1,023 research outputs found
Dynamical mean field theory of optical third harmonic generation
We formulate the third harmonic generation (THG) within the dynamical mean
field theory (DMFT) approximation of the Hubbard model. In the limit of large
dimensions, where DMFT becomes exact, the vertex corrections to current
vertices are identically zero, and hence the calculation of the THG spectrum
reduces to a time-ordered convolution, followd by appropriate analytic
continuuation. We present the typical THG spectrum of the Hubbard model
obtained by this method. Within our DMFT calculation, we observe a nontrivial
approximate {\em scaling} function describing the THG spectra in all Mott
insulators, independent of the gap magnitude.Comment: 4 eps figure
Comment on "Origin of Giant Optical Nonlinearity in Charge-Transfer--Mott Insulators: A New Paradigm for Nonlinear Optics"
Comment on Phys. Rev. Lett. 86, 2086 (2001)Comment: 1 page, 1 eps figur
Monte Carlo Simulation of Secondary Electrons in Solids and its Application for Scanning Electron Microscopy
A new Monte Carlo calculation model is introduced to simulate not only the primary electron behavior but also the secondary electron cascade in a specimen bombarded with an electron beam. Either the primary or the generated electron in a specimen having energy greater than 0.1 keV is defined as a fast electron and the single scattering model is used in the simulation which employs the Mott elastic scattering cross section and the Rao Sahib-Wittry energy loss equation. The electron having energy smaller than 0.1 keV is defined as a slow electron and the cascade model is used which takes into account the classical binary collision with the conduction electrons. The performance of this simulation is verified in comparison with experiments for energy and angular distributions of slow secondary electrons (\u3c50eV). Then, this simulation is applied in a discussion of the quantitative signal variation of the secondary and the backscattered electrons depending on a specimen surface topography. The maximum intensity of the secondary electron signal is obtained where the scanning electron beam reaches around 1nm beside the top edge of a surface step made of Cu with the vertical side wall of 500nm in height
Nonlinear Optical Response of Spin Density Wave Insulators
We calculate the third order nonlinear optical response in the Hubbard model
within the spin density wave (SDW) mean field ansatz in which the gap is due to
onsite Coulomb repulsion. We obtain closed-form analytical results in one
dimension (1D) and two dimension (2D), which show that nonlinear optical
response in SDW insulators in 2D is stronger than both 3D and 1D. We also
calculate the two photon absorption (TPA) arising from the stress tensor term.
We show that in the SDW, the contribution from stress tensor term to the
low-energy peak corresponding to two photon absorption becomes identically zero
if we consider the gauge invariant current properly.Comment: we use \psfrag in figur
Relaxation Dynamics of Photocarriers in One-Dimensional Mott Insulators Coupled to Phonons
We examine recombination processes of photocarriers in one-dimensional Mott
insulators coupled to phonons. Performing density matrix renormalization group
calculations, we find that, even for small electron-phonon coupling, many
phonons are generated dynamically, which cause initial relaxation process after
the irradiation. At the same time, spin-charge coupling coming from mixing of
high- and low-energy states by the irradiation is suppressed. We discuss
differences between Mott and band insulators in terms of relaxation dynamics.Comment: 5 pages, 3 figure
Angle-resolved photoemission study of MX-chain compound [Ni(chxn)Br]Br
We report on the results of angle-resolved photoemission experiments on a
quasi-one-dimensional -chain compound [Ni(chxn)Br]Br (chxn =
1,2-cyclohexanediamine), a one-dimensional Heisenberg system with
and K, which shows a gigantic non-linear optical effect. A "band"
having about 500 meV energy dispersion is found in the first half of the
Brillouin zone , but disappears at . Two
dispersive features, expected from the spin-charge separation, as have been
observed in other quasi-one-dimensional systems like SrCuO, are not
detected. These characteristic features are well reproduced by the -
chain model calculations with a small charge-transfer energy compared
with that of one-dimensional Cu-O based compounds. We propose that this smaller
is the origin of the absence of clear spin- and charge-separation in
the photoemission spectra and strong non-linear optical effect in
[Ni(chxn)Br]Br.Comment: 4 pages, 3 figure
Shear bands in granular flow through a mixing length model
We discuss the advantages and results of using a mixing-length, compressible
model to account for shear banding behaviour in granular flow. We formulate a
general approach based on two function of the solid fraction to be determined.
Studying the vertical chute flow, we show that shear band thickness is always
independent from flowrate in the quasistatic limit, for Coulomb wall boundary
conditions. The effect of bin width is addressed using the functions developed
by Pouliquen and coworkers, predicting a linear dependence of shear band
thickness by channel width, while literature reports contrasting data. We also
discuss the influence of wall roughness on shear bands. Through a Coulomb wall
friction criterion we show that our model correctly predicts the effect of
increasing wall roughness on the thickness of shear bands. Then a simple
mixing-length approach to steady granular flows can be useful and
representative of a number of original features of granular flow.Comment: submitted to EP
Nonlinear Optical Response Functions of Mott Insulators Based on Dynamical Mean Field Approximation
We investigate the nonlinear optical susceptibilities of Mott insulators with
the dynamical mean field approximation. The two-photon absorption (TPA) and the
third-harmonic generation (THG) spectra are calculated, and the classification
by the types of coupling to external fields shows different behavior from
conventional semiconductors. The direct transition terms are predominant both
in the TPA and THG spectra, and the importance of taking all types of
interaction with the external field into account is illustrated in connection
with the THG spectrum and dcKerr effect. The dependence of the TPA and THG
spectra on the Coulomb interaction indicate a scaling relation. We apply this
relation to the quantitative evaluation and obtain results comparable to those
of experiments.Comment: 14 pages, 12 figure
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