9,033 research outputs found
Understanding fragility in supercooled Lennard-Jones mixtures. II. Potential energy surface
We numerically investigated the connection between isobaric fragility and the
properties of high-order stationary points of the potential energy surface in
different supercooled Lennard-Jones mixtures. The increase of effective
activation energies upon supercooling appears to be driven by the increase of
average potential energy barriers measured by the energy dependence of the
fraction of unstable modes. Such an increase is sharper, the more fragile is
the mixture. Correlations between fragility and other properties of high-order
stationary points, including the vibrational density of states and the
localization features of unstable modes, are also discussed.Comment: 13 pages, 13 figures, minor revisions, one figure adde
Understanding fragility in supercooled Lennard-Jones mixtures. I. Locally preferred structures
We reveal the existence of systematic variations of isobaric fragility in
different supercooled Lennard-Jones binary mixtures by performing molecular
dynamics simulations. The connection between fragility and local structures in
the bulk is analyzed by means of a Voronoi construction. We find that clusters
of particles belonging to locally preferred structures form slow, long-lived
domains, whose spatial extension increases by decreasing temperature. As a
general rule, a more rapid growth, upon supercooling, of such domains is
associated to a more pronounced super-Arrhenius behavior, hence to a larger
fragility.Comment: 14 pages, 14 figures, minor revisions, one figure adde
Pairing correlations of cold fermionic gases at overflow from a narrow to a wide harmonic trap
Within the context of Hartree-Fock-Bogoliubov theory, we study the behavior
of superfluid Fermi systems when they pass from a small to a large container.
Such systems can be now realized thanks to recent progress in experimental
techniques. It will allow to better understand pairing properties at overflow
and in general in rapidly varying external potentials
Giant Monopole Resonances and nuclear incompressibilities studied for the zero-range and separable pairing interactions
Background: Following the 2007 precise measurements of monopole strengths in
tin isotopes, there has been a continuous theoretical effort to obtain a
precise description of the experimental results. Up to now, there is no
satisfactory explanation of why the tin nuclei appear to be significantly
softer than 208Pb.
Purpose: We determine the influence of finite-range and separable pairing
interactions on monopole strength functions in semi-magic nuclei.
Methods: We employ self-consistently the Quasiparticle Random Phase
Approximation on top of spherical Hartree-Fock-Bogolyubov solutions. We use the
Arnoldi method to solve the linear-response problem with pairing.
Results: We found that the difference between centroids of Giant Monopole
Resonances measured in lead and tin (about 1 MeV) always turns out to be
overestimated by about 100%. We also found that the volume incompressibility,
obtained by adjusting the liquid-drop expression to microscopic results, is
significantly larger than the infinite-matter incompressibility.
Conclusions: The zero-range and separable pairing forces cannot induce
modifications of monopole strength functions in tin to match experimental data.Comment: 11 RevTeX pages, 16 figures, 1 table, extended versio
Collective vibrational states with fast iterative QRPA method
An iterative method we previously proposed to compute nuclear strength
functions is developed to allow it to accurately calculate properties of
individual nuclear states. The approach is based on the
quasi-particle-random-phase approximation (QRPA) and uses an iterative
non-hermitian Arnoldi diagonalization method where the QRPA matrix does not
have to be explicitly calculated and stored. The method gives substantial
advantages over conventional QRPA calculations with regards to the
computational cost. The method is used to calculate excitation energies and
decay rates of the lowest lying 2+ and 3- states in Pb, Sn, Ni and Ca isotopes
using three different Skyrme interactions and a separable gaussian pairing
force.Comment: 10 pages, 11 figure
Jamming phase diagram for frictional particles
The non-equilibrium transition from a fluid-like state to a disordered
solid-like state, known as the jamming transition, occurs in a wide variety of
physical systems, such as colloidal suspensions and molecular fluids, when the
temperature is lowered or the density increased. Shear stress, as temperature,
favors the fluid-like state, and must be also considered to define the system
'jamming phase diagram' [1-4]. Frictionless athermal systems [1], for instance,
can be described by the zero temperature plane of the jamming diagram in the
temperature, density, stress space. Here we consider the jamming of athermal
frictional systems [8-13] such as granular materials, which are important to a
number of applications from geophysics to industry. At constant volume and
applied shear stress[1, 2], we show that while in absence of friction a system
is either fluid-like or jammed, in the presence of friction a new region in the
density shear-stress plane appears, where new dynamical regimes are found. In
this region a system may slip, or even flow with a steady velocity for a long
time in response to an applied stress, but then eventually jams. Jamming in
non-thermal frictional systems is described here by a phase diagram in the
density, shear-stress and friction space
Near-infrared spectroscopy study of tourniquet-induced forearm ischaemia in patients with coronary artery disease
Near-Infrared Spectroscopy (NIR) can be employed to monitor local changes in haemodynamics and oxygenation of human tissues. A preliminary study has been performed in order to evaluate the NIRS transmittance response to induced forearm ischaemia in patients with coronary artery disease (CAD). The population consists in 40 patients with cardiovascular risk factors and angiographically documented CAD, compared to a group of 13 normal subjects. By inflating and subsequently deflating a cuff placed around the patient arm, an ischaemia has been induced and released, and the patients have been observed until recovery of the basal conditions. A custom LAIRS spectrometer (IRIS) has been used to collect the backscattered light intensities from the patient forearm throughout the ischaemic and the recovery phase. The time dependence of the near-infrared transmittance on the control group is consistent with the available literature. On the contrary, the magnitude and dynamics of the NIRS signal on the CAD patients show deviations from the documented normal behavior, which can be tentatively attributed to abnormal vessel stiffness. These preliminary results, while validating the performance of the IRIS spectrometer, are strongly conducive towards the applicability of the NIRS technique to ischaemia analysis and to endothelial dysfunction characterization in CAD patients with cardiovascular risk factors.Publisher PD
Finite-size effects and collective vibrations in the inner crust of neutron stars
We study the linear response of the inner crust of neutron stars within the
Random Phase Approximation, employing a Skyrme-type interaction as effective
interaction. We adopt the Wigner-Seitz approximation, and consider a single
unit cell of the Coulomb lattice which constitutes the inner crust, with a
nucleus at its center, surrounded by a sea of free neutrons. With the use of an
appropriate operator, it is possible to analyze in detail the properties of the
vibrations of the surface of the nucleus and their interaction with the modes
of the sea of free neutrons, and to investigate the role of shell effects and
of resonant states
Characterization of two new alleles at the goat CSN1S2 locus.
Two novel alleles at the goat CSN1S2 locus have been identified: CSN1S2(F) and CSN1S2(D). Sequence analyses revealed that the CSN1S2(F) allele is characterized by a G --> A transition at the 13th nucleotide in exon 3 changing the seventh amino acid of the mature protein from Val to Ile. The CSN1S2(D) allele, apparently associated with a decreased synthesis of alpha s2-casein, is characterized by a 106-bp deletion, involving the last 11 bp of the exon 11 and the first 95 bp of the following intron. Methods (PCR-RFLP and PCR) for identification of carriers of these alleles have been developed
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