630 research outputs found
An ab initio study of magneto-electric coupling of
The present paper proposes the direct calculation of the microscopic
contributions to the magneto-electric coupling, using ab initio methods. The
electrostrictive and the Dzyaloshinskii-Moriya contributions were evaluated
individually. For this purpose a specific method was designed, combining DFT
calculations and embedded fragments, explicitely correlated, quantum chemical
calculations. This method allowed us to calculate the evolution of the magnetic
couplings as a function of an applied electric field. We found that in the Dzyaloshinskii-Moriya contribution to the magneto-electric effect
is three orders of magnitude weaker than the electrostrictive contribution.
Strictive effects are thus dominant in the magnetic exchange evolution under an
applied electric field, and by extension on the magneto-electric effect. These
effects remain however quite small and the modifications of the magnetic
excitations under an applied electric field will be difficult to observe
experimentally. Another important conclusion is that the amplitude of the
magneto-electric effect is very small. Indeed, it can be shown that the linear
magneto-electric tensor is null due to the inter-layer symmetry operations.Comment: J. Phys. Cond. Matter 201
Correlation and Dimerization Effects on the Physical Behavior of the Charge Transfer Salts : A DMRG Study of the Quarter-Filling t-J Model
The present work studies the quasi one-dimensional -based
compounds within a correlated model. More specifically, we focus our attention
on the composed influence of the electronic dimerization-factor and the
repulsion, on the transport properties and the localization of the electronic
density in the ground-state. Those properties are studied through the
computation of the charge gaps (difference between the ionization potential and
the electro-affinity: IP-EA) and the long- and short-bond orders of an infinite
quarter-filled chain within a model. The comparison between the
computed gaps and the experimental activation energy of the semiconductor
allows us to estimate the on-site electronic
repulsion of the molecule to .Comment: 13 pages, 4 figures, RevTe
Ultrafast non-linear optical signal from a single quantum dot: exciton and biexciton effects
We present results on both the intensity and phase-dynamics of the transient
non-linear optical response of a single quantum dot (SQD).
The time evolution of the Four Wave Mixing (FWM) signal on a subpicosecond
time scale is dominated by biexciton effects. In particular, for the
cross-polarized excitation case a biexciton bound state is found. In this
latter case, mean-field results are shown to give a poor description of the
non-linear optical signal at small times. By properly treating exciton-exciton
effects in a SQD, coherent oscillations in the FWM signal are clearly
demonstrated. These oscillations, with a period corresponding to the inverse of
the biexciton binding energy, are correlated with the phase dynamics of the
system's polarization giving clear signatures of non-Markovian effects in the
ultrafast regime.Comment: 10 pages, 3 figure
Phase distortions of attosecond pulses produced by resonance-enhanced high harmonic generation
Resonant enhancement of high harmonic generation can be obtained in plasmas
containing ions with strong radiative transitions resonant with harmonic
orders. The mechanism for this enhancement is still debated. We perform the
first temporal characterization of the attosecond emission from a tin plasma
under near-resonant conditions for two different resonance detunings. We show
that the resonance considerably changes the relative phase of neighbouring
harmonics. For very small detunings, their phase locking may even be lost,
evidencing strong phase distortions in the emission process and a modified
attosecond structure. These features are well reproduced by our simulations,
allowing their interpretation in terms of the phase of the recombination dipole
moment
ArI2(X)→Ar+I2(B) photodissociation: Comparison between linear and T-shaped isomers dynamics
An attempt was made to compute the spectra and product state distributions on semiempirical and ab initio electronic potential energy surfaces for both linear and T-shaped ArI2(X) isomers. It was shown that whereas the T-shaped isomer dynamics is dominated by resonances, dissociation of the linear isomer involves competition between a fast direct process and slower vibrational predissociation of near-linear resonances.This work has been supported by DGICYT @Ministerio de Educación y Ciencia (MEC), Spain, under Grant No. PB95- 0071, INTAS under Grant No. 97-31573 and the SpanishFrench PICASSO project No. HF1999-0132. One of authors (A.A.B) also thanks MEC for sabbatical fellowship.Peer Reviewe
Phonons in the multiferroic langasite BaNbFeSiO : evidences for symmetry breaking
The chiral langasite BaNbFeSiO is a multiferroic
compound. While its magnetic order below T=27 K is now well characterised,
its polar order is still controversial. We thus looked at the phonon spectrum
and its temperature dependence to unravel possible crystal symmetry breaking.
We combined optical measurements (both infrared and Raman spectroscopy) with ab
initio calculations and show that signatures of a polar state are clearly
present in the phonon spectrum even at room temperature. An additional symmetry
lowering occurs below 120~K as seen from emergence of softer phonon modes in
the THz range. These results confirm the multiferroic nature of this langasite
and open new routes to understand the origin of the polar state
Real-Time Non-Rigid Surface Detection
On-line diagnostics of power system components is an important area since it allows the diagnostics to be performed at regular intervals during the normal operation of the components. This combined with reliability centered maintenance could lead to reduced customer outages. In this thesis the on-line diagnostic methods for medium voltage cross-linked polyethylene (XLPE) cables are investigated based on Time Domain Reflectometry (TDR). Degradation of XLPE insulated power cables by water-trees (WT) is a primary cause of failure of these cables. The detection of WT and information about the severity of the degradation can be obtained with off-line measurements using dielectric spectroscopy. In many situations only a limited part of the cable may be degraded by the WT. In such a situation a method for localization of this WT section would be desirable. The developed high frequency measurements superimposed on HV system is presented. It was used to measure the propagation constant of the WT aged cables as a function of the applied HV. This was done in order to study the diagnostic criteria, which could be used for on-line TDR diagnostics of WT aged cables. A physically based dielectric model of WT was developed in order to explain qualitatively and quantitatively the permittivity and loss of WT at different frequencies and voltages. The sensors applicable for the on-line TDR were investigated in terms of sensitivity and bandwidth. High frequency models were built and the simulation results in frequency and time domains were verified by measurements. The developed on-line TDR systems are presented. Their applicability to detect water penetration under the cable sheath and localize the broken screen wires was investigated during the measurements in laboratory environment. The results of field measurements with on-line TDR are presented. Variations due to load cycling of the cable were observed, where an increase in the cable temperature cause an increase of the pulse propagation velocity in the cable. The temperature dependent wave propagation in the cable is investigated and explained by modeling.QC 20100709</p
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