1,367 research outputs found
Low distortion automatic phase control circuit
A voltage controlled phase shifter is rendered substantially harmonic distortion free over a large dynamic input range by employing two oppositely poled, equally biased varactor diodes as the voltage controlled elements which adjust the phase shift. Control voltages which affect the bias of both diodes equally are used to adjust the phase shift without increasing distortion. A feedback stabilized phase shifter is rendered substantially frequency independent by employing a phase detector to control the phase shift of the voltage controlled phase shifter
Low distortion automatic phase control circuit
Circuit for generation and demodulation of quadrature double side band signals in frequency division multiplexing system is described. Circuit is designed to produce low distortion automatic phase control. Illustration of circuit and components is included
Electronic Structure of Superconducting Ba6c60
We report the results of first-principles electronic-structure calculations
for superconducting Ba6C60. Unlike the A3C60 superconductors, this new compound
shows strong Ba-C hybridization in the valence and conduction regions, mixed
covalent/ionic bonding character, partial charge transfer, and insulating
zero-gap band structure.Comment: 11 pages + 4 figures (1 appended, others on request), LaTeX with
REVTE
Density-functional-based predictions of Raman and IR spectra for small Si clusters
We have used a density-functional-based approach to study the response of silicon clusters to applied electric fields. For the dynamical response, we have calculated the Raman activities and infrared (IR) intensities for all of the vibrational modes of several clusters (SiN with N=3-8, 10, 13, 20, and 21) using the local density approximation (LDA). For the smaller clusters (N=3-8) our results are in good agreement with previous quantum-chemical calculations and experimental measurements, establishing that LDA-based IR and Raman data can be used in conjunction with measured spectra to determine the structure of clusters observed in experiment. To illustrate the potential of the method for larger clusters, we present calculated IR and Raman data for two low-energy isomers of Si10 and for the lowest-energy structure of Si13 found to date. For the static response, we compare our calculated polarizabilities for N=10, 13, 20, and 21 to recent experimental measurements. The calculated results are in rough agreement with experiment, but show less variation with cluster size than the measurements. Taken together, our results show that LDA calculations can offer a powerful means for establishing the structures of experimentally fabricated clusters and nanoscale systems
Three-dimensional electronic instabilities in polymerized solid A1C60
The low-temperature structure of A1C60 (A=K, Rb) is an ordered array of
polymerized C60 chains, with magnetic properties that suggest a non-metallic
ground state. We study the paramagnetic state of this phase using
first-principles electronic-structure methods, and examine the magnetic
fluctuations around this state using a model Hamiltonian. The electronic and
magnetic properties of even this polymerized phase remain strongly three
dimensional, and the magnetic fluctuations favor an unusual three-dimensional
antiferromagnetically ordered structure with a semi-metallic electronic
spectrum.Comment: REVTeX 3.0, 10 pages, 4 figures available on request from
[email protected]
Comparative Density Functional Theory Study of Magnetic Exchange Coupling in Di-nuclear Transition Metal Complexes
Multi-center transition metal complexes (MCTMs) with magnetically interacting
ions have been proposed as components for information processing devices and
storage units. For any practical application of MCTMs as magnetic units, it is
crucial to characterize their magnetic behavior, and in particular the
isotropic magnetic exchange coupling, J, between its magnetic centers. Due to
the large size of typical MCTMs, density functional theory (DFT) is the only
practical electronic structure method for evaluating the J coupling. Here we
assess the accuracy of different density functional approximations for
predicting the magnetic couplings of seven di-metal transition metal complexes
with known reliable experimental J couplings spanning from ferromagnetic to
strong antiferromagnetic. The density functionals considered include global
hybrid functionals which mix semilocal density functional approximations and
exact exchange with a fixed admixing parameter, six local hybrid functionals
where the admixing parameters are extended to be spatially dependent, the SCAN
and rSCAN meta-generalized gradient approximations (GGAs), and two widely
used GGAs. We found that global hybrids have a tendency to over-correct the
error in magnetic coupling parameters from the Perdew-Burke-Ernzerhof (PBE)
GGA, while the performance of local hybrid density functionals is scattered
without a clear trend, suggesting that more efforts are needed for the
extension from global to local hybrid density functionals for this particular
property. The SCAN and rSCAN meta-GGAs are found to perform as well or
better than the global and local hybrids on most tested complexes. We further
analyze the charge density redistribution of meta-GGAs as well as global and
local hybrid density functionals with respect to that of PBE, in connection to
the self-interaction error (SIE) or delocalization error
Using Fill Terraces to Understand Incision Rates and Evolution of the Colorado River in Eastern Grand Canyon, Arizona
The incision and aggradation of the Colorado River in eastern Grand Canyon through middle to late Quaternary time can be traced in detail using well-exposed fill terraces dated by a combination of optically stimulated luminescence, uranium series, and cosmogenic nuclide dating. This fluvial history provides the best bedrock incision rate for this important landscape and highlights the complications and advantages of fill terrace records for understanding river long-profile evolution and incision. The use of fill terraces, as distinct from strath terraces, for calculating incision rates is complicated by the cyclic alluviation and incision they record. In the example of the Grand Canyon this has led to various rates being reported by different workers and rates that tend to be overestimates in shorter records. We illustrate that a meaningful long-term bedrock incision rate of 140 m/m.y. can be extracted from the Grand Canyon record by linking episodes when the Colorado River is floored on bedrock. Variable incision rates reported in the greater region may be, to some degree, due to inconsistent calculations. Our data also highlight that the Colorado River has been a mixed alluvial-bedrock river through both time and space and has been a bedrock river for less than half of its Pleistocene history. This strong temporal variation, combined with the varying bedrock the river encounters on its path, heightens the challenge of understanding the tectonic, climatic, and drainage integration controls on the form and evolution of the Colorado River’s long profile
Variational Hilbert space truncation approach to quantum Heisenberg antiferromagnets on frustrated clusters
We study the spin- Heisenberg antiferromagnet on a series of
finite-size clusters with features inspired by the fullerenes. Frustration due
to the presence of pentagonal rings makes such structures challenging in the
context of quantum Monte-Carlo methods. We use an exact diagonalization
approach combined with a truncation method in which only the most important
basis states of the Hilbert space are retained. We describe an efficient
variational method for finding an optimal truncation of a given size which
minimizes the error in the ground state energy. Ground state energies and
spin-spin correlations are obtained for clusters with up to thirty-two sites
without the need to restrict the symmetry of the structures. The results are
compared to full-space calculations and to unfrustrated structures based on the
honeycomb lattice.Comment: 22 pages and 12 Postscript figure
Malting Barley Grade Factors
This archival publication may not reflect current scientific knowledge or recommendations. Current information available from the University of Minnesota Extension: https://www.extension.umn.edu
Quasi one dimensional He inside carbon nanotubes
We report results of diffusion Monte Carlo calculations for both He
absorbed in a narrow single walled carbon nanotube (R = 3.42 \AA) and strictly
one dimensional He. Inside the tube, the binding energy of liquid He is
approximately three times larger than on planar graphite. At low linear
densities, He in a nanotube is an experimental realization of a
one-dimensional quantum fluid. However, when the density increases the
structural and energetic properties of both systems differ. At high density, a
quasi-continuous liquid-solid phase transition is observed in both cases.Comment: 11 pages, 3ps figures, to appear in Phys. Rev. B (RC
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