2,796 research outputs found
Analysis of Accordion DNA Stretching Revealed by The Gold Cluster Ruler
A promising new method for measuring intramolecular distances in solution
uses small-angle X-ray scattering interference between gold nanocrystal labels
(Mathew-Fenn et al, Science, 322, 446 (2008)). When applied to double stranded
DNA, it revealed that the DNA length fluctuations are strikingly strong and
correlated over at least 80 base pair steps. In other words, the DNA behaves as
accordion bellows, with distant fragments stretching and shrinking concertedly.
This hypothesis, however, disagrees with earlier experimental and computational
observations. This Letter shows that the discrepancy can be rationalized by
taking into account the cluster exclusion volume and assuming a moderate
long-range repulsion between them. The long-range interaction can originate
from an ion exclusion effect and cluster polarization in close proximity to the
DNA surface.Comment: 9 pages, 4 figures, to appear in Phys. Rev.
Boundary correlation function of fixed-to-free bcc operators in square-lattice Ising model
We calculate the boundary correlation function of fixed-to-free boundary
condition changing operators in the square-lattice Ising model. The correlation
function is expressed in four different ways using block Toeplitz
determinants. We show that these can be transformed into a scalar Toeplitz
determinant when the size of the matrix is even. To know the asymptotic
behavior of the correlation function at large distance we calculate the
asymptotic behavior of this scalar Toeplitz determinant using the Szeg\"o's
theorem and the Fisher-Hartwig theorem. At the critical temperature we confirm
the power-law behavior of the correlation function predicted by conformal field
theory
Microphase separation in polyelectrolytic diblock copolymer melt : weak segregation limit
We present a generalized theory of microphase separation for charged-neutral
diblock copolymer melt. Stability limit of the disordered phase for salt-free
melt has been calculated using Random Phase Approximation (RPA) and
self-consistent field theory (SCFT). Explicit analytical free energy
expressions for different classical ordered microstructures (lamellar, cylinder
and sphere) are presented. We demonstrate that chemical mismatch required for
the onset of microphase separation () in charged-neutral
diblock melt is higher and the period of ordered microstructures is lower than
those for the corresponding neutral-neutral diblock system. Theoretical
predictions on the period of ordered structures in terms of Coulomb
electrostatic interaction strength, chain length, block length, and the
chemical mismatch between blocks are presented. SCFT has been used to go beyond
the stability limit, where electrostatic potential and charge distribution are
calculated self-consistently. Stability limits calculated using RPA are in
perfect agreement with the corresponding SCFT calculations. Limiting laws for
stability limit and the period of ordered structures are presented and
comparisons are made with an earlier theory. Also, transition boundaries
between different morphologies have been investigated
Equation of Motion for the Solvent Polarization Apparent Charges in the Polarizable Continuum Model: Application to Time-Dependent CI
The dynamics of the electrons for a molecule in solution is coupled to the
dynamics of its polarizable environment, i.e., the solvent. To theoretically
investigate such electronic dynamics, we have recently developed equations of
motion (EOM) for the apparent solvent polarization charges that generate the
reaction field in the Polarizable Continuum Model (PCM) for solvation and we
have coupled them to a real-time time-dependent density functional theory (RT
TDDFT) description of the solute [Corni et al. J. Phys. Chem. A 119, 5405
(2014)]. Here we present an extension of the EOM-PCM approach to a
Time-Dependent Configuration Interaction (TD CI) description of the solute
dynamics, which is free from the qualitative artifacts of RT TDDFT in the
adiabatic approximation. As tests of the developed approach, we investigate the
solvent Debye relaxation after an electronic excitation of the solute obtained
either by a pulse of light or by assuming the idealized sudden promotion
to the excited state. Moreover, we present EOM for the Onsager solvation model
and we compare the results with PCM. The developed approach provides
qualitatively correct real-time evolutions and is promising as a general tool
to investigate the electron dynamics elicited by external electromagnetic
fields for molecules in solution.Comment: This is the final peer-reviewed manuscript accepted for publication
in The Journal of Chemical Physics. Copyright by AIP, the final published
version can be found at
http://scitation.aip.org/content/aip/journal/jcp/146/6/10.1063/1.497562
Real-Time Description of the Electronic Dynamics for a Molecule close to a Plasmonic Nanoparticle
The optical properties of molecules close to plasmonic nanostructures greatly
differ from their isolated molecule counterparts. To theoretically investigate
such systems in a Quantum Chemistry perspective, one has to take into account
that the plasmonic nanostructure (e.g., a metal nanoparticle - NP) is often too
large to be treated atomistically. Therefore, a multiscale description, where
the molecule is treated by an ab initio approach and the metal NP by a lower
level description, is needed. Here we present an extension of one such
multiscale model [Corni, S.; Tomasi, J. {\it J. Chem. Phys.} {\bf 2001}, {\it
114}, 3739] originally inspired by the Polarizable Continuum Model, to a
real-time description of the electronic dynamics of the molecule and of the NP.
In particular, we adopt a Time-Dependent Configuration Interaction (TD CI)
approach for the molecule, the metal NP is described as a continuous dielectric
of complex shape characterized by a Drude-Lorentz dielectric function and the
molecule- NP electromagnetic coupling is treated by an equation-of-motion (EOM)
extension of the quasi-static Boundary Element Method (BEM). The model includes
the effects of both the mutual molecule- NP time-dependent polarization and the
modification of the probing electromagnetic field due to the plasmonic
resonances of the NP. Finally, such an approach is applied to the investigation
of the light absorption of a model chromophore, LiCN, in the presence of a
metal NP of complex shape.Comment: This is the final peer-reviewed manuscript accepted for publication
of an open access article published under an ACS AuthorChoice License, which
permits copying and redistribution of the article or any adaptations for
non-commercial purposes. Link to the original article:
http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.6b1108
Evidence of growing spatial correlations at the glass transition from nonlinear response experiments
The ac nonlinear dielectric response of glycerol was
measured close to its glass transition temperature to investigate the
prediction that supercooled liquids respond in an increasingly non-linear way
as the dynamics slows down (as spin-glasses do). We find that
indeed displays several non trivial features. It is peaked
as a function of the frequency and obeys scaling as a function of
, with the relaxation time of the liquid. The height
of the peak, proportional to the number of dynamically correlated molecules
, increases as the system becomes glassy, and decays as a
power-law of over several decades beyond the peak. These findings
confirm the collective nature of the glassy dynamics and provide the first
direct estimate of the dependence of .Comment: 22 pages, 6 figures. With respect to v1, a few new sentences were
added in the introduction and conclusion, references were updated, some typos
corrected
Gender gap in the ERASMUS mobility program
Studying abroad has become very popular among students. The ERASMUS mobility
program is one of the largest international student exchange programs in the
world, which has supported already more than three million participants since
1987. We analyzed the mobility pattern within this program in 2011-12 and found
a gender gap across countries and subject areas. Namely, for almost all
participating countries, female students are over-represented in the ERASMUS
program when compared to the entire population of tertiary students. The same
tendency is observed across different subject areas. We also found a gender
asymmetry in the geographical distribution of hosting institutions, with a bias
of male students in Scandinavian countries. However, a detailed analysis
reveals that this latter asymmetry is rather driven by subject and consistent
with the distribution of gender ratios among subject areas
Random Matrix Theory and Entanglement in Quantum Spin Chains
We compute the entropy of entanglement in the ground states of a general
class of quantum spin-chain Hamiltonians - those that are related to quadratic
forms of Fermi operators - between the first N spins and the rest of the system
in the limit of infinite total chain length. We show that the entropy can be
expressed in terms of averages over the classical compact groups and establish
an explicit correspondence between the symmetries of a given Hamiltonian and
those characterizing the Haar measure of the associated group. These averages
are either Toeplitz determinants or determinants of combinations of Toeplitz
and Hankel matrices. Recent generalizations of the Fisher-Hartwig conjecture
are used to compute the leading order asymptotics of the entropy as N -->
infinity . This is shown to grow logarithmically with N. The constant of
proportionality is determined explicitly, as is the next (constant) term in the
asymptotic expansion. The logarithmic growth of the entropy was previously
predicted on the basis of numerical computations and conformal-field-theoretic
calculations. In these calculations the constant of proportionality was
determined in terms of the central charge of the Virasoro algebra. Our results
therefore lead to an explicit formula for this charge. We also show that the
entropy is related to solutions of ordinary differential equations of
Painlev\'e type. In some cases these solutions can be evaluated to all orders
using recurrence relations.Comment: 39 pages, 1 table, no figures. Revised version: minor correction
The synchrotron peak shift during high-energy flares of blazars
A prediction for the energy shift of the synchrotron spectrum of
flat-spectrum radio quasars (FSRQs) during high-energy flares is presented. If
the -ray emission of FSRQs is produced by Comptonization of external
radiation, then the peak of the synchrotron spectrum is predicted to move to
lower energies in the flare state. This is opposite to the well-known broadband
spectral behavior of high-frequency peaked BL-Lac objects where the external
radiation field is believed to be weak and synchrotron-self Compton scattering
might be the dominant -ray radiation mechanism. The synchrotron peak
shift, if observed in FSRQs, can thus be used as a diagnostic to determine the
dominant radiation mechanism in these objects. I suggest a few FSRQs as
promising candidates to test the prediction of the external-Comptonization
model.Comment: 9 pages, including 2 figures; uses epsf.sty, rotate.sty; accepted for
ApJ Letters; minor revision
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