The Quantized Hall Effect

This variation on a classical phenomenon makes it possible, even in an irregular sample, to measure fundamental constants with an accuracy rivaling that of the most precise measurements yet made F ew areas of modern science have effect, a planar sample of conductive been studied as intensively as the physics of semiconducting mate-material is placed in a magnetic field and thus obtain quantities that arc in-perpendicular to the sample's surface. dependent of the current flow. The ra-rials. Semiconductors are key components of the electronic devices that are Then, by means of electrodes at each tio of the voltage drop along the sam-end of the sample, a small electric cur-ple to the current is just the ordinary the hallmark of our era. In develop-rent is made to pass from one end to electrical resistance of the sample. The ing these devices, investigators have ratio of the voltage drop across the learned how to grow nearly perfect the other. (An electric current is simply the motion of charged particles; in the sample to the current flowing along it is called the Hall resistance. semiconductor crystals: how to modify the electronic properties of such case of most solids the current-carrying particles are electrons.) In his 1980 experiment von Klitzing crystals by the addition of a few parts was studying the Hall effect in certain Two measurements are then made. per million of foreign atoms, and how devices in which the electrons free to To make one measurement, contacts to understand so well the properties of carry current are confined within a thin the modified crystals that they can in-" ear both ends of the sample are con-layer. He found that when such de-nected to a voltmeter, which measures vices are cooled to within about a de-corporate them into circuit elements the voltage drop from one end of the whose thickness is measured in tens of sample to the other, that is, in the di-gree of absolute zero and placed in ex-atomic layers and whose area is so rectio " parallel to the Row of current. tremely strong magnetic fields, the be-small that a million individual ele-(This is essentially a measure of the havior of the ordinary resistance and ments can fit on a chip measuring one difference between the energy of the the Hall resistance was dramatically different from the behavior described square centimeter. electrons entering the sample and the It might seem …