The theory of spin exchange between optically pumped alkali-Inetal atoms and noble-gas nuclei is presented. Spin exchange with heavy noble gases is dominated by interactions in long-lived van der Waals molecules. The main spin interactions are assumed to be the spin-rotation interactions yN S between the rotational angular momentum N of the alkali-metalnoble-gas pair and the electron spin S of the alkali-metal atom, and the contact hyperfine interaction aK S between the nuclear spin K of the noble-gas atom and the electron spin S. Arbitrary values for EC and for the nuclear spin I of the alkali-metal atom are assumed. Precise formal expressions for spin transfer coefficients are given along with convenient approximations based on a perturbation expansion in powers of (o. '/yX), a quantity which has been shown to be small by experiment.
The results of theoretical and experimental investigation of an initially curved clamped-clamped microbeam actuated by a distributed electrostatic force are presented. Reduced-order Galerkin and consistently constructed lumped models of the shallow Euler-Bernoulli arch were built and verified by numerical analysis, and the influence of various parameters on the stability was investigated. Due to the unique combination of generic mechanical and electrostatic nonlinearities, the voltage-deflection characteristic of the device may have two maxima implying the existence of sequential snap-through buckling and pull-in instability and of bistability of the beam. The first critical voltage can be higher or lower than the second one, while the stable deflections are significantly larger than in a straight beam. The minimal initial elevation required for the appearance of the snap-through in the electrostatically actuated beam is smaller than in the case of uniform deflection-independent loading; a closed-form approximation of this elevation was evaluated. The devices were fabricated from silicon on insulator (SOI) wafer using deep reactive ion etching and in-plane responses were characterized by means of optical and scanning electron microscopy. Model results obtained for the actual dimensions of the device were in good agreement with the experimental data.
Electrothermal linear micromotors fabricated by deep reactive ion etching of silicon-on-insulator wafers are presented. These high-aspect-ratio motors are powered by thermal actuator arrays with a height of 50 µm. Synchronized arrays, each containing ten actuators connected by a midpoint yoke, are used to advance a slider through frictional contact. Forces of 6.7 mN have been demonstrated at a voltage of 12 V using motors measuring 2.5 mm by 2.1 mm. Unidirectional motors have been successfully operated at speeds of up to 1 mm s −1 over a range in excess of 2 mm. Motors are found to be well suited for positioning compliant mechanisms and similar applications requiring large forces and displacements at low drive voltages.
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