807 research outputs found
Diffractoid grating configuration for X-ray and ultraviolet focusing
An aspheric grating is described which is operable to image local or distant point sources sharply in a designated wavelength, i.e., produce a perfectly stigmatic image in the given wavelength at grazing angles of incidence. The grating surface comprises a surface of revolution defined by a curve which does not have a constant radius of curvature but is defined by a nonlinear differential equation
On three-dimensional reconstruction of optically thin solar emission sources
Calculations are given for constructing the three dimensional distribution of optically thin EUV emission sources associated with solar active regions, from two dimensional observations (projections) recorded by the spectroheliograph on the OSO 7 satellite. The relation of the method to other image reconstruction methods is briefly discussed as well as the special requirements imposed in the solar case such as a knowledge of the true solar rotation function. A useful correlation criterion for establishing the physical validity of solutions is given
Intercalation and Staging Behavior in Super-Oxygenated
A high temperature electrochemical oxidation process has been used to produce
large single crystals of suitable for neutron scattering
experiments. Below room temperature the oxygen-rich phases have structural
superlattice scattering peaks which indicate new periodicities ranging from 2
to 6.6 layers perpendicular to the copper oxide planes. A model structure
originally proposed for can account for the superlattice
peaks as a result of anti-phase domain boundaries between different tilt
directions of the CuO octahedra. Within this model, the changes in CuO
tilt directions are induced by segregated layers of interstitial oxygen which
order in a manner similar to intercalants in graphite. This structural model
thus clarifies previous work and establishes as a unique
lamellar superconducting system with annealed disorder.Comment: 23 pages, latex, 6 figures (not including Figures 2 & 7 and Table 1
which were not submitted but are available upon request to the Authors at:
[email protected]
Charge occupancy of two interacting electrons on artificial molecules - exact results
We present exact solutions for two interacting electrons on an artificial
atom and on an artificial molecule made by one and two (single level) quantum
dots connected by ideal leads. Specifically, we calculate the accumulated
charge on the dots as function of the gate voltage, for various strengths of
the electron-electron interaction and of the hybridization between the dots and
the (one-dimensional) leads. With increasing of the (negative) gate voltage,
the accumulated charge in the two-electron ground state increases in gradual
steps from 0 to 1 and then to 2. The value 0 represents an "insulating" state,
where both electrons are bound to shallow states on the impurities. The value
of 1 corresponds to a "metal", with one electron localized on the dots and the
other extended on the leads. The value of 2 corresponds to another "insulator",
with both electrons strongly localized. The width of the "metallic" regime
diverges with strength of the electron-electron interaction for the single dot,
but remains very narrow for the double dot. These results are contrasted with
the simple Coulomb blockade picture.Comment: 12 pages, 7 figure
Physics of -Meson Condensation and High Temperature Cuprate Superconductors
The idea of condensation of the Goldstone -meson field in nuclear matter
had been put forward a long time ago. However, it was established that the
normal nuclear density is too low, it is not sufficient to condensate
-mesons. This is why the -condensation has never been observed.
Recent experimental and theoretical studies of high temperature cuprate
superconductors have revealed condensation of Goldstone magnons, the effect
fully analogous to the -condensation. The magnon condensation has been
observed. It is clear now that quantum fluctuations play a crucial role in the
condensation, in particular they drive a quantum phase transition that destroys
the condensate at some density of fermions
Direct Observation of the Quantum Energy Gap in S = 1/2 Tetragonal Cuprate Antiferromagnets
Using an electron spin resonance spectrometer covering a wide range of
frequency and magnetic field, we have measured the low energy excitations of
the S=1/2 tetragonal antiferromagnets, Sr_{2}CuO_{2}Cl_{2} and
Sr_{2}Cu_{3}O_{4}Cl_{2}. Our observation of in-plane energy gaps of order 0.1
meV at zero external magnetic field are consistent with a spin wave
calculation, which includes several kinds of quantum fluctuations that remove
frustration. Results agree with other experiments and with exchange anisotropy
parameters determined from a five band Hubbard model.Comment: 4 pages, 3 figure
Diffusion Monte Carlo study of circular quantum dots
We present ground and excited state energies obtained from Diffusion Monte
Carlo (DMC) calculations, using accurate multiconfiguration wave functions, for
electrons () confined to a circular quantum dot. We analyze the
electron-electron pair correlation functions and compare the density and
correlation energies to the predictions of local spin density approximation
theory (LSDA). The DMC estimated change in electrochemical potential as
function of the number of electrons in the dot is compared to that from LSDA
and Hartree-Fock (HF) calculations.Comment: 7 pages, 4 eps figures. To be published in Phys. Rev. B, September
15th 2000. See erratum cond-mat/030571
Addition Spectra of Chaotic Quantum Dots: Interplay between Interactions and Geometry
We investigate the influence of interactions and geometry on ground states of
clean chaotic quantum dots using the self-consistent Hartree-Fock method. We
find two distinct regimes of interaction strength: While capacitive energy
fluctuations follow approximately a random matrix prediction for
weak interactions, there is a crossover to a regime where is
strongly enhanced and scales roughly with interaction strength. This
enhancement is related to the rearrangement of charges into ordered states near
the dot edge. This effect is non-universal depending on dot shape and size. It
may provide additional insight into recent experiments on statistics of Coulomb
blockade peak spacings.Comment: 4 pages, final version to appear in Phys. Rev. Let
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