Possible observation of impurity effects on conductance quantization

Abstract

We report an experimental observation of impurity-induced conductance dips in quantized channels as predicted by previous theoretical studies. Our experiments use quantum point contacts on a two-dimensional electron gas in a modulation-doped GaAs/Al Gai -"As heterostructure. The electron gas has a sheet density of 1.2x10" cm 2 and a mobility of 4. 6x10' cm2/Vs, measured at 50 mK. Our data, which are qualitatively very similar to those calculated with use of a two-dimensional Anderson model, strongly suggest that we are observing both the erosion of conductance quantization, and localization in the presence of an impurity-induced random potential. Conductance measurements (van Wees et al. ' and Wharam et al. ) with so-called quantum point contacts (QPC) on a two-dimensional (2D) electron gas in modulation-dopedGaAs/A1, Gai "As heterostructures have demonstrated the existence of plateaus at integer multiples of the elementary conductance Go q /xh. The plateaus observed in those experiments are fairly flat and well defined, pointing to nearly ideal conditions for their observation. It is therefore of great interest to understand what "ideal" means in this particular context, and how various factors such as the potential distribution in the constriction affect the quantization. As impurities, intentional or not, are unavoidable, it is of particular interest to investigate their influence. One suspects for example that, since the remote doping layer in the Al"Gai -, As affects the mobility of the 2D electron gas, it will also affect the quantization in some way. The worst case, one may think, would be that of unintentional impurity centers being located directly in the confined channel.Such questions are now attracting widespread interest among theoreticians. Different groups have recently provided an extensive study of the case of an impurity (attractive or repulsive) being located in the confined channel. From a set of interesting results, these groups have shown that attractive impurities, in particular, exhibit a clear signature in the form of a deep dip appearing at the end of each conductance plateau, followed by a sharp rise to the next plateau as the channel width is increased. Using a somewhat different approach, other theoreticians have studied the crossover from ballistic to diffusive transport in quantized channels as a function of disorder produced by a random, attractive impurity potential. Their results show a rounding of the conductance steps with increasing disorder, loss of the quantization in units of Go, and the occurrence of pronounced conductance dips at each new channel opening. These finding are consistent with the single-impurity case cited above. Kander, Imry, and Sivan interpret the conductance dips as a localization e6'ect occurring each time a new channel is opened. This Rapid Communication reports experimental observation of impurity-induced conductance dips in quantized channels as predicted by the authors cited above. Our 2p, m C) 3 (3 C9 .8 0.4 0 -0.4 g4 gf r l . / V (V) FIG. 1. Point-...

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