Surface acoustic waves are used to actuate and process smallest amounts of fluids on the planar surface of a piezoelectric chip. Chemical modification of the chip surface is employed to create virtual wells and tubes to confine the liquids. Lithographically modulated wetting properties of the surface define a fluidic network, in analogy to the wiring of an electronic circuit. Acoustic radiation pressure exerted by the surface wave leads to internal streaming in the fluid and eventually to actuation of small droplets along predetermined trajectories. This way, in analogy to microelectronic circuitry, programmable biochips for a variety of assays on a chip have been realized.
The interaction between surface acoustic waves and quasi-two-dimensional inversion electron systems on GaAs/Al"Ga& As heterojunctions is investigated in high magnetic fields and at low temperatures. The interaction of the surface acoustic wave with high-mobility inversion electrons leads to strong quantum oscillations in both the transmitted surface wave intensity as well as in the sound velocity, rejecting the quantum oscillations of the magnetoconductivity as a function of an applied magnetic field. We study the dependence of thip interaction on the magnetic field and on the surface-acoustic-wave power and frequency, and discuss the results using simple models. The inhuence of slight spatial inhomogeneities in the carrier density on the line shape of the quantum oscillations is analyzed in detail and related to their inAuence on the quantum Hall effect. First experimental results on the interaction of surface acoustic waves with two-dimensional electron systems in gated heterojunctions providing an adjustable carrier density are presented.define the piezoelectric coupling coefficient 2(U -Uo) K =
The in-plane effective electron mass (m ʈ ) in narrow Ga 0.47 In 0.53 As/InP quantum wells is strongly dependent on the quantization energy. Cyclotron resonance in a series of quantum wells with well widths down to 15 Å reveals a mass enhancement of up to 50% (m ʈ ϭ0.065m 0 ) over the bulk value of Ga 0.47 In 0.53 As. This effect is caused by the nonparabolicity of the conduction band and wave function penetration into the barrier material. Our experimental findings are in good agreement with calculations performed within the framework of k-p theory. We obtain an easy-to-use relation between the mass and the quantization energy m 0 /m ʈ (⑀)ϭ(1Ϫ1.96⑀/eV)/0.044.
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