134,156 research outputs found
Modulation of the dephasing time for a magnetoplasma in a quantum well
We investigate the femtosecond kinetics of optically excited 2D
magneto-plasma. We calculate the femtosecond dephasing and relaxation kinetics
of the laser pulse excited magneto-plasma due to bare Coulomb potential
scattering, because screening is under these conditions of minor importance. By
taking into account four Landau subbands in both the conduction band and the
valence band, we are now able to extend our earlier study [Phys. Rev. B {\bf
58}, 1998,in print (see also cond-mat/9808073] to lower magnetic fields. We can
also fix the magnetic field and change the detuning to further investigate the
carrier density-dependence of the dephasing time. For both cases, we predict
strong modulation in the dephasing time.Comment: RevTex, 3 figures, to be published in Solid. Stat. Commu
Riemann zeros, prime numbers and fractal potentials
Using two distinct inversion techniques, the local one-dimensional potentials
for the Riemann zeros and prime number sequence are reconstructed. We establish
that both inversion techniques, when applied to the same set of levels, lead to
the same fractal potential. This provides numerical evidence that the potential
obtained by inversion of a set of energy levels is unique in one-dimension. We
also investigate the fractal properties of the reconstructed potentials and
estimate the fractal dimensions to be for the Riemann zeros and for the prime numbers. This result is somewhat surprising since the
nearest-neighbour spacings of the Riemann zeros are known to be chaotically
distributed whereas the primes obey almost poisson-like statistics. Our
findings show that the fractal dimension is dependent on both the
level-statistics and spectral rigidity, , of the energy levels.Comment: Five postscript figures included in the text. To appear in Phys. Rev.
Strongly modulated transmissions in gapped armchair graphene nanoribbons with sidearm or on-site gate voltage
We propose two schemes of field-effect transistor based on gapped armchair
graphene nanoribbons connected to metal leads, by introducing sidearms or
on-site gate voltages. We make use of the band gap to reach excellent
switch-off character. By introducing one sidearm or on-site gate to the
graphene nanoribbon, conduction peaks appear inside the gap regime. By further
applying two sidearms or on-site gates, these peaks are broadened to conduction
plateaus with a wide energy window, thanks to the resonance from the dual
structure. The position of the conduction windows inside the gap can be fully
controlled by the length of the sidearms or the on-site gate voltages, which
allows "on" and "off" operations for a specific energy window inside the gap
regime. The high robustness of both the switch-off character and the conduction
windows is demonstrated and shows the feasibility of the proposed dual
structures for real applications.Comment: 6 pages, 6 figure
Comment on "Photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal at room temperature"
We comment on the conclusion by Ma et al. [Appl. Phys. Lett. {\bf 94}, 241112
(2009)] that the Elliott-Yafet mechanism is more important than the
D'yakonov-Perel' mechanism at high carrier density in intrinsic bulk CdTe at
room temperature. We point out that the spin relaxation is solely from the
D'yakonov-Perel' mechanism. The observed peak in the density dependence of spin
relaxation time is exactly what we predicted in a recent work [Phys. Rev. B
{\bf 79}, 125206 (2009)].Comment: 2 page
SOS-convex Semi-algebraic Programs and its Applications to Robust Optimization: A Tractable Class of Nonsmooth Convex Optimization
In this paper, we introduce a new class of nonsmooth convex functions called
SOS-convex semialgebraic functions extending the recently proposed notion of
SOS-convex polynomials. This class of nonsmooth convex functions covers many
common nonsmooth functions arising in the applications such as the Euclidean
norm, the maximum eigenvalue function and the least squares functions with
-regularization or elastic net regularization used in statistics and
compressed sensing. We show that, under commonly used strict feasibility
conditions, the optimal value and an optimal solution of SOS-convex
semi-algebraic programs can be found by solving a single semi-definite
programming problem (SDP). We achieve the results by using tools from
semi-algebraic geometry, convex-concave minimax theorem and a recently
established Jensen inequality type result for SOS-convex polynomials. As an
application, we outline how the derived results can be applied to show that
robust SOS-convex optimization problems under restricted spectrahedron data
uncertainty enjoy exact SDP relaxations. This extends the existing exact SDP
relaxation result for restricted ellipsoidal data uncertainty and answers the
open questions left in [Optimization Letters 9, 1-18(2015)] on how to recover a
robust solution from the semi-definite programming relaxation in this broader
setting
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