Path methods for strong shift equivalence of positive matrices

In the early 1990's, Kim and Roush developed path methods for establishing strong shift equivalence (SSE) of positive matrices over a dense subring U of the real numbers R. This paper gives a detailed, unified and generalized presentation of these path methods. New arguments which address arbitrary dense subrings U of R are used to show that for any dense subring U of R, positive matrices over U which have just one nonzero eigenvalue and which are strong shift equivalent over U must be strong shift equivalent over U_+. In addition, we show positive real matrices on a path of shift equivalent positive real matrices are SSE over R_+; positive rational matrices which are SSE over R_+ must be SSE over Q_+; and for any dense subring U of R, within the set of positive matrices over U which are conjugate over U to a given matrix, there are only finitely many SSE-U_+ classes.Comment: This version adds a 3-part program for studying SEE over the reals. One part is handled by the arxiv post "Strong shift equivalence and algebraic K-theory". This version is the author version of the paper published in the Kim memorial volume. From that, my short lifestory of Kim (and more) is on my web page http://www.math.umd.edu/~mboyle/papers/index.htm

High-performance Schottky diodes endure high temperatures

Fabrication process and aluminum/GaAs (gallium arsenide) coupling are used to produce Schottky diodes that have high cutoff frequencies and can withstand operating temperatures in excess of 500 C

Elevated temperature crack growth

Critical gas turbine engine hot section components such as blades, vanes, and combustor liners tend to develop minute cracks during early stages of operations. The ability of currently available path-independent (P-I) integrals to correlate fatigue crack propagation under conditions that simulate the turbojet engine combustor liner environment was determined. To date, an appropriate specimen design and a crack displacement measurement method were determined. Alloy 718 was selected as the analog material based on its ability to simulate high temperature behavior at lower temperatures in order to facilitate experimental measurements. Available P-I integrals were reviewed and the best approaches are being programmed into a finite element post processor for eventual comparison with experimental data. The experimental data will include cyclic crack growth tests under thermomechanical conditions, and, additionally, thermal gradients

Turbulent transport and dynamo in sheared MHD turbulence with a non-uniform magnetic field

We investigate three-dimensional magnetohydrodynamics turbulence in the presence of velocity and magnetic shear (i.e., with both a large-scale shear flow and a nonuniform magnetic field). By assuming a turbulence driven by an external forcing with both helical and nonhelical spectra, we investigate the combined effect of these two shears on turbulence intensity and turbulent transport represented by turbulent diffusivities (turbulent viscosity, α and β effect) in Reynolds-averaged equations. We show that turbulent transport (turbulent viscosity and diffusivity) is quenched by a strong flow shear and a strong magnetic field. For a weak flow shear, we further show that the magnetic shear increases the turbulence intensity while decreasing the turbulent transport. In the presence of a strong flow shear, the effect of the magnetic shear is found to oppose the effect of flow shear (which reduces turbulence due to shear stabilization) by enhancing turbulence and transport, thereby weakening the strong quenching by flow shear stabilization. In the case of a strong magnetic field (compared to flow shear), magnetic shear increases turbulence intensity and quenches turbulent transport

Electron Removal Self Energy and its application to Ca2CuO2Cl2

We propose using the self energy defined for the electron removal Green's function. Starting from the electron removal Green's function, we obtained expressions for the removal self energy Sigma^ER (k,omega) that are applicable for non-quasiparticle photoemission spectral functions from a single band system. Our method does not assume momentum independence and produces the self energy in the full k-omega space. The method is applied to the angle resolved photoemission from Ca_2CuO_2Cl_2 and the result is found to be compatible with the self energy value from the peak width of sharp features. The self energy is found to be only weakly k-dependent. In addition, the Im Sigma shows a maximum at around 1 eV where the high energy kink is located.Comment: 5 pages, 3 figure

Analysis of the backscatter spectrum in an ionospheric modification experiment

Predictions of the backscatter spectrum, including effects of ionospheric inhomogeneity, are compared with experimental observations of incoherent backscatter from an artificially heated region. Our calculations show that the strongest backscatter echo received is not from the reflection level, but from a region some distance below. Certain asymmetrical features are explained of the up-shifted and down-shifted plasma lines in the backscatter spectrum, and the several satellite peaks accompanying them

High-fidelity ion-trap quantum computing with hyperfine clock states

We propose the implementation of a geometric-phase gate on magnetic-field-insensitive qubits with $\hat{\sigma}^z$-dependent forces for trapped ion quantum computing. The force is exerted by two laser beams in a Raman configuration. Qubit-state dependency is achieved by a small frequency detuning from the virtually-excited state. Ion species with excited states of long radiative lifetimes are used to reduce the chance of a spontaneous photon emission to less than 10$^{-8}$ per gate-run. This eliminates the main source of gate infidelity of previous implementations. With this scheme it seems possible to reach the fault tolerant threshold.Comment: 4 pages, 1 figur