Landau band conductivity in a two-dimensional electron system modulated by an artificial one-dimensional superlattice potential

Abstract

We investigate the magnetoresistance of a quasi-two-dimensional electron system subject to a onedimensional superlattice potential created by the field effect in gated AlGaAs/GaAs heterojunctions. At low temperatures this potential gives rise to a new type of magnetoresistance oscillation with a period governed by the ratio of the classical cyclotron diameter 2Rc to the superlattice period a. The oscillations are quantitatively explained in period, phase, and magnitude by the formation of Landau bands in the two-dimensional electron system under the influence of the periodic potential.PACS numbers: 73.40. Kp, 71.25.He Until recently experimental investigations of the effect of a lateral superlattice potential on the electronic transport properties of a two-dimensional electron system (2D ES) have been limited to ''natural" superlattices in metal oxide-semiconductor structures on high-index surfaces of Si^'^ or InSb.^ Modulation-doped AlGaAs/GaAs heterojunctions with a 2D ES of high mobility now make it possible to study lateral superlattice phenomena using artificially created potentials with periods in the submicrometer range. As a first manifestation of such onedimensional (ID) superlattice effects Weiss et al.^ recently observed a new type of oscillation of the lowtemperature magneotresistance at magnetic fields B < 1 T applied perpendicularly to the 2D ES of an AlGaAs/ GaAs heterojunction with a weak periodic electron density modulation. They created the modulation via the persistent photoeffect by exposing the sample to a light grating of submicrometer period a. Similar to Shubnikov-de Haas (SdH) oscillations these new resistance oscillations are periodic in l/B. Weiss et al. found that apart from a phase factor 0, maxima in the magnetoresistance arise whenever the classical cyclotron diameter IRc at the Fermi energy is a multiple of the period a such that IRc = (m-l-0)a, m = 1,2,. . .In their report, however, they did not identify the mechanism causing the oscillations.Here we present experimental studies of these novel magnetoresistance oscillations on AlGaAs/GaAs heterojunctions with a periodically microstructured gate electrode, in which the field effect is used to create a ID periodic potential. We demonstrate that the magnetoresistance oscillations reflect the formation of Landau bands in the 2D ES under the influence of the superlattice potential. Calculating the magnetic band structure in the presence of the periodic potential, ^'^ we show that the bandwidth of the Landau bands at the Fermi energy oscillates periodically with period IRcIa and gives rise to an oscillatory band conductivity. We calculate the magnetoresistance caused by this mechanism and find good agreement with the observed oscillations in period, phase, and magnitude.The samples are prepared on conventional AlGaAs/ GaAs heterojunctions grown by molecular-beam epitaxy. The heterojunctions consist of a l.6-//m-thick GaAs buffer on top of a semi-insulating GaAs substrate, a 20nm nominally undoped Al;cGai-;cAs (x=0.33) spacer, a 50-nm...

Magnetoquantum Oscillations in a Lateral Superlattice

“…This agrees with our finding that the number of oscillation periods resolved depends on the mobility of the sample: the higher the mobility the more oscillations are observable since the electrons can traverse more periods of the potential ballistically. Similar experimental results have been observed recently by Winkler et al (1989) using conventionally microstructured samples. The key for the explanation of this novel oscillatory behaviour of pxx and pyy in such periodically modulated 2-DEG's is a modification of the Landau level spectrum discussed in the next section.…”

“…The temperature dependence of these oscillations is much weaker than that of SdH oscillations, since the relevant energy is the distance between flat bands, which is much larger .than the mean distance between adjacent bands. Winkler et al (1989) have explained the oscillations in pxx -measured in samples with microstructured gates -along very similar theoretical lines. Using the quasi-classical large-n limit, which means…”

Quantum transport theory of Weiss oscillations quenching of a two-dimensional electron gas in a strong periodic potential

“…Several closely related explanations of the Weiss oscillations have been given from both a classical 3 and a quantummechanical point of view. 1, 4,5 But all of the works quoted above have focused on the effects of a weak applied potential. Recently, Beton et al 7 have performed magnetoresistance measurements of 2DEG subjected to a periodic potential of variable amplitude.…”

“…[1][2][3][4][5][6] The modulated 2DEG is realized in highmobility Al x Ga 1Ϫx As/GaAs heterojunctions by illuminating samples with interfering laser beams or by a periodic voltage-gate array on top of heterojunction films. It has been observed that at magnetic fields (B) higher than 0.5 T, Shubnikov-de Haas-type oscillations appear in the resistivity tensor element xx and yy .…”

Quantum computation in quantum-Hall systems

“…It is possible to generate by holographic and other methods [48][49][50][51] a narrow strip of conductance at each spin in turn, see Figure 1, and send a current of spin-polarized electrons through it. The observed current can be pre-calibrated to enable high certainty determination of whether there was a spin-exchange scattering event thus determining the nuclear spin's direction, resembling the spin-diode [38,52] techniques.…”