321,831 research outputs found
More than one dynamic crossover in protein hydration water
Studies of liquid water in its supercooled region have led to many insights
into the structure and behavior of water. While bulk water freezes at its
homogeneous nucleation temperature of approximately 235 K, for protein
hydration water, the binding of water molecules to the protein avoids
crystallization. Here we study the dynamics of the hydrogen bond (HB) network
of a percolating layer of water molecules, comparing measurements of a hydrated
globular protein with the results of a coarse-grained model that has been shown
to successfully reproduce the properties of hydration water. With dielectric
spectroscopy we measure the temperature dependence of the relaxation time of
protons charge fluctuations. These fluctuations are associated to the dynamics
of the HB network of water molecules adsorbed on the protein surface. With
Monte Carlo (MC) simulations and mean--field (MF) calculations we study the
dynamics and thermodynamics of the model. In both experimental and model
analyses we find two dynamic crossovers: (i) one at about 252 K, and (ii) one
at about 181 K. The agreement of the experiments with the model allows us to
relate the two crossovers to the presence of two specific heat maxima at
ambient pressure. The first is due to fluctuations in the HB formation, and the
second, at lower temperature, is due to the cooperative reordering of the HB
network
Diffusing opinions in bounded confidence processes
We study the effects of diffusing opinions on the Deffuant et al. model for
continuous opinion dynamics. Individuals are given the opportunity to change
their opinion, with a given probability, to a randomly selected opinion inside
an interval centered around the present opinion. We show that diffusion induces
an order-disorder transition. In the disordered state the opinion distribution
tends to be uniform, while for the ordered state a set of well defined opinion
clusters are formed, although with some opinion spread inside them. If the
diffusion jumps are not large, clusters coalesce, so that weak diffusion favors
opinion consensus. A master equation for the process described above is
presented. We find that the master equation and the Monte-Carlo simulations do
not always agree due to finite-size induced fluctuations. Using a linear
stability analysis we can derive approximate conditions for the transition
between opinion clusters and the disordered state. The linear stability
analysis is compared with Monte Carlo simulations. Novel interesting phenomena
are analyzed
Basis set effects on the hyperpolarizability of CHCl_3: Gaussian-type orbitals, numerical basis sets and real-space grids
Calculations of the hyperpolarizability are typically much more difficult to
converge with basis set size than the linear polarizability. In order to
understand these convergence issues and hence obtain accurate ab initio values,
we compare calculations of the static hyperpolarizability of the gas-phase
chloroform molecule (CHCl_3) using three different kinds of basis sets:
Gaussian-type orbitals, numerical basis sets, and real-space grids. Although
all of these methods can yield similar results, surprisingly large, diffuse
basis sets are needed to achieve convergence to comparable values. These
results are interpreted in terms of local polarizability and
hyperpolarizability densities. We find that the hyperpolarizability is very
sensitive to the molecular structure, and we also assess the significance of
vibrational contributions and frequency dispersion
Phonon transport in large scale carbon-based disordered materials: Implementation of an efficient order-N and real-space Kubo methodology
We have developed an efficient order-N real-space Kubo approach for the
calculation of the phonon conductivity which outperforms state-of-the-art
alternative implementations based on the Green's function formalism. The method
treats efficiently the time-dependent propagation of phonon wave packets in
real space, and this dynamics is related to the calculation of the thermal
conductance. Without loss of generality, we validate the accuracy of the method
by comparing the calculated phonon mean free paths in disordered carbon
nanotubes (isotope impurities) with other approaches, and further illustrate
its upscalability by exploring the thermal conductance features in large width
edge-disordered graphene nanoribbons (up to ~20 nm), which is out of the reach
of more conventional techniques. We show that edge-disorder is the most
important scattering mechanism for phonons in graphene nanoribbons with
realistic sizes and thermal conductance can be reduced by a factor of ~10.Comment: Accepted for publication in Physical Review B - Rapid Communication
An exact expression to calculate the derivatives of position-dependent observables in molecular simulations with flexible constraints
In this work, we introduce an algorithm to compute the derivatives of
physical observables along the constrained subspace when flexible constraints
are imposed on the system (i.e., constraints in which the hard coordinates are
fixed to configuration-dependent values). The presented scheme is exact, it
does not contain any tunable parameter, and it only requires the calculation
and inversion of a sub-block of the Hessian matrix of second derivatives of the
function through which the constraints are defined. We also present a practical
application to the case in which the sought observables are the Euclidean
coordinates of complex molecular systems, and the function whose minimization
defines the constraints is the potential energy. Finally, and in order to
validate the method, which, as far as we are aware, is the first of its kind in
the literature, we compare it to the natural and straightforward
finite-differences approach in three molecules of biological relevance:
methanol, N-methyl-acetamide and a tri-glycine peptideComment: 13 pages, 8 figures, published versio
Quantum Simulation of Quantum Field Theories in Trapped Ions
We propose the quantum simulation of a fermion and an antifermion field modes
interacting via a bosonic field mode, and present a possible implementation
with two trapped ions. This quantum platform allows for the scalable add-up of
bosonic and fermionic modes, and represents an avenue towards quantum
simulations of quantum field theories in perturbative and nonperturbative
regimes.Comment: To be published in Physical Review Letter
Spin canted magnetism, decoupling of charge and spin ordering in NdNiO
We report detailed magnetization measurements on the perovskite oxide
NdNiO. This system has a first order metal-insulator (M-I) transition at
about 200 K which is associated with charge ordering. There is also a
concurrent paramagnetic to antiferromagnetic spin ordering transition in the
system. We show that the antiferromagnetic state of the nickel sublattice is
spin canted. We also show that the concurrency of the charge ordering and spin
ordering transitions is seen only while warming up the system from low
temperature. The transitions are not concurrent while cooling the system
through the M-I transition temperature. This is explained based on the fact
that the charge ordering transition is first order while the spin ordering
transition is continuous. In the magnetically ordered state the system exhibits
ZFC-FC irreversibilities, as well as history-dependent magnetization and aging.
Our analysis rules out the possibility of spin-glass or superparamagnetism and
suggests that the irreversibilities originate from magnetocrystalline
anisotropy and domain wall pinning.Comment: 8 pages, 7 figure
Tuning laser-induced bandgaps in graphene
Could a laser field lead to the much sought-after tunable bandgaps in
graphene? By using Floquet theory combined with Green's functions techniques,
we predict that a laser field in the mid-infrared range can produce observable
bandgaps in the electronic structure of graphene. Furthermore, we show how they
can be tuned by using the laser polarization. Our results could serve as a
guidance to design opto-electronic nano-devices.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter
Self-assembly scenarios of patchy colloidal particles
The rapid progress in precisely designing the surface decoration of patchy
colloidal particles offers a new, yet unexperienced freedom to create building
entities for larger, more complex structures in soft matter systems. However,
it is extremely difficult to predict the large variety of ordered equilibrium
structures that these particles are able to undergo under the variation of
external parameters, such as temperature or pressure. Here we show that, by a
novel combination of two theoretical tools, it is indeed possible to predict
the self-assembly scenario of patchy colloidal particles: on one hand, a
reliable and efficient optimization tool based on ideas of evolutionary
algorithms helps to identify the ordered equilibrium structures to be expected
at T = 0; on the other hand, suitable simulation techniques allow to estimate
via free energy calculations the phase diagram at finite temperature. With
these powerful approaches we are able to identify the broad variety of emerging
self-assembly scenarios for spherical colloids decorated by four patches and we
investigate and discuss the stability of the crystal structures on modifying in
a controlled way the tetrahedral arrangement of the patches.Comment: 11 pages, 7 figures, Soft Matter Communication (accepted
Spin-dependent effective interactions for halo nuclei
We discuss the spin-dependence of the effective two-body interactions
appropriate for three-body computations. The only reasonable choice seems to be
the fine and hyperfine interactions known for atomic electrons interacting with
the nucleus. One exception is the nucleon-nucleon interaction imposing a
different type of symmetry. We use the two-neutron halo nucleus 11Li as
illustration. We demonstrate that models with the wrong spin-dependence are
basically without predictive power. The Pauli forbidden core and valence states
must be consistently treated.Comment: TeX file, 6 pages, 3 postscript figure
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