Raman scattering from arrays of GaAs quantum cylinders with radii down to 30 nm has been observed. The spectra show an enhanced TO phonon intensity due to a change in the first-order selection rules. In addition, the smallest cylinders exhibit an extra spectral feature between the TO and LO phonons. The behaviour of this additional peak as a function of the scattering geometry, the cylinder size and the variation of the medium surrounding the cylinders is consistent with a theoretical model of the surface phonons of a cylinder.
We have investigated the magnetoresistance of a heterostructure containing a near surface two-dimensional electron gas subject to a periodic magnetic field that alternates in sign. The field is produced by an array of submicrometer ferromagnets fabricated on the surface of the heterostructure. We observe a giant low-field magnetoresistance due to electrons propagating in open orbits along lines of zero magnetic field. We are able to account for the observed form and magnitude of this magnetoresistance in a semiclassical model.
͓S0163-1829͑97͒51424-X͔The nature of the electron states of two-dimensional systems ͑2DES's͒ in a nonuniform magnetic field has attracted considerable interest recently. The existence of extended states in a random magnetic field is still a matter of controversy. 1-4 In a magnetic field that varies spatially about a mean of zero, the existence of delocalized 1D states propagating along the contours of zero magnetic field has been predicted theoretically. 5-7 Magnetoresistance commensurability oscillations have recently been observed in experimental studies of 2DES's in periodically modulated magnetic fields 8-10 and it has been suggested that an additional lowfield magnetoresistance might be associated with such 1D states. 9 Large-period, sign-alternating magnetic modulations have been studied for nonplanar 2DES's in the quantum regime. 11,12 In this paper, we report the observation of a very large, low-field magnetoresistance ͑MR͒ that provides clear evidence for the channeling of 2D electrons in open orbits 5-7 along lines of zero magnetic field. A periodic, signalternating magnetic modulation is produced by a submicrometer ferromagnetic grating fabricated above a nearsurface 2DES. By tilting the applied external magnetic field with respect to the plane of the 2DES, we are able to study systematically the dependence of the MR on the amplitude of the magnetic modulation. Our experimental results are shown to be in good quantitative agreement with a semiclassical model. The semiclassical origin of the MR is confirmed by the fact that it is still observable above 200 K.The device used is illustrated schematically in Fig. 1. The 2DES is formed in a 22-nm-wide GaAs/͑AlGa͒As quantum well, the center of which is only 35 nm beneath the surface of the heterostructure. After infrared illumination, the electron density saturates at 4.8ϫ10 15 m Ϫ2 , while the electron mobility of 70 m 2 V Ϫ1 s Ϫ1 corresponds to an electron mean free path of 1 e ϭ7.7 m. An array of nickel stripes with period aϭ500 nm has been fabricated by electron-beam lithography directly on the surface of the heterostructure. The stripes are taken to be along the y direction. The stripes have nominal width dϭ200 nm and height hϭ100 nm. In order to avoid any strain-induced electric modulation at the 2DES ͑Refs. 8-10͒ due to the differential thermal contraction of Ni and GaAs, the stripes are oriented normal to the ͓100͔ direction which is nonpiezoelectric. 13 The grating covers the entire active area of the Hall bar devices, which a...
A novel microelectronic "pill" has been developed for in situ studies of the gastro-intestinal tract, combining microsensors and integrated circuits with system-level integration technology. The measurement parameters include real-time remote recording of temperature, pH, conductivity, and dissolved oxygen. The unit comprises an outer biocompatible capsule encasing four microsensors, a control chip, a discrete component radio transmitter, and two silver oxide cells (the latter providing an operating time of 40 h at the rated power consumption of 12.1 mW). The sensors were fabricated on two separate silicon chips located at the front end of the capsule. The robust nature of the pill makes it adaptable for use in a variety of environments related to biomedical and industrial applications.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.