Equilibrium orbit analysis in a free-electron laser with a coaxial wiggler

An analysis of single-electron orbits in combined coaxial wiggler and axial guide magnetic fields is presented. Solutions of the equations of motion are developed in a form convenient for computing orbital velocity components and trajectories in the radially dependent wiggler. Simple analytical solutions are obtained in the radially-uniform-wiggler approximation and a formula for the derivative of the axial velocity $v_{\|}$ with respect to Lorentz factor $\gamma$ is derived. Results of numerical computations are presented and the characteristics of the equilibrium orbits are discussed. The third spatial harmonic of the coaxial wiggler field gives rise to group $III$ orbits which are characterized by a strong negative mass regime.Comment: 13 pages, 9 figures, to appear in phys. rev.

Composite Fermions in Modulated Structures: Transport and Surface Acoustic Waves

Motivated by a recent experiment of Willett et al. [Phys. Rev. Lett. 78, 4478 (1997)], we employ semiclassical composite-fermion theory to study the effect of a periodic density modulation on a quantum Hall system near Landau level filling factor nu=1/2. We show that even a weak density modulation leads to dramatic changes in surface-acoustic-wave (SAW) propagation, and propose an explanation for several key features of the experimental observations. We predict that properly arranged dc transport measurements would show a structure similar to that seen in SAW measurements.Comment: Version published in Phys. Rev. Lett. Figures changed to show SAW velocity shift. LaTeX, 5 pages, two included postscript figure

Do the Cognitive Skills of School Dropouts Matter in the Labor Market?

Does the U.S. labor market reward cognitive skill differences among high school dropouts, the members of the labor force with the least educational attainments? This paper reports the results of an exploration of this question, using a new data set that provides information on the universe of dropouts who last attempted the GED exams in Florida and New York between 1984 and 1990. The design of the sample reduces variation in unmeasured variables such as motivation that are correlated with cognitive skills. We examine the labor market returns to basic cognitive skills as measured by GED test scores. We explore whether the returns differ by gender and race. The results indicate quite large earnings returns to cognitive skills for both male and female dropouts, and for white and non-white dropouts. The earnings payoff to skills increases with age.

Who Benefits from Obtaining a GED? Evidence from High School and Beyond

This paper examines the value of the GED credential and the conventional high school diploma in explaining the earnings of 27-year-old males in the early 1990s. The data base is the High School & Beyond sophomore cohort. We replicate the basic findings of prior studies that implicitly assume the labor market value of the GED credential does not depend on the skills with which dropouts left school. We show that these average effects mask a more complicated pattern. Obtaining a GED is associated with higher earnings at age 27 for those male dropouts who had very weak cognitive skills as tenth graders, but not for those who had stronger cognitive skills as tenth graders.

Experimental Demonstration of Fermi Surface Effects at Filling Factor 5/2

Using small wavelength surface acoustic waves (SAW) on ultra-high mobility heterostructures, Fermi surface properties are detected at 5/2 filling factor at temperatures higher than those at which the quantum Hall state forms. An enhanced conductivity is observed at 5/2 by employing sub 0.5 micron wavelength SAW, indicating a quasiparticle mean-free-path substantially smaller than that in the lowest Landau level. These findings are consistent with the presence of a filled Fermi sea of composite fermions, which may pair at lower temperatures to form the 5/2 ground state.Comment: 11 pages, 4 figure

The Taft-Hartley Law

Address delivered at Evansville at the Annual Meeting of the Indiana State Bar Association, September 5, 1947

The Taft-Hartley Law

Address delivered at Evansville at the Annual Meeting of the Indiana State Bar Association, September 5, 1947

Improving Brain–Machine Interface Performance by Decoding Intended Future Movements

Objective. A brain–machine interface (BMI) records neural signals in real time from a subject\u27s brain, interprets them as motor commands, and reroutes them to a device such as a robotic arm, so as to restore lost motor function. Our objective here is to improve BMI performance by minimizing the deleterious effects of delay in the BMI control loop. We mitigate the effects of delay by decoding the subject\u27s intended movements a short time lead in the future. Approach. We use the decoded, intended future movements of the subject as the control signal that drives the movement of our BMI. This should allow the user\u27s intended trajectory to be implemented more quickly by the BMI, reducing the amount of delay in the system. In our experiment, a monkey (Macaca mulatta) uses a future prediction BMI to control a simulated arm to hit targets on a screen. Main Results. Results from experiments with BMIs possessing different system delays (100, 200 and 300 ms) show that the monkey can make significantly straighter, faster and smoother movements when the decoder predicts the user\u27s future intent. We also characterize how BMI performance changes as a function of delay, and explore offline how the accuracy of future prediction decoders varies at different time leads. Significance. This study is the first to characterize the effects of control delays in a BMI and to show that decoding the user\u27s future intent can compensate for the negative effect of control delay on BMI performance

Composite fermions in the Fractional Quantum Hall Effect: Transport at finite wavevector

We consider the conductivity tensor for composite fermions in a close to half-filled Landau band in the temperature regime where the scattering off the potential and the trapped gauge field of random impurities dominates. The Boltzmann equation approach is employed to calculate the quasiclassical transport properties at finite effective magnetic field, wavevector and frequency. We present an exact solution of the kinetic equation for all parameter regimes. Our results allow a consistent description of recently observed surface acoustic wave resonances and other findings.Comment: REVTEX, 4 pages, 1 figur