13,739 research outputs found
The vortex dynamics of a Ginzburg-Landau system under pinning effect
It is proved that the vortices are attracted by impurities or inhomogeities
in the superconducting materials. The strong H^1-convergence for the
corresponding Ginzburg-Landau system is also proved.Comment: 23page
Signal enhancement of the in-plane and out-of-plane Rayleigh wave components
Several groups have reported an enhancement of the ultrasonic Rayleigh wave when scanning close to a surface-breaking defect in a metal sample. This enhancement may be explained as an interference effect where the waves passing directly between source and receiver interfere with those waves reflected back from the defect. We present finite element models of the predicted enhancement when approaching a defect, along with experiments performed using electromagnetic acoustic transducers sensitive to either in-plane or out-of-plane motion. A larger enhancement of the in-plane motion than the out-of-plane motion is observed and can be explained by considering ultrasonic reflections and mode conversion at the defect
Nonequilibrium Green's function approach to mesoscopic thermal transport
We present a formulation of a nonequilibrium Green's function method for
thermal current in nanojunction atomic systems with nonlinear interactions.
This first-principle approach is applied to the calculation of the thermal
conductance in carbon nanotube junctions. It is shown that nonlinearity already
becomes important at low temperatures. Nonlinear interactions greatly suppress
phonon transmission at room temperature. The peak of thermal conductance is
found to be around 400K, in good agreement with experiments. High-order phonon
scattering processes are important for diffusive heat transport.Comment: 4 pages, 4 figure
Oscillatory Magneto-Thermopower and Resonant Phonon Drag in a High-Mobility 2D Electron Gas
Experimental and theoretical evidence is presented for new low-magnetic-field
( kG) 1/B-oscillations in the thermoelectric power of a high-mobility
GaAs/AlGaAs two-dimensional (2D) electron gas. The oscillations result from
inter-Landau-Level resonances of acoustic phonons carrying a momentum equal to
twice the Fermi wavenumber at . Numerical calculations show that both 3D
and 2D phonons can contribute to this effect.Comment: 4 pages, 5 figure
Shear wave generation using a spiral electromagnetic acoustic transducer
A spiral electromagnetic acoustic transducer (EMAT) is efficient in eddy current generation and has been used for surface defect inspection using Rayleigh waves or thickness gauging based on plane waves in echo mode. Measured and calculated particle velocities and directivities are presented. It is found that the shear wave is not predominantly a plane wave. It has zero amplitude on the axis of the generation EMAT and has maximum amplitude at the critical angle. The shear wave could be used in the steel industry for both internal and surface defect inspections together with Rayleigh wave
Weakly nonlinear quantum transport: an exactly solvable model
We have studied the weakly non-linear quantum transport properties of a
two-dimensional quantum wire which can be solved exactly. The non-linear
transport coefficients have been calculated and interesting physical properties
revealed. In particular we found that as the incoming electron energy
approaches a resonant point given by energy , where the transport is
characterized by a complete reflection, the second order non-linear conductance
changes its sign. This has interesting implications to the current-voltage
characteristics. We have also investigated the establishment of the gauge
invariance condition. We found that for systems with a finite scattering
region, correction terms to the theoretical formalism are needed to preserve
the gauge invariance. These corrections were derived analytically for this
model.Comment: 15 pages, LaTeX, submitted to Phys. Rev.
Spins of the supermassive black hole in M87: new constraints from TeV observations
The rapid TeV ray variability detected in the well-known nearby
radio galaxy M87 implies an extremely compact emission region (5-10
Schwarzschild radii) near the horizon of the supermassive black hole in the
galactic center. TeV photons are affected by dilution due to interaction with
the radiation field of the advection-dominated accretion flow (ADAF) around the
black hole, and can thus be used to probe the innermost regions around the
black hole. We calculate the optical depth of the ADAF radiation field to the
TeV photons and find it strongly depends on the spin of the black hole. We find
that transparent radii of 10 TeV photons are of and
for the maximally rotating and non-rotating black holes, respectively. With the
observations, the calculated transparent radii strongly suggest the black hole
is spinning fast in the galaxy. TeV photons could be used as a powerful
diagnostic for estimating black hole spins in galaxies in the future.Comment: 4 pages, 4 figures. to appear in ApJ
Quantum Electronic Transport through a Precessing Spin
The conductance through a local nuclear spin precessing in a magnetic field
is studied by using the equations-of-motion approach. The characteristics of
the conductance is determined by the tunneling matrix and the position of
equilibrium chemical potential. We find that the spin flip coupling between the
electrons on the spin site and the leads produces the conductance oscillation.
When the spin is precessing in the magnetic field at Larmor frequency
(), the conductance develops the oscillation with the frequency of
both and 2 components, the relative spectrum weight of
which can be tuned by the chemical potential and the spin flip coupling.Comment: 5 pages, 3 figure
Isospin dependence of pseudospin symmetry in nuclear resonant states
The relativistic mean field theory in combination with the analytic
continuation in the coupling constant method is used to determine the energies
and widths of single-particle resonant states in Sn isotopes. It is shown that
there exists clear shell structure in the resonant levels as appearing in the
bound levels. In particular, the isospin dependence of pseudospin symmetry is
clearly shown in the resonant states, is consistent with that in the bound
states, where the splittings of energies and widths between pseudospin doublets
are found in correlation with the quantum numbers of single-particle states, as
well as the nuclear mass number. The similar phenomenon also emerges in the
spin partners.Comment: 7 pages, 6 figure
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