103,764 research outputs found
A skyrmion-based spin-torque nano-oscillator
A model for a spin-torque nano-oscillator based on the self-sustained
oscillation of a magnetic skyrmion is presented. The system involves a circular
nanopillar geometry comprising an ultrathin film free magnetic layer with a
strong Dzyaloshinkii-Moriya interaction and a polariser layer with a
vortex-like spin configuration. It is shown that spin-transfer torques due to
current flow perpendicular to the film plane leads to skyrmion gyration that
arises from a competition between geometric confinement due to boundary edges
and the vortex-like polarisation of the spin torques. A phenomenology for such
oscillations is developed and quantitative analysis using micromagnetics
simulations is presented. It is also shown that weak disorder due to random
anisotropy variations does not influence the main characteristics of the
steady-state gyration.Comment: 15 pages, 6 figure
The BFKL Pomeron within Physical Renormalization Schemes and Scales
In this lecture the next-to-leading order (NLO) corrections to the QCD
Pomeron intercept obtained from the Balitsky-Fadin-Kuraev-Lipatov (BFKL)
equation are discussed. It is shown that the BFKL Pomeron intercept when
evaluated in non-Abelian physical renormalization schemes with
Brodsky-Lepage-Mackenzie (BLM) optimal scale setting does not exhibit the
serious problems encountered in the modified minimal subtraction (bar{MS})
scheme. The results obtained provide an opportunity for applications of the NLO
BFKL resummation to high-energy phenomenology.
One of such applications for virtual gamma-gamma total cross section shows a
good agreement with preliminary data at CERN LEP.Comment: Presented at XXXXV PNPI Winter School, Repino, St.Petersburg, Russia,
19-25 Feb., 2001; Latex, 16 pages, 5 figure
A note on q-Bernstein polynomials
In this paper we constructed new q-extension of Bernstein polynomials. Fron
those q-Berstein polynomials, we give some interesting properties and we
investigate some applications related this q-Bernstein polynomials.Comment: 13 page
Schwinger Pair Production at Finite Temperature in Scalar QED
In scalar QED we study the Schwinger pair production from an initial ensemble
of charged bosons when an electric field is turned on for a finite period
together with or without a constant magnetic field. The scalar QED Hamiltonian
depends on time through the electric field, which causes the initial ensemble
of bosons to evolve out of equilibrium. Using the Liouville-von Neumann method
for the density operator and quantum states for each momentum mode, we
calculate the Schwinger pair-production rate at finite temperature, which is
the pair-production rate from the vacuum times a thermal factor of the
Bose-Einstein distribution.Comment: RevTex 10 pages, no figure; replaced by the version accepted in Phys.
Rev. D; references correcte
Temperature dependence of nonlinear auto-oscillator linewidths: Application to spin-torque nano-oscillators
The temperature dependence of the generation linewidth for an auto-oscillator
with a nonlinear frequency shift is calculated. It is shown that the frequency
nonlinearity creates a finite correlation time, tau, for the phase
fluctuations. In the low-temperature limit in which the spectral linewidth is
smaller than 1/tau, the line shape is approximately Lorentzian and the
linewidth is linear in temperature. In the opposite high-temperature limit in
which the linewidth is larger than 1/tau, the nonlinearity leads to an apparent
"inhomogeneous broadening" of the line, which becomes Gaussian in shape and has
a square-root dependence on temperature. The results are illustrated for the
spin-torque nano-oscillator.Comment: 4 pages, 1 figur
D_s (2317) as a four-quark state in QCD sum rules
We perform a QCD sum rule study of the open-charmed as a
four-quark state. Using the diquark-antidiquark picture for the four-quark
state, we consider four possible interpolating fields for , namely,
scalar-scalar, pseudoscalar-pseudoscalar, vector-vector, and
axial-vector--axial-vector types. We test all four currents by constructing
four separate sum rules. The sum rule with the scalar-scalar current gives a
stable value for the mass which qualitatively agrees with the
experimental value, and the result is not sensitive to the continuum threshold.
The vector-vector sum rule also gives a stable result with small sensitivity to
the continuum threshold and the extracted mass is somewhat lower than the
scalar-scalar current value. On the other hand, the two sum rules in the
pseudoscalar and axial-vector channels are found to yield the mass highly
sensitive to the continuum threshold, which implies that a four-quark state
with the combination of pseudoscalar-pseudoscalar or axial-vector--axial-vector
type would be disfavored. These results would indicate that is a
bound state of scalar-diquark and scalar-antidiquark and/or vector-diquark and
vector-antidiquark.Comment: 10 pages, 4 figures, REVTeX, 1 fig. added. to appear in Phys. Rev.
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