Dynamics of Vortex Nucleation in Nanomagnets with Broken Symmetry

We investigate fundamental processes that govern dynamics of vortex nucleation in sub-100 nm mesoscopic magnets. We focus on a structure with broken symmetry - Pacman-like nanomagnet shape - in which we study micromagnetic behavior both by means of a simple model and numerically. We show that it is possible to establish desired vortex chirality and polarity by applying only quasi-static in-plane magnetic field along specific directions. We identify the modes of vortex nucleation that are very robust against external magnetic field noise. These vortex nucleation modes are common among wide range of sub-100 nm magnets with broken rotational symmetry.Comment: 5 pages, 4 figure

Frictional drag between quantum wells mediated by phonon exchange

We use the Kubo formalism to evaluate the contribution of acoustic phonon exchange to the frictional drag between nearby two-dimensional electron systems. In the case of free phonons, we find a divergent drag rate ($\tau_{D}^{-1}$). However, $\tau_{D}^{-1}$ becomes finite when phonon scattering from either lattice imperfections or electronic excitations is accounted for. In the case of GaAs quantum wells, we find that for a phonon mean free path $\ell_{ph}$ smaller than a critical value, imperfection scattering dominates and the drag rate varies as $ln (\ell_{ph}/d)$ over many orders of magnitude of the layer separation $d$. When $\ell_{ph}$ exceeds the critical value, the drag rate is dominated by coupling through an electron-phonon collective mode localized in the vicinity of the electron layers. We argue that the coupled electron-phonon mode may be observable for realistic parameters. Our theory is in good agreement with experimental results for the temperature, density, and $d$-dependence of the drag rate.Comment: 45 pages, LaTeX, 8 postscript file figure