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...

We report alternative types of magnetoresistance oscillations in high mobility two-dimensional electron systems subjected to large amplitude one-dimensional periodic magnetic modulations, of period 500 nm to 1 m. We observe Shubnikov-de Haas oscillations that are strongly modified in amplitude and phase, Hall resistance oscillations, and aperiodic magnetoresistance oscillations. These effects are shown to arise from the internal structure of overlapping Landau bands and are well accounted for by perturbation calculations.

We report geometry-induced fractal behaviour in the low-field magneto-conductance fluctuations of a mesoscopic semiconductor billiard. Such fractal behaviour was recently predicted to be induced by the mixed (chaotic/regular) phase space generated by the soft-walled billiard potential, and our results constitute a possible experimental observation of the infinite hierarchical nature of this mixed phase space. Preliminary investigations of the effects of temperature and gate bias, which directly control the electron coherence and billiard potential profile, are presented.

No abstract

No abstract