30,460 research outputs found
Acoustic waves and heating due to molecular energy transfer in an electric discharge CO laser
This paper summarizes analytical studies and the interpretation of experimental results for the compression and rarefaction waves generated in the cavity of a pulsed CO electric discharge laser. A one-dimensional analysis of acoustic waves is applied to a transversely excited laser. The influences of heating in the cathode fall, heat transfer to the cathode, flow through both the anode and cathode, and bulk heating of the plasma are included. The analysis is used to relate the bulk heating rate to observable features of the pressure and density waves. Data obtained from interferograms and reported elsewhere are used to infer the bulk heating rates in a pulsed CO laser. Results are presented for CO/Ar, CO/N2, and N2 plasmas. Comparison of the data with recent theoretical results for the heating due to electron/ neutral collisions and the anharmonic defect associated with V-V energy transfer shows substantial differences at lower values of total energy deposition. The change of heating with E/N is in fairly good agreement with predicted values
Spatial solitons under competing linear and nonlinear diffractions
We introduce a general model which augments the one-dimensional nonlinear
Schr\"{o}dinger (NLS) equation by nonlinear-diffraction terms competing with
the linear diffraction. The new terms contain two irreducible parameters and
admit a Hamiltonian representation in a form natural for optical media. The
equation serves as a model for spatial solitons near the supercollimation point
in nonlinear photonic crystals. In the framework of this model, a detailed
analysis of the fundamental solitary waves is reported, including the
variational approximation (VA), exact analytical results, and systematic
numerical computations. The Vakhitov-Kolokolov (VK) criterion is used to
precisely predict the stability border for the solitons, which is found in an
exact analytical form, along with the largest total power (norm) that the waves
may possess. Past a critical point, collapse effects are observed, caused by
suitable perturbations. Interactions between two identical parallel solitary
beams are explored by dint of direct numerical simulations. It is found that
in-phase solitons merge into robust or collapsing pulsons, depending on the
strength of the nonlinear diffraction
Formation of energy gap in higher dimensional spin-orbital liquids
A Schwinger boson mean field theory is developed for spin liquids in a
symmetric spin-orbital model in higher dimensions. Spin, orbital and coupled
spin-orbital operators are treated equally. We evaluate the dynamic correlation
functions and collective excitations spectra. As the collective excitations
have a finite energy gap, we conclude that the ground state is a spin-orbital
liquid with a two-fold degeneracy, which breaks the discrete spin-orbital
symmetry. Possible relevence of this spin liquid state to several realistic
systems, such as CaVV and NaSbTiO, are discussed.Comment: 4 pages with 1 figur
Triplet-singlet relaxation in semiconductor single and double quantum dots
We study the triplet-singlet relaxation in two-electron semiconductor quantum
dots. Both single dots and vertically coupled double dots are discussed. In our
work, the electron-electron Coulomb interaction, which plays an important role
in the electronic structure, is included. The spin mixing is caused by
spin-orbit coupling which is the key to the triplet-singlet relaxation. We show
that the selection rule widely used in the literature is incorrect unless near
the crossing/anticrossing point in single quantum dots. The triplet/singlet
relaxation in double quantum dots can be markedly changed by varying barrier
height, inter-dot distance, external magnetic field and dot size.Comment: 7 pages, 4 figures, PRB in pres
Anti-shielding Effect and Negative Temperature in Instantaneously Reversed Electric Fields and Left-Handed Media
The connections between the anti-shielding effect, negative absolute
temperature and superluminal light propagation in both the instantaneously
reversed electric field and the left-handed media are considered in the present
paper. The instantaneous inversion of the exterior electric field may cause the
electric dipoles into the state of negative absolute temperature and therefore
give rise to a negative effective mass term of electromagnetic field (i. e.,
the electromagnetic field propagating inside the negative-temperature medium
will acquire an imaginary rest mass), which is said to result in the potential
superluminality effect of light propagation in this anti-shielding dielectric.
In left-handed media, such phenomena may also arise.Comment: 9 pages, Late
Generalized Valence Bond State and Solvable Models for Spin-1/2 Systems with Orbital degeneracy
A spin-1/2 system with double orbital degeneracy may possess SU(4) symmetry.
According to the group theory a global SU(4) singelt state can be expressed as
a linear combination of all possible configurations consisting of four-site
SU(4) singlets. Following P. W. Andersion's idea for spin 1/2 system, we
propose that the ground state for the antiferromagnetic SU(4) model is SU(4)
resonating valence bond (RVB) state. A short-range SU(4) RVB state is a spin
and orbital liquid, and its elementary excitations has an energy gap. We
construct a series of solvale models which ground states are short-range SU(4)
RVB states. The results can be generalized to the antiferromagnetic SU(N)
models.Comment: 4 page
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