In February 1980, Klaus von Klitzing made a discovery that opened up one of the most exciting chapters in physics history. He had prepared a semiconductor device containing electrons confined to a single layer. This so-called 2D electron gas was already well known to physicists. But when von Klitzing subjected it to very low temperature and very high magnetic field, he found that an intrinsic electronic property, the Hall resistance, occurred only at quantized values that were integer multiples of h∕e2 [1]. The exceptional precision of those values, and their observed insensitivity to sample impurities, ultimately led to the quantum Hall resistance being used to redefine a unit in terms of fundamental constants (see Kilogram Untethered from Early Objects). A perhaps lesser known fact is that physicists have been pursuing a 3D version of the quantum Hall effect (QHE) for 30 years. Experiments achieved success in 2019 [2]. Now, theorists are explaining those results with a model that involves a wave-like electron density. Their picture could help expand the realm of the QHE in 3D [3].

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