9,447 research outputs found
A microscopic model for solidification
We present a novel picture of a non isothermal solidification process
starting from a molecular level, where the microscopic origin of the basic
mechanisms and of the instabilities characterizing the approach to equilibrium
is rendered more apparent than in existing approaches based on coarse grained
free energy functionals \`a la Landau.
The system is composed by a lattice of Potts spins, which change their state
according to the stochastic dynamics proposed some time ago by Creutz. Such a
method is extended to include the presence of latent heat and thermal
conduction.
Not only the model agrees with previous continuum treatments, but it allows
to introduce in a consistent fashion the microscopic stochastic fluctuations.
These play an important role in nucleating the growing solid phase in the melt.
The approach is also very satisfactory from the quantitative point of view
since the relevant growth regimes are fully characterized in terms of scaling
exponents.Comment: 7 pages Latex +3 figures.p
Evidence of a pressure-induced metallization process in monoclinic VO
Raman and combined trasmission and reflectivity mid infrared measurements
have been carried out on monoclinic VO at room temperature over the 0-19
GPa and 0-14 GPa pressure ranges, respectively. The pressure dependence
obtained for both lattice dynamics and optical gap shows a remarkable stability
of the system up to P*10 GPa. Evidence of subtle modifications of V ion
arrangements within the monoclinic lattice together with the onset of a
metallization process via band gap filling are observed for PP*. Differently
from ambient pressure, where the VO metal phase is found only in
conjunction with the rutile structure above 340 K, a new room temperature
metallic phase coupled to a monoclinic structure appears accessible in the high
pressure regime, thus opening to new important queries on the physics of
VO.Comment: 5 pages, 3 figure
Anisotropic excitonic effects in the energy loss function of hexagonal boron nitride
We demonstrate that the valence energy-loss function of hexagonal boron
nitride (hBN) displays a strong anisotropy in shape, excitation energy and
dispersion for momentum transfer q parallel or perpendicular to the hBN layers.
This is manifested by e.g. an energy shift of 0.7 eV that cannot be captured by
single-particle approaches and is a demonstration of a strong anisotropy in the
two-body electron-hole interaction. Furthermore, for in-plane directions of q
we observe a splitting of the -plasmon in the M direction that is absent in the
K direction and this can be traced back to band-structure effects.Comment: 10 pages, 4 figure
Anomalous Aharonov--Bohm gap oscillations in carbon nanotubes
The gap oscillations caused by a magnetic flux penetrating a carbon nanotube
represent one of the most spectacular observation of the Aharonov-Bohm effect
at the nano--scale. Our understanding of this effect is, however, based on the
assumption that the electrons are strictly confined on the tube surface, on
trajectories that are not modified by curvature effects. Using an ab-initio
approach based on Density Functional Theory we show that this assumption fails
at the nano-scale inducing important corrections to the physics of the
Aharonov-Bohm effect. Curvature effects and electronic density spilled out of
the nanotube surface are shown to break the periodicity of the gap
oscillations. We predict the key phenomenological features of this anomalous
Aharonov-Bohm effect in semi-conductive and metallic tubes and the existence of
a large metallic phase in the low flux regime of Multi-walled nanotubes, also
suggesting possible experiments to validate our results.Comment: 7 figure
Fluctuation-Induced Casimir Forces in Granular Fluids
We have numerically investigated the behavior of driven non-cohesive granular
media and found that two fixed large intruder particles, immersed in a sea of
small particles, experience, in addition to a short range depletion force, a
long range repulsive force. The observed long range interaction is
fluctuation-induced and we propose a mechanism similar to the Casimir effect
that generates it: the hydrodynamic fluctuations are geometrically confined
between the intruders, producing an unbalanced renormalized pressure. An
estimation based on computing the possible Fourier modes explains the repulsive
force and is in qualitative agreement with the simulations.Comment: 4 pages, 3 figures. Accepted in Phys. Rev. Letter
Sequential fissions of heavy nuclear systems
In Xe+Sn central collisions from 12 to 20 MeV/A measured with the INDRA
4 multidetector, the three-fragment exit channel occurs with a significant
cross section. In this contribution, we show that these fragments arise from
two successive binary splittings of a heavy composite system. Strong Coulomb
proximity effects are observed in the three-fragment final state. By comparison
with Coulomb trajectory calculations, we show that the time scale between the
consecutive break-ups decreases with increasing bombarding energy, becoming
compatible with quasi-simultaneous multifragmentation above 18 MeV/A.Comment: 6 pages, 5 figures, contribution to conference proceedings of the
Fifth International Workshop on Nuclear fission and Fission-Product
Spectroscop
Pharmacokinetic and clinical considerations for monitoring asparaginase activity levels during pegaspargase therapy
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111166/1/pbc25426.pd
Experimental determination of the quasi-projectile mass with measured neutrons
The investigation of the isospin dependence of multifragmentation reactions
relies on precise reconstruction of the fragmenting source. The criteria used
to assign free emitted neutrons, detected with the TAMU Neutron Ball, to the
quasi-projectile source are investigated in the framework of two different
simulation codes. Overall and source-specific detection efficiencies for
multifragmentation events are found to be model independent. The equivalence of
the two different methods used to assign experimentally detected charged
particles and neutrons to the emitting source is shown. The method used
experimentally to determine quasi-projectile emitted free neutron multiplicity
is found to be reasonably accurate and sufficiently precise as to allow for the
study of well-defined quasi-projectile sources.Comment: 10 pages, 8 figures. To be submitted to Nucl. Instr. and Meth.
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