6,375 research outputs found
Classification of phase singularities for complex scalar waves
Motivated by the importance and universal character of phase singularities
which are clarified recently, we study the local structure of equi-phase loci
near the dislocation locus of complex valued planar and spatial waves, from the
viewpoint of singularity theory of differentiable mappings, initiated by H.
Whitney and R. Thom. The classification of phase-singularities are reduced to
the classification of planar curves by radial transformations due to the theory
of A. du Plessis, T. Gaffney, and L. Wilson. Then fold singularities are
classified into hyperbolic and elliptic singularities. We show that the
elliptic singularities are never realized by any Helmholtz waves, while the
hyperbolic singularities are realized in fact. Moreover, the classification and
realizability of Whitney's cusp, as well as its bifurcation problem are
considered in order to explain the three points bifurcation of phase
singularities. In this paper, we treat the dislocation of linear waves mainly,
developing the basic and universal method, the method of jets and
transversality, which is applicable also to non-linear waves.Comment: 17 pages, 5 figure
S-wave bottom tetraquarks
The relativistic four-quark equations are found in the framework of
coupled-channel formalism. The dynamical mixing of the meson-meson states with
the four-quark states is considered. The four-quark amplitudes of the
tetraquarks, including , , and bottom quarks, are constructed. The
poles of these amplitudes determine the masses and widths of -wave bottom
tetraquarks.Comment: 8 pages, late
Linear-response theory of the longitudinal spin Seebeck effect
We theoretically investigate the longitudinal spin Seebeck effect, in which
the spin current is injected from a ferromagnet into an attached nonmagnetic
metal in a direction parallel to the temperature gradient. Using the fact that
the phonon heat current flows intensely into the attached nonmagnetic metal in
this particular configuration, we show that the sign of the spin injection
signal in the longitudinal spin Seebeck effect can be opposite to that in the
conventional transverse spin Seebeck effect when the electron-phonon
interaction in the nonmagnetic metal is sufficiently large. Our linear-response
approach can explain the sign reversal of the spin injection signal recently
observed in the longitudinal spin Seebeck effect.Comment: Proc. of ICM 2012 (Accepted for publication in J. Korean Phys. Soc.),
typos correcte
Evaporative Deposition Patterns Revisited: Spatial Dimensions of the Deposit
A model accounting for finite spatial dimensions of the deposit patterns in
the evaporating sessile drops of colloidal solution on a plane substrate is
proposed. The model is based on the assumption that the solute particles occupy
finite volume and hence these dimensions are of the steric origin. Within this
model, the geometrical characteristics of the deposition patterns are found as
functions of the initial concentration of the solute, the initial geometry of
the drop, and the time elapsed from the beginning of the drying process. The
model is solved analytically for small initial concentrations of the solute and
numerically for arbitrary initial concentrations of the solute. The agreement
between our theoretical results and the experimental data is demonstrated, and
it is shown that the observed dependence of the deposit dimensions on the
experimental parameters can indeed be attributed to the finite dimensions of
the solute particles. These results are universal and do not depend on any free
or fitting parameters; they are important for understanding the evaporative
deposition and may be useful for creating controlled deposition patterns.Comment: 34 pages, 14 figures, LaTeX; submitted to Physical Review
Calculation of pure dephasing for excitons in quantum dots
Pure dephasing of an exciton in a small quantum dot by optical and acoustic
phonons is calculated using the ``independent boson model''. Considering the
case of zero temperature the dephasing is shown to be only partial which
manifests itself in the polarization decaying to a finite value. Typical
dephasing times can be assigned even though the spectra exhibits strongly
non-Lorentzian line shapes. We show that the dephasing from LO phonon
scattering, occurs on a much larger time scale than that of dephasing due to
acoustic phonons which for low temperatures are also a more efficient dephasing
mechanism. The typical dephasing time is shown to strongly depend on the
quantum dot size whereas the electron phonon ``coupling strength'' and external
electric fields tend mostly to effect the residual coherence. The relevance of
the dephasing times for current quantum information processing implementation
schemes in quantum dots is discussed
Learning maps: a design-based approach for capacity building in tertiary online learning and teaching
This paper addresses the importance of creating high quality and contextualized resources for capacity building of academics for online learning and teaching. Drawing on a design-based research framework, the paper presents work-in-progress learning maps. Learning maps are an increasingly popular concept and resource among learning designers which capture and organize various theories and resources for the target learners. In a climate where the tertiary sector struggles to provide quality resources and support for teaching and learning practice, we argue that the creation and implementation of learning maps poses clear advantages and a successful model for teacher capacity building, and subsequently improves student learning
A numerical investigation of a piezoelectric surface acoustic wave interaction with a one-dimensional channel
We investigate the propagation of a piezoelectric surface acoustic wave (SAW)
across a GaAs/AlGaAs heterostructure surface, on which there is
fixed a metallic split-gate. Our method is based on a finite element
formulation of the underlying equations of motion, and is performed in
three-dimensions fully incorporating the geometry and material composition of
the substrate and gates. We demonstrate attenuation of the SAW amplitude as a
result of the presence of both mechanical and electrical gates on the surface.
We show that the incorporation of a simple model for the screening by the
two-dimensional electron gas (2DEG), results in a total electric potential
modulation that suggests a mechanism for the capture and release of electrons
by the SAW. Our simulations suggest the absence of any significant turbulence
in the SAW motion which could hamper the operation of SAW based quantum devices
of a more complex geometry.Comment: 8 pages, 8 figure
Eight-band calculations of strained InAs/GaAs quantum dots compared with one, four, and six-band approximations
The electronic structure of pyramidal shaped InAs/GaAs quantum dots is
calculated using an eight-band strain dependent Hamiltonian. The
influence of strain on band energies and the conduction-band effective mass are
examined. Single particle bound-state energies and exciton binding energies are
computed as functions of island size. The eight-band results are compared with
those for one, four and six bands, and with results from a one-band
approximation in which m(r) is determined by the local value of the strain. The
eight-band model predicts a lower ground state energy and a larger number of
excited states than the other approximations.Comment: 8 pages, 7 figures, revtex, eps
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