9,537 research outputs found
The Exchange Gate in Solid State Spin Quantum Computation: The Applicability of the Heisenberg Model
Solid state quantum computing proposals rely on adiabatic operations of the
exchange gate among localized spins in nanostructures. We study corrections to
the Heisenberg interaction between lateral semiconductor quantum dots in an
external magnetic field. Using exact diagonalization we obtain the regime of
validity of the adiabatic approximation. We also find qualitative corrections
to the Heisenberg model at high magnetic fields and in looped arrays of spins.
Looped geometries of localized spins generate flux dependent, multi-spin terms
which go beyond the basic Heisenberg model.Comment: 13 pages, 8 figure
Simplification of Manipulator Dynamic Model for Nonlinear Decoupled Control
This paper presents the development of simplified manipulator dynamic models which satisfy the desired steady-state error specification in the joint-variable space or in the Cartesian space under a nonlinear decoupled controller. The formulae which relate the tracking errors of joint variables in the joint-variable space or the manipulator hand in the Cartesian space to the dynamic modeling errors are first developed. Using these formulae, we derive the maximum error tolerance for each dynamic coefficient of the equations of motion. Then each simplified dynamic coefficient of the equations of motion can be expressed as a linear combination of the product terms of sinusoidal and polynomial basis functions. To illustrate the approach, a computer simulation has been carried out to obtain two simplified dynamic models of a Stanford robot arm which satisfy the specified error tolerances in the joint-variable space and in the Cartesian space under respective nonlinear decoupled controllers. Finally, to measure the time complexity of simplified models, the number of mathematical operations in terms of multiplication and addition for computing the joint torques is tabulated and discussed with the parallel computation result of Newton-Euler equations of motion
Generalized Approach for the Control of Constrained Robot Systems
This paper presents a generalized approach for controlling various cases of the constrained robot system. To accomplish specific tasks successfully by a constrained robot system, both the constraint forces/torques and the motion of the manipulator end-effector must be specified and controlled. Using the Jacobian matrix of the constraint function, the generalized coordinates of the constrained robot system can be partitioned into two sets; this leads to partitioning the constrained robot system into two subsystems. The constraint forces/torques in each subsystem can be decomposed into two components: the motion-independent and the motion-dependent forces/torques. Using the constraint function in the Cartesian space, the motion-independent forces/torques can be expressed by a generalized multiplier vector and the Jacobian matrix of the constraint function. The motion-dependent forces/torques can be determined by the motion of the manipulator end-effector, the motion-independent forces/torques, and other known quantities. This decomposition of the constraint robot system into subsystems leads to the design of a nonlinear decoupled controller with a simple structure, which takes the constraints into consideration for controlling the constrained robot system. Applying the proposed nonlinear decoupled controller to each subsystem and using the relation between the motion-independent forces/torques in the subsystems, we can show that both the errors in the manipulator end-effector motion and the constraint forces/torques approach zero asymptotically. Typical examples of the constrained robot systems are analyzed and discussed
Isotropic-Nematic Transition in Liquid-Crystalline Elastomers
In liquid-crystalline elastomers, the nematic order parameter and the induced
strain vary smoothly across the isotropic-nematic transition, without the
expected first-order discontinuity. To investigate this smooth variation, we
measure the strain as a function of temperature over a range of applied stress,
for elastomers crosslinked in the nematic and isotropic phases, and analyze the
results using a variation on Landau theory. This analysis shows that the smooth
variation arises from quenched disorder in the elastomer, combined with the
effects of applied stress and internal stress.Comment: 4 pages, including 4 postscript figures, uses REVTeX
Spin-Orbit Coupling in Iridium-Based 5d Compounds Probed by X-ray Absorption Spectroscopy
We have performed x-ray absorption spectroscopy (XAS) measurements on a
series of Ir-based 5d transition metal compounds, including Ir, IrCl3, IrO2,
Na2IrO3, Sr2IrO4, and Y2Ir2O7. By comparing the intensity of the "white-line"
features observed at the Ir L2 and L3 absorption edges, it is possible to
extract valuable information about the strength of the spin-orbit coupling in
these systems. We observe remarkably large, non-statistical branching ratios in
all Ir compounds studied, with little or no dependence on chemical composition,
crystal structure, or electronic state. This result confirms the presence of
strong spin-orbit coupling effects in novel iridates such as Sr2IrO4, Na2IrO3,
and Y2Ir2O7, and suggests that even simple Ir-based compounds such as IrO2 and
IrCl3 may warrant further study. In contrast, XAS measurements on Re-based 5d
compounds, such as Re, ReO2, ReO3, and Ba2FeReO6, reveal statistical branching
ratios and negligible spin-orbit coupling effects.Comment: 9 pages, 4 figure
Reheating and turbulence
We show that the ''turbulent'' particle spectra found in numerical
simulations of the behavior of matter fields during reheating admit a simple
interpretation in terms of hydrodynamic models of the reheating period. We
predict a particle number spectrum with for Comment: 10 pages, one figure included in tex
Using conditional kernel density estimation for wind power density forecasting
Of the various renewable energy resources, wind power is widely recognized as one of the most promising. The management of wind farms and electricity systems can benefit greatly from the availability of estimates of the probability distribution of wind power generation. However, most research has focused on point forecasting of wind power. In this paper, we develop an approach to producing density forecasts for the wind power generated at individual wind farms. Our interest is in intraday data and prediction from 1 to 72 hours ahead. We model wind power in terms of wind speed and wind direction. In this framework, there are two key uncertainties. First, there is the inherent uncertainty in wind speed and direction, and we model this using a bivariate VARMA-GARCH (vector autoregressive moving average-generalized autoregressive conditional heteroscedastic) model, with a Student t distribution, in the Cartesian space of wind speed and direction. Second, there is the stochastic nature of the relationship of wind power to wind speed (described by the power curve), and to wind direction. We model this using conditional kernel density (CKD) estimation, which enables a nonparametric modeling of the conditional density of wind power. Using Monte Carlo simulation of the VARMA-GARCH model and CKD estimation, density forecasts of wind speed and direction are converted to wind power density forecasts. Our work is novel in several respects: previous wind power studies have not modeled a stochastic power curve; to accommodate time evolution in the power curve, we incorporate a time decay factor within the CKD method; and the CKD method is conditional on a density, rather than a single value. The new approach is evaluated using datasets from four Greek wind farms
Low-Temperature Spin Diffusion in a Spin-Polarized Fermi Gas
We present a finite temperature calculation of the transverse spin-diffusion
coefficient, , in a dilute degenerate Fermi gas in the presence of a
small external magnetic field, . While the longitudinal diffusion
coefficient displays the conventional low-temperature Fermi-liquid behavior,
, the corresponding results for show three
separate regimes: (a) for ; (b) , for and large spin-rotation
parameter , and (c) for and . Our results are qualitatively consistent with the available
experimental data in weakly spin-polarized and mixtures.Comment: 13 pages, REVTEX, 3 figures available upon request, RU-94-4
Effective theories for real-time correlations in hot plasmas
We discuss the sequence of effective theories needed to understand the
qualitative, and quantitative, behavior of real-time correlators
in ultra-relativistic plasmas. We analyze in detail the case where A is a
gauge-invariant conserved current. This case is of interest because it includes
a correlation recently measured in lattice simulations of classical, hot,
SU(2)-Higgs gauge theory. We find that simple perturbation theory, free kinetic
theory, linearized kinetic theory, and hydrodynamics are all needed to
understand the correlation for different ranges of time. We emphasize how
correlations generically have power-law decays at very large times due to
non-linear couplings to long-lived hydrodynamic modes.Comment: 28 pages, Latex, uses revtex, epsf macro packages [Revised version: t
-> sqrt{t} in a few typos on p. 10.
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