313 research outputs found
From spinons to magnons in explicit and spontaneously dimerized antiferromagnetic chains
We reconsider the excitation spectra of a dimerized and frustrated
antiferromagnetic Heisenberg chain. This model is taken as the simpler example
of compiting spontaneous and explicit dimerization relevant for Spin-Peierls
compounds. The bosonized theory is a two frequency Sine-Gordon field theory. We
analize the excitation spectrum by semiclassical methods. The elementary
triplet excitation corresponds to an extended magnon whose radius diverge for
vanishing dimerization. The internal oscilations of the magnon give rise to a
series of excited state until another magnon is emited and a two magnon
continuum is reached. We discuss, for weak dimerization, in which way the
magnon forms as a result of a spinon-spinon interaction potential.Comment: 5 pages, latex, 3 figures embedded in the tex
Excitations with fractional spin less than 1/2 in frustrated magnetoelastic chains
We study the magnetic excitations on top of the plateaux states recently
discovered in spin-Peierls systems in a magnetic field. We show by means of
extensive density matrix renormalization group (DMRG) computations and an
analytic approach that one single spin-flip on top of
() plateau decays into elementary excitations each carrying a
fraction of the spin. This fractionalization goes beyond the
well-known decay of one magnon into two spinons taking place on top of the M=0
plateau. Concentrating on the plateau (N=3) we unravel the
microscopic structure of the domain walls which carry fractional
spin-, both from theory and numerics. These excitations are shown to
be noninteracting and should be observable in x-ray and nuclear magnetic
resonance experiments.Comment: 6 pages, 5 figures. Accepted to be published in Phys. Rev.
Pressure dependence of the melting mechanism at the limit of overheating in Lennard-Jones crystals
We study the pressure dependence of the melting mechanism of a surface free
Lennard-Jones crystal by constant pressure Monte Carlo simulation. The
difference between the overheating temperature() and the
thermodynamical melting point() increase for increasing pressure. When
particles move into the repulsive part of the potential the properties at
change. There is a crossover pressure where the volume jump becomes
pressure-independent. The overheating limit is pre-announced by thermal
excitation of big clusters of defects. The temperature zone where the system is
dominated by these big clusters of defects increases with increasing pressure.
Beyond the crossover pressure we find that excitation of defects and clusters
of them start at the same temperature scale related with .Comment: 6 pages, 5 figures. Accepted for publication in Physical Review
Mixing of magnetic and phononic excitations in incommensurate Spin-Peierls systems
We analyze the excitation spectra of a spin-phonon coupled chain in the
presence of a soliton. This is taken as a microscopic model of a Spin-Peierls
material placed in a high magnetic field. We show, by using a semiclassical
approximation in the bosonized representation of the spins that a trapped
magnetic state obtained in the adiabatic approximation is destroyed by
dynamical phonons. Low energy states are phonons trapped by the soliton. When
the magnetic gap is smaller than the phonon frequencies the only low energy
state is a mixed magneto-phonon state with the energy of the gap. We emphasize
that our results are relevant for the Raman spectra of the inorganic
Spin-Peierls material CuGeO.Comment: 5 pages, latex, 2 figures embedded in the tex
Liquefaction Potential of Recent Fills versus Natural Sands Located in High-Seismicity Regions Using Shear-Wave Velocity
The liquefaction potential of clean and silty sands is examined on the basis of the field measurement of the shear-wave velocity, Vs. The starting point is the database of 225 case histories supporting the Andrus-Stokoe Vs-based liquefaction chart for sands, silts, and gravels. Only clean and silty sands with nonplastic fines are considered, resulting in a reduced database of 110 case histories, which are plotted separately by type of deposit. A line of constant cyclic shear strain, γcl≈0.03%, is recommended for liquefaction evaluation of recent uncompacted clean and silty sand fills and earthquake magnitude, Mw=7.5. The geologically recent natural silty sand sites in the Imperial Valley of southern California have significantly higher liquefaction resistance as a result of preshaking caused by the high seismic activity in the valley. A line of constant cyclic shear strain, γcl≈0.1–0.2%, is recommended for practical use in the Imperial Valley. Additional research including revisiting available Vs-based and penetration-based databases is proposed to generalize the results of the paper and develop liquefaction charts that account more realistically for deposit type, seismic history, and geologic age
Microscopic theory for the incommensurate transition in TiOCl
We propose a microscopic mechanism for the incommensurate phase in TiOX
compounds. The model includes the antiferromagnetic chains of Ti ions immersed
in the phonon bath of the bilayer structure. Making use of the Cross-Fisher
theory, we show that the geometrically frustrated character of the lattice is
responsible for the structural instability which leads the chains to an
incommensurate phase without an applied magnetic field. In the case of TiOCl,
we show that our model is consistent with the measured phonon frequencies at
and the value of the incommensuration vector at the transition
temperature. Moreover, we find that the dynamical structure factor shows a
progressive softening of an incommensurate phonon near the zone boundary as the
temperature decreases. This softening is accompanied by a broadening of the
peak which gets asymmetrical as well when going towards the transition
temperature. These features are in agreement with the experimental inelastic
X-ray measurements.Comment: 6 pages, 5 figures. Published versio
Influence of the anion potential on the charge ordering in quasi-one dimensional charge transfer salts
We examine the various instabilities of quarter-filled strongly correlated
electronic chains in the presence of a coupling to the underlying lattice. To
mimic the physics of the (TMTTF)X Bechgaard-Fabre salts we also include
electrostatic effects of intercalated anions. We show that small displacements
of the anion can stabilize new mixed Charged Density Wave-Bond Order Wave
phases in which central symmetry centers are suppressed. This finding is
discussed in the context of recent experiments. We suggest that the recently
observed charge ordering is due to a cooperative effect between the Coulomb
interaction and the coupling of the electronic stacks to the anions. On the
other hand, the Spin-Peierls instability at lower temperature requires a
Peierls-like lattice coupling.Comment: Latex, 4 pages, 4 postscript figure
Modeling seismic wave propagation and amplification in 1D/2D/3D linear and nonlinear unbounded media
To analyze seismic wave propagation in geological structures, it is possible
to consider various numerical approaches: the finite difference method, the
spectral element method, the boundary element method, the finite element
method, the finite volume method, etc. All these methods have various
advantages and drawbacks. The amplification of seismic waves in surface soil
layers is mainly due to the velocity contrast between these layers and,
possibly, to topographic effects around crests and hills. The influence of the
geometry of alluvial basins on the amplification process is also know to be
large. Nevertheless, strong heterogeneities and complex geometries are not easy
to take into account with all numerical methods. 2D/3D models are needed in
many situations and the efficiency/accuracy of the numerical methods in such
cases is in question. Furthermore, the radiation conditions at infinity are not
easy to handle with finite differences or finite/spectral elements whereas it
is explicitely accounted in the Boundary Element Method. Various absorbing
layer methods (e.g. F-PML, M-PML) were recently proposed to attenuate the
spurious wave reflections especially in some difficult cases such as shallow
numerical models or grazing incidences. Finally, strong earthquakes involve
nonlinear effects in surficial soil layers. To model strong ground motion, it
is thus necessary to consider the nonlinear dynamic behaviour of soils and
simultaneously investigate seismic wave propagation in complex 2D/3D geological
structures! Recent advances in numerical formulations and constitutive models
in such complex situations are presented and discussed in this paper. A crucial
issue is the availability of the field/laboratory data to feed and validate
such models.Comment: of International Journal Geomechanics (2010) 1-1
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