Tricky Anomaly

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Science  27 Sep 2013:
Vol. 341, Issue 6153, pp. 1431
DOI: 10.1126/science.341.6153.1431-c

A quantum point contact is a narrow conducting channel that can be shaped by applying variable voltage to electrodes placed above a two-dimensional electron gas in semiconductor nanostructures. As a function of the voltage, the conductance of the contact exhibits equally spaced plateaus corresponding to sequential openings of transport channels; however, an additional feature that appears at the conductance value of roughly 0.7 of the step still lacks a detailed theoretical explanation. Bauer et al. argue that it is a consequence of the structure of the local density of states. They developed a one-dimensional tight-binding model—in which a singularity in the density of states resulted in enhanced electronic interactions—that was able to account for data taken in varying experimental conditions. Iqbal et al. ascribe the feature to localized spins and the Kondo effect. They performed their experiments on quantum point contacts of varying length and found that the conductance of the anomaly varied periodically with the length; in their model, the number of localized states increased with the length, explaining the periodicity. Both models offer considerable and complementary insights into the problem.

Nature 501, 73; 79 (2013).

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