714 research outputs found
Leading off-diagonal contribution to the spectral form factor of chaotic quantum systems
We semiclassically derive the leading off-diagonal correction to the spectral
form factor of quantum systems with a chaotic classical counterpart. To this
end we present a phase space generalization of a recent approach for uniformly
hyperbolic systems (M. Sieber and K. Richter, Phys. Scr. T90, 128 (2001); M.
Sieber, J. Phys. A: Math. Gen. 35, L613 (2002)). Our results coincide with
corresponding random matrix predictions. Furthermore, we study the transition
from the Gaussian orthogonal to the Gaussian unitary ensemble.Comment: 8 pages, 2 figures; J. Phys. A: Math. Gen. (accepted for publication
On the spectrum of a bent chain graph
We study Schr\"odinger operators on an infinite quantum graph of a chain form
which consists of identical rings connected at the touching points by
-couplings with a parameter . If the graph is "straight",
i.e. periodic with respect to ring shifts, its Hamiltonian has a band spectrum
with all the gaps open whenever . We consider a "bending"
deformation of the chain consisting of changing one position at a single ring
and show that it gives rise to eigenvalues in the open spectral gaps. We
analyze dependence of these eigenvalues on the coupling and the
"bending angle" as well as resonances of the system coming from the bending. We
also discuss the behaviour of the eigenvalues and resonances at the edges of
the spectral bands.Comment: LaTeX, 23 pages with 7 figures; minor changes, references added; to
appear in J. Phys. A: Math. Theo
A study of the selective hydroconversion of biocarboxylic acids to bioalcohols over novel indium-nickel/zeolite catalysts using octanoic acid as model reactant
Octanoic acid (OA) was hydrotreated in a flow-through reactor at 21 bar total pressure and 240-340 °C over supported metal catalysts prepared from Ni-zeolites (A, X, P) by indium modification. The Ni-zeolites were activated first in H2 flow at 21 bar and 450 °C. While a fraction of the nickel got fully reduced, the zeolite structure became partially destructed. However, some nickel cations remained unreduced, therefore a large fraction of the crystalline zeolite structure was retained. The indium modification of the reduced Ni-zeolites generated bimetallic NiIn/Ni,H-zeolite catalysts having higher stability, hydroconversion activity, octanol selectivity, and lower hydrodecarbonylation activity than the parent partially destructed Ni-zeolite and the Adkins-type catalysts, commercially applied for the conversion of fatty acids to alcohols. © 2013 Akadémiai Kiadó, Budapest, Hungary
G-Quadruplex Dynamics Contribute To Regulation Of Mitochondrial Gene Expression
Single-stranded DNA or RNA sequences rich in guanine (G) can adopt non-canonical structures known as G-quadruplexes (G4). Mitochondrial DNA (mtDNA) sequences that are predicted to form G4 are enriched on the heavy-strand and have been associated with formation of deletion breakpoints. Increasing evidence supports the ability of mtDNA to form G4 in cancer cells; however, the functional roles of G4 structures in regulating mitochondrial nucleic acid homeostasis in non-cancerous cells remain unclear. Here, we demonstrate by live cell imaging that the G4-ligand RHPS4 localizes primarily to mitochondria at low doses. We find that low doses of RHPS4 do not induce a nuclear DNA damage response but do cause an acute inhibition of mitochondrial transcript elongation, leading to respiratory complex depletion. We also observe that RHPS4 interferes with mtDNA levels or synthesis both in cells and isolated mitochondria. Importantly, a mtDNA variant that increases G4 stability and anti-parallel G4-forming character shows a stronger respiratory defect in response to RHPS4, supporting the conclusion that mitochondrial sensitivity to RHPS4 is G4-mediated. Taken together, our results indicate a direct role for G4 perturbation in mitochondrial genome replication, transcription processivity, and respiratory function in normal cells
Shot noise from action correlations
We consider universal shot noise in ballistic chaotic cavities from a
semiclassical point of view and show that it is due to action correlations
within certain groups of classical trajectories. Using quantum graphs as a
model system we sum these trajectories analytically and find agreement with
random-matrix theory. Unlike all action correlations which have been considered
before, the correlations relevant for shot noise involve four trajectories and
do not depend on the presence of any symmetry.Comment: 4 pages, 2 figures (a mistake in version 1 has been corrected
Effect of pitchfork bifurcations on the spectral statistics of Hamiltonian systems
We present a quantitative semiclassical treatment of the effects of
bifurcations on the spectral rigidity and the spectral form factor of a
Hamiltonian quantum system defined by two coupled quartic oscillators, which on
the classical level exhibits mixed phase space dynamics. We show that the
signature of a pitchfork bifurcation is two-fold: Beside the known effect of an
enhanced periodic orbit contribution due to its peculiar -dependence at
the bifurcation, we demonstrate that the orbit pair born {\em at} the
bifurcation gives rise to distinct deviations from universality slightly {\em
above} the bifurcation. This requires a semiclassical treatment beyond the
so-called diagonal approximation. Our semiclassical predictions for both the
coarse-grained density of states and the spectral rigidity, are in excellent
agreement with corresponding quantum-mechanical results.Comment: LaTex, 25 pp., 14 Figures (26 *.eps files); final version 3, to be
published in Journal of Physics
All electron and pseudopotential study of the spin polarization of the V (001) surface: LDA versus GGA
The spin-polarization at the V(001) surface has been studied by using
different local (LSDA) and semilocal (GGA) approximations to the
exchange-correlation potential of DFT within two ab initio methods: the
all-electron TB-LMTO-ASA and the pseudopotential LCAO code SIESTA (Spanish
Initiative for Electronic Simulations with Thousands of Atoms). A comparative
analysis is performed first for the bulk and then for a N-layer V(001) film (7
< N < 15). The LSDA approximation leads to a non magnetic V(001) surface with
both theoretical models in agreement (disagreement) with magneto-optical Kerr
(electron-capture spectroscopy) experiments. The GGA within the pseudopotential
method needs thicker slabs than the LSDA to yield zero moment at the central
layer, giving a high surface magnetization (1.70 Bohr magnetons), in contrast
with the non magnetic solution obtained by means of the all-electron code.Comment: 12 pages, 1 figure. Latex gzipped tar fil
Exchange coupling in Eu monochalcogenides from first principles
Using a density functional method with explicit account for strong Coulomb
repulsion within the 4f shell, we calculate effective exchange parameters and
the corresponding ordering temperatures of the (ferro)magnetic insulating Eu
monochalcogenides (EuX; X=O,S,Se,Te) at ambient and elevated pressure
conditions. Our results provide quantitative account of the many-fold increase
of the Curie temperatures with applied pressure and reproduce well the
enhancement of the tendency toward ferromagnetic ordering across the series
from telluride to oxide, including the crossover from antiferromagnetic to
ferromagnetic ordering under pressure in EuTe and EuSe. The first and second
neighbor effective exchange are shown to follow different functional
dependencies. Finally, model calculations indicate a significant contribution
of virtual processes involving the unoccupied f states to the effective
exchange.Comment: 4 pages, 6 figure
Periodic-orbit theory of universal level correlations in quantum chaos
Using Gutzwiller's semiclassical periodic-orbit theory we demonstrate
universal behaviour of the two-point correlator of the density of levels for
quantum systems whose classical limit is fully chaotic. We go beyond previous
work in establishing the full correlator such that its Fourier transform, the
spectral form factor, is determined for all times, below and above the
Heisenberg time. We cover dynamics with and without time reversal invariance
(from the orthogonal and unitary symmetry classes). A key step in our reasoning
is to sum the periodic-orbit expansion in terms of a matrix integral, like the
one known from the sigma model of random-matrix theory.Comment: 44 pages, 11 figures, changed title; final version published in New
J. Phys. + additional appendices B-F not included in the journal versio
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