Diffusion of Monochromatic Classical Waves

We study the diffusion of monochromatic classical waves in a disordered acoustic medium by scattering theory. In order to avoid artifacts associated with mathematical point scatterers, we model the randomness by small but finite insertions. We derive expressions for the configuration-averaged energy flux, energy density, and intensity for one, two and three dimensional (1D, 2D and 3D) systems with an embedded monochromatic source using the ladder approximation to the Bethe-Salpeter equation. We study the transition from ballistic to diffusive wave propagation and obtain results for the frequency-dependence of the medium properties such as mean free path and diffusion coefficient as a function of the scattering parameters. We discover characteristic differences of the diffusion in 2D as compared to the conventional 3D case, such as an explicit dependence of the energy flux on the mean free path and quite different expressions for the effective transport velocity.Comment: 11 pages, 2 figure

Charge Density Wave Ratchet

We propose to operate a locally-gated charge density wave as an electron pump. Applying an oscillating gate potential with frequency $f$ causes equally spaced plateaux in the sliding charge density wave current separated by $\Delta I=2eNf,$ where $N$ is the number of parallel chains. The effects of thermal noise are investigated.Comment: To be published in Applied Physics Letter

Feedback control of noise in spin valves by the spin-transfer torque

The miniaturisation of magnetic read heads and random access memory elements makes them vulnerable to thermal fluctuations. We demonstrate how current-induced spin-transfer torques can be used to suppress the effects of thermal fluctuations. This enhances the fidelity of perpendicular magnetic spin valves. The simplest realization is a dc current to stabilize the free magnetic layers. The power can be significantly reduced without losing fidelity by simple control schemes, in which the stabilizing current-induced spin-transfer torque is controlled by the instantaneous resistance.Comment: 4pages, 2 figure

Spin-transfer in diffusive ferromagnet-normal metal systems with spin-flip scattering

The spin-transfer in biased disordered ferromagnet (F) - normal metal (N) systems is calculated by the diffusion equation. For F1-N2-F2 and N1-F1-N2-F2-N3 spin valves, the effect of spin-flip processes in the normal metal and ferromagnet parts are obtained analytically. Spin-flip in the center metal N2 reduces the spin-transfer, whereas spin-flip in the outer normal metals N1 and N3 can increase it by effectively enhancing the spin polarization of the device.Comment: 9 pages, 3 figure