in the linear and spherical
approximation, we find no Berry phase in C, and
presence/absence of Berry phase for coupling of one hole to an
/ vibration. We study in particular the special equal-coupling case
(), which is reduced to the motion of a particle on a 5-dimensional
sphere. In the icosahedral molecule, the final outcome assesses the
presence/absence of a Berry phase of for the hole coupled to
/ vibrations. Some qualitative consequences on ground-state symmetry,
low-lying excitations, and electron emission from C are spelled out.Comment: 31 pages (RevTeX), 3 Postscript figures (uuencoded
Scaling of Berry's Phase Close to the Dicke Quantum Phase Transition
We discuss the thermodynamic and finite size scaling properties of the
geometric phase in the adiabatic Dicke model, describing the super-radiant
phase transition for an qubit register coupled to a slow oscillator mode.
We show that, in the thermodynamic limit, a non zero Berry phase is obtained
only if a path in parameter space is followed that encircles the critical
point. Furthermore, we investigate the precursors of this critical behavior for
a system with finite size and obtain the leading order in the 1/N expansion of
the Berry phase and its critical exponent
Enhanced Electron Pairing in a Lattice of Berry Phase Molecules
We show that electron hopping in a lattice of molecules possessing a Berry
phase naturally leads to pairing. Our building block is a simple molecular site
model inspired by C, but realized in closer similarity with Na. In
the resulting model electron hopping must be accompanied by orbital operators,
whose function is to switch on and off the Berry phase as the electron number
changes. The effective hamiltonians (electron-rotor and electron-pseudospin)
obtained in this way are then shown to exhibit a strong pairing phenomenon, by
means of 1D linear chain case studies. This emerges naturally from numerical
studies of small -site rings, as well as from a BCS-like mean-field theory
formulation. The pairing may be explained as resulting from the exchange of
singlet pairs of orbital excitations, and is intimately connected with the
extra degeneracy implied by the Berry phase when the electron number is odd.
The relevance of this model to fullerides, to other molecular superconductors,
as well as to present and future experiments, is discussed.Comment: 30 pages, RevTe
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