685 research outputs found
Quantum coherence in a degenerate two-level atomic ensemble: for a transition
For a transition driven by a linearly polarized
light and probed by a circularly light, quantum coherence effects are
investigated. Due to the coherence between the drive Rabi frequency and Zeeman
splitting, electromagnetically induced transparency, electromagnetically
induced absorption, and the transition from positive to negative dispersion are
obtained, as well as the populations coherently oscillating in a wide spectral
region. At the zero pump-probe detuning, the subluminal and superluminal light
propagation is predicted. Finally, coherent population trapping states are not
highly sensitive to the refraction and absorption in such ensemble.Comment: 9 pages, 6 figure
Fusion rules and singular vectors of the osp(1|2) current algebra
The fusion of Verma modules of the osp(1|2) current algebra is studied. In
the framework of an isotopic formalism, the singular vector decoupling
conditions are analyzed. The fusion rules corresponding to the admissible
representations of the osp(1|2) algebra are determined. A relation between the
characters of these last representations and those corresponding to the minimal
superconformal models is found. A series of equations that relate the
descendants of the highest weight vectors resulting from a fusion of Verma
modules are obtained. Solving these equations the singular vectors of the
theory can be determined.Comment: 63 pages, phyzz
Theory of nuclear induced spectral diffusion: Spin decoherence of phosphorus donors in Si and GaAs quantum dots
We propose a model for spectral diffusion of localized spins in
semiconductors due to the dipolar fluctuations of lattice nuclear spins. Each
nuclear spin flip-flop is assumed to be independent, the rate for this process
being calculated by a method of moments. Our calculated spin decoherence time
ms for donor electron spins in Si:P is a factor of two longer than
spin echo decay measurements. For P nuclear spins we show that spectral
diffusion is well into the motional narrowing regime. The calculation for GaAs
quantum dots gives s depending on the quantum dot size. Our
theory indicates that nuclear induced spectral diffusion should not be a
serious problem in developing spin-based semiconductor quantum computer
architectures.Comment: 15 pages, 9 figures. Accepted for publication in Phys. Rev.
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New Molecular Collisional Interaction Effect in Low-Energy Sputtering
Y. Yao, Z. Hargitai, M. Albert, R. G. Albridge, A. V. Barnes, J. M. Gilligan, B. Pratt Ferguson,
G. Lüpke, V. D. Gordon (currently with UT Austin), and N. H. Tolk are with the
Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235 -- J. C. Tully is with the
Department of Physics and Department of Chemistry, Yale University, New Haven, Connecticut 06520 -- G. Betz and W. Husinsky are with the
Institut für Allgemeine Physik, Technische Universität Wien, A-1040 Vienna, AustriaAn unexpected pronounced enhancement is observed in sputtering yields per atom for N2+
compared
to N+ from a polycrystalline gold target. This effect is seen when the kinetic energy per projectile
atom is below 500 eV and increases as projectile energy decreases to near-threshold energies.
Enhancements for O2+
over O+ begin at even lower kinetic energies below 100 eV per atom. This
new molecular interaction effect may be explained qualitatively by invoking a simple energy transfer
model which involves the vibrational frequency of the molecule and the collisional interaction time.
[S0031-9007(98)06668-X]Chemistr
Ground-state energy and spin in disordered quantum dots
We investigate the ground-state energy and spin of disordered quantum dots
using spin-density-functional theory. Fluctuations of addition energies
(Coulomb-blockade peak spacings) do not scale with average addition energy but
remain proportional to level spacing. With increasing interaction strength, the
even-odd alternation of addition energies disappears, and the probability of
non-minimal spin increases, but never exceeds 50%. Within a two-orbital model,
we show that the off-diagonal Coulomb matrix elements help stabilize a ground
state of minimal spin.Comment: 10 pages, 2 figure
Electron spin coherence in semiconductors: Considerations for a spin-based solid state quantum computer architecture
We theoretically consider coherence times for spins in two quantum computer
architectures, where the qubit is the spin of an electron bound to a P donor
impurity in Si or within a GaAs quantum dot. We show that low temperature
decoherence is dominated by spin-spin interactions, through spectral diffusion
and dipolar flip-flop mechanisms. These contributions lead to 1-100 s
calculated spin coherence times for a wide range of parameters, much higher
than former estimates based on measurements.Comment: Role of the dipolar interaction clarified; Included discussion on the
approximations employed in the spectral diffusion calculation. Final version
to appear in Phys. Rev.
Dynamical 1/N approach to time-dependent currents through quantum dots
A systematic truncation of the many-body Hilbert space is implemented to
study how electrons in a quantum dot attached to conducting leads respond to
time-dependent biases. The method, which we call the dynamical 1/N approach, is
first tested in the most unfavorable case, the case of spinless fermions (N=1).
We recover the expected behavior, including transient ringing of the current in
response to an abrupt change of bias. We then apply the approach to the
physical case of spinning electrons, N=2, in the Kondo regime for the case of
infinite intradot Coulomb repulsion. In agreement with previous calculations
based on the non-crossing approximation (NCA), we find current oscillations
associated with transitions between Kondo resonances situated at the Fermi
levels of each lead. We show that this behavior persists for a more realistic
model of semiconducting quantum dots in which the Coulomb repulsion is finite.Comment: 18 pages, 7 eps figures, discussion extended for spinless electrons
and typo
Molecular dynamics simulation of the order-disorder phase transition in solid NaNO
We present molecular dynamics simulations of solid NaNO using pair
potentials with the rigid-ion model. The crystal potential surface is
calculated by using an \emph{a priori} method which integrates the \emph{ab
initio} calculations with the Gordon-Kim electron gas theory. This approach is
carefully examined by using different population analysis methods and comparing
the intermolecular interactions resulting from this approach with those from
the \emph{ab initio} Hartree-Fock calculations. Our numerics shows that the
ferroelectric-paraelectric phase transition in solid NaNO is triggered by
rotation of the nitrite ions around the crystallographical c axis, in agreement
with recent X-ray experiments [Gohda \textit{et al.}, Phys. Rev. B \textbf{63},
14101 (2000)]. The crystal-field effects on the nitrite ion are also addressed.
Remarkable internal charge-transfer effect is found.Comment: RevTeX 4.0, 11 figure
General Adaptive Neighborhood Image Restoration, Enhancement and Segmentation
12 pagesInternational audienceThis paper aims to outline the General Adaptive Neighborhood Image Processing (GANIP) approach [1–3], which has been recently introduced. An intensity image is represented with a set of local neighborhoods defined for each point of the image to be studied. These so-called General Adaptive Neighborhoods (GANs) are simultaneously adaptive with the spatial structures, the analyzing scales and the physical settings of the image to be addressed and/or the human visual system. After a brief theoretical introductory survey, the GANIP approach will be successfully applied on real application examples in image restoration, enhancement and segmentation
Geodesic flows on Riemannian g.o. spaces
We prove the integrability of geodesic flows on the Riemannian g.o. spaces of
compact Lie groups, as well as on a related class of Riemannian homogeneous
spaces having an additional principal bundle structure.Comment: 12 pages, minor corrections, final versio
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