165 research outputs found
A mathematical model for the Fermi weak interactions
We consider a mathematical model of the Fermi theory of weak interactions as
patterned according to the well-known current-current coupling of quantum
electrodynamics. We focuss on the example of the decay of the muons into
electrons, positrons and neutrinos but other examples are considered in the
same way. We prove that the Hamiltonian describing this model has a ground
state in the fermionic Fock space for a sufficiently small coupling constant.
Furthermore we determine the absolutely continuous spectrum of the Hamiltonian
and by commutator estimates we prove that the spectrum is absolutely continuous
away from a small neighborhood of the thresholds of the free Hamiltonian. For
all these results we do not use any infrared cutoff or infrared regularization
even if fermions with zero mass are involved
The vibrational dynamics of vitreous silica: Classical force fields vs. first-principles
We compare the vibrational properties of model SiO_2 glasses generated by
molecular-dynamics simulations using the effective force field of van Beest et
al. (BKS) with those obtained when the BKS structure is relaxed using an ab
initio calculation in the framework of the density functional theory. We find
that this relaxation significantly improves the agreement of the density of
states with the experimental result. For frequencies between 14 and 26 THz the
nature of the vibrational modes as determined from the BKS model is very
different from the one from the ab initio calculation, showing that the
interpretation of the vibrational spectra in terms of calculations using
effective potentials can be very misleading.Comment: 7 pages of Latex, 4 figure
On the Infrared Problem for the Dressed Non-Relativistic Electron in a Magnetic Field
We consider a non-relativistic electron interacting with a classical magnetic
field pointing along the -axis and with a quantized electromagnetic field.
The system is translation invariant in the -direction and we consider the
reduced Hamiltonian associated with the total momentum along the
-axis. For a fixed momentum sufficiently small, we prove that
has a ground state in the Fock representation if and only if
, where is the derivative of the map . If , we obtain the
existence of a ground state in a non-Fock representation. This result holds for
sufficiently small values of the coupling constant
Universal electric-field-driven resistive transition in narrow-gap Mott insulators
One of today's most exciting research frontier and challenge in condensed
matter physics is known as Mottronics, whose goal is to incorporate strong
correlation effects into the realm of electronics. In fact, taming the Mott
insulator-to-metal transition (IMT), which is driven by strong electronic
correlation effects, holds the promise of a commutation speed set by a quantum
transition, and with negligible power dissipation. In this context, one
possible route to control the Mott transition is to electrostatically dope the
systems using strong dielectrics, in FET-like devices. Another possibility is
through resistive switching, that is, to induce the insulator-to-metal
transition by strong electric pulsing. This action brings the correlated system
far from equilibrium, rendering the exact treatment of the problem a difficult
challenge. Here, we show that existing theoretical predictions of the
off-equilibrium manybody problem err by orders of magnitudes, when compared to
experiments that we performed on three prototypical narrow gap Mott systems
V2-xCrxO3, NiS2-xSex and GaTa4Se8, and which also demonstrate a striking
universality of this Mott resistive transition (MRT). We then introduce and
numerically study a model based on key theoretically known physical features of
the Mott phenomenon in the Hubbard model. We find that our model predictions
are in very good agreement with the observed universal MRT and with a
non-trivial timedelay electric pulsing experiment, which we also report. Our
study demonstrates that the MRT can be associated to a dynamically directed
avalanche
Dissolution–precipitation processes governing the carbonation and silicification of the serpentinite sole of the New Caledonia ophiolite
International audienceThe weathering of mantle peridotite tectoni-cally exposed to the atmosphere leads commonly to natural carbonation processes. Extensive cryptocrystalline mag-nesite veins and stock-work are widespread in the ser-pentinite sole of the New Caledonia ophiolite. Silica is systematically associated with magnesite. It is commonly admitted that Mg and Si are released during the laterization of overlying peridotites. Thus, the occurrence of these veins is generally attributed to a per descensum mechanism that involves the infiltration of meteoric waters enriched in dissolved atmospheric CO 2. In this study, we investigate serpentinite carbonation processes, and related silicifica-tion, based on a detailed petrographic and crystal chemical study of serpentinites. The relationships between serpen-tine and alteration products are described using an original method for the analysis of micro-X-ray fluorescence images performed at the centimeter scale. Our investigations highlight a carbonation mechanism, together with precipitation of amorphous silica and sepiolite, based on a dis-solution–precipitation process. In contrast with the per descensum Mg/Si-enrichment model that is mainly concentrated in rock fractures, dissolution–precipitation process is much more pervasive. Thus, although the texture of rocks remains relatively preserved, this process extends more widely into the rock and may represent a major part of total carbonation of the ophiolite
A Uniform Approach to Antiferromagnetic Heisenberg Spins on Low Dimensional Lattices
Using group theoretical methods we show for both the triangular and square
lattices that in the continuum limit the antiferromagnetic order parameter
lives on SO3 without respect of the initial lattice. For the antiferromagnetic
chain we recover the Haldane decomposition. This order parameter interacts with
a local gauge field rather than with a global one as implicitly suggested in
the literature which in our approach appears in a rather natural manner. In
fact this merely corresponds to a novel extension of the spin group by a local
gauge field. This analysis based on the real division algebras applies to low
dimensional lattices.Comment: 5 pages; REVTeX
Electronic redistribution around oxygen atoms in silicate melts by ab initio molecular dynamics simulation
The structure around oxygen atoms of four silicate liquids (silica, rhyolite,
a model basalt and enstatite) is evaluated by ab initio molecular dynamics
simulation. Thanks to the use of maximally localized Wannier orbitals to
represent the electronic ground state of the simulated system, one is able to
quantify the redistribution of electronic density around oxygen atoms as a
function of the cationic environment and melt composition. It is shown that the
structure of the melt in the immediate vicinity of the oxygen atoms modulates
the distribution of the Wannier orbitals associated with oxygen atoms. In
particular the evaluation of the distances between the oxygen-core and the
orbital Wannier centers and their evolution with the nature of the cation
indicates that the Al-O bond in silicate melts is certainly less covalent than
the Si-O bond while for the series Mg-O, Ca-O, Na-O and K-O the covalent
character of the M-O bond diminishes rapidly to the benefit of the ionic
character. Furthermore it is found that the distribution of the oxygen dipole
moment coming from the electronic polarization is only weakly dependent on the
melt composition, a finding which could explain why some empirical force fields
can exhibit a high degree of transferability with melt composition.Comment: 27 pages, 7 figures. To be published in Journal of Non-Crystalline
Solid
Coupling between magnetic field and curvature in Heisenberg spins on surfaces with rotational symmetry
We study the nonlinear -model in an external magnetic field applied
on curved surfaces with rotational symmetry. The Euler-Lagrange equations
derived from the Hamiltonian yield the double sine-Gordon equation (DSG)
provided the magnetic field is tuned with the curvature of the surface. A
skyrmion appears like a solution for this model and surface deformations
are predicted at the sector where the spins point in the opposite direction to
the magnetic field. We also study some specific examples by applying the model
on three rotationally symmetric surfaces: the cylinder, the catenoid and the
hyperboloid. The coupling between a magnetic field and the curvature of the
substract is an interesting result and we believe that this issue may be
relevant to be applied in condensed matter systems, e.g., superconductors,
nematic liquid crystals, graphene and topological insulators.Comment: To be published in Physics Letters
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