6,367 research outputs found
Effect of Impurities and Effective Masses on Spin-Dependent Electrical Transport in Ferromagnet-Normal Metal-Ferromagnet Hybrid Junctions
The effect of nonmagnetic impurities and the effective masses on the
spin-dependent transport in a ferromagnet-normal metal-ferromagnet junction is
investigated on the basis of a two-band model. Our results show that impurities
and the effective masses of electrons in two ferromagnetic electrodes have
remarkable effects on the behaviors of the conductance, namely, both affect the
oscillating amplitudes, periods, as well as the positions of the resonant peaks
of the conductance considerably. The impurity tends to suppress the amplitudes
of the conductance, and makes the spin-valve effect less obvious, but under
certain conditions the phenomenon of the so-called impurity-induced resonant
tunneling is clearly observed. The impurity and the effective mass both can
lead to nonmonotonous oscillation of the junction magnetoresistance (JMR) with
the incident energy and the thickness of the normal metal. It is also observed
that a smaller difference of the effective masses of electrons in two
ferromagnetic electrodes would give rise to a larger amplitude of the JMR.Comment: Revtex, 10 figure
Singularity formation in three-dimensional vortex sheets
We study singularity formation of three-dimensional (3-D) vortex sheets without surface tension using a new approach. First, we derive a leading order approximation to the boundary integral equation governing the 3-D vortex sheet. This leading order equation captures the most singular contributions of the integral equation. By introducing an appropriate change of variables, we show that the leading order vortex sheet equation degenerates to a two-dimensional vortex sheet equation in the direction of the tangential velocity jump. This change of variables is guided by a careful analysis based on properties of certain singular integral operators, and is crucial in identifying the leading order singular behavior. Our result confirms that the tangential velocity jump is the physical driving force of the vortex sheet singularities. We also show that the singularity type of the three-dimensional problem is similar to that of the two-dimensional problem. Moreover, we introduce a model equation for 3-D vortex sheets. This model equation captures the leading order singularity structure of the full 3-D vortex sheet equation, and it can be computed efficiently using fast Fourier transform. This enables us to perform well-resolved calculations to study the generic type of 3-D vortex sheet singularities. We will provide detailed numerical results to support the analytic prediction, and to reveal the generic form of the vortex sheet singularity
Negative refraction in nonlinear wave systems
People have been familiar with the phenomenon of wave refraction for several
centuries. Recently, a novel type of refraction, i.e., negative refraction,
where both incident and refractory lines locate on the same side of the normal
line, has been predicted and realized in the context of linear optics in the
presence of both right- and left-handed materials. In this work, we reveal, by
theoretical prediction and numerical verification, negative refraction in
nonlinear oscillatory systems. We demonstrate that unlike what happens in
linear optics, negative refraction of nonlinear waves does not depend on the
presence of the special left-handed material, but depends on suitable physical
condition. Namely, this phenomenon can be observed in wide range of oscillatory
media under the Hopf bifurcation condition. The complex Ginzburg-Landau
equation and a chemical reaction-diffusion model are used to demonstrate the
feasibility of this nonlinear negative refraction behavior in practice
Thermoelectric Properties of Silicon Carbide Nanowires with Nitrogen Dopants and Vacancies
The thermoelectric properties of cubic zincblend silicon carbide nanowires
(SiCNWs) with nitrogen impurities and vacancies along [111] direction are
theoretically studied by means of atomistic simulations. It is found that the
thermoelectric figure of merit ZT of SiCNWs can be significantly enhanced by
doping N impurities together with making Si vacancies. Aiming at obtaining a
large ZT, we study possible energetically stable configurations, and disclose
that, when N dopants locate at the center, a small number of Si vacancies at
corners are most favored for n-type nanowires, while a large number of Si
vacancies spreading into the flat edge sites are most favored for p-type
nanowires. For the SiCNW with a diameter of 1.1 nm and a length of 4.6 nm, the
ZT value for the n-type is shown capable of reaching 1.78 at 900K. The
conditions to get higher ZT values for longer SiCNWs are also addressed.Comment: 9 pages, 10 figure
Density-functional calculations of the electronic structure and lattice dynamics of superconducting LaOFBiS: Evidence for an electron-phonon interaction near the charge-density-wave instability
We discuss the electronic structure, lattice dynamics and electron-phonon
interaction of newly discovered superconductor LaOFBiS
using density functional based calculations. A strong Fermi surface nesting at
=(,,0) suggests a proximity to charge density wave
instability and leads to imaginary harmonic phonons at this point
associated with in-plane displacements of S atoms. Total energy analysis
resolves only a shallow double-well potential well preventing the appearance of
static long-range order. Both harmonic and anharmonic contributions to
electron-phonon coupling are evaluated and give a total coupling constant
prompting this material to be a conventional
superconductor contrary to structurally similar FeAs materials.Comment: Supplementary Materials is adde
Dynamic Pattern of Finite-Pulsed Beams inside One-dimensional Photonic Band Gap Materials
The dynamics of two-dimensional electromagnetic (EM) pulses through
one-dimensional photonic crystals (1DPC) has been theoretically studied.
Employing the time expectation integral over the Poynting vector as the arrival
time [Phys. Rev. Lett. 84, 2370, (2000)], we show that the superluminal
tunneling process of EM pulses is the propagation of the net forward-going
Poynting vector through the 1DPC, and the Hartman effect is due to the
saturation effect of the arrival time (smaller and smaller time accumulated) of
the net forward energy flow caused by the interference effect of the forward
and the backward field (from the interfaces of each layer) happened in the
region before the 1DPC and in the front part of the 1DPC.Comment: 18 pages, 4 figure
Signatures of Non-commutative QED at Photon Colliders
In this paper we study non-commutative (NC) QED signatures at photon
colliders through pair production of charged leptons and
charged scalars . The NC corrections for the fermion pair production
can be easily obtained since NC QED with fermions has been extensively studied
in the literature. NC QED with scalars is less studied. To obtain the cross
section for productions, we first investigate the structure of NC QED
with scalars, and then study the corrections due to the NC geometry to the
ordinary QED cross sections. Finally by folding in the photon spectra for a
collider with laser back-scattered photons from the
machine, we obtain 95% CL lower bound on the NC scale using the above two
processes. We find that, with , and TeV and
integrated luminosity , the NC scale up to 0.7, 1.2, and 1.6
TeV can be probed, respectively, while, for monochromatic photon beams, these
numbers become 1.1, 1.7, 2.6 TeV, respectively.Comment: 16 pages, 7 figure
Single-walled carbon nanotube bundle under hydrostatic pressure studied by the first-principles calculations
The structural, electronic, optical and vibrational properties of the
collapsed (10,10) single-walled carbon nanotube bundle under hydrostatic
pressure have been studied by the first-principles calculations. Some features
are observed in the present study: First, a collapsed structure is found, which
is distinct from both of the herringbone and parallel structures obtained
previously. Secondly, a pseudo-gap induced by the collapse appears along the
symmetry axis \textit{X}. Thirdly, the relative orientation between
the collapsed tubes has an important effect on their electronic, optical and
vibrational properties, which provides an efficient experimental method to
distinguish unambiguously three different collapsed structures.Comment: 14 pages, 6 figure
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