What Is the Quantum Hall Effect?

The quantum Hall effect is a well-accepted theory in physics describing the behavior of electrons within a magnetic field at extremely low temperatures. Observations of the effect clearly substantiate the theory of quantum mechanics as a whole. The results are so precise that the standard for the measurement of electrical resistance uses the quantum Hall effect, which also underpins the work done on superconductors.

The Hall effect, discovered by Edwin Hall in 1879, is observed when a current of electricity passes through a conductor placed in a magnetic field. Charge carriers, which are usually electrons but can be protons, scatter to the side of the conductor due to the influence of the magnetic field. The phenomenon can be visualized as a series of cars pushed sideways due to a strong wind while going down a highway. The cars take a curved path as they attempt to drive forward but are forced sideways.

A potential difference between the sides of the conductor develops. The voltage difference is quite small and is a function of the composition of the conductor. Amplification of the signal is necessary to make useful instruments based on the Hall effect. This imbalance in electrical potential is the principle behind a Hall probe that measures magnetic fields.

With the popularity of semiconductors, physicists became interested in examining the Hall effect in foils so thin, the charge carriers were essentially restricted to motion in two dimensions. They applied current to conductive foils under strong magnetic fields and low temperatures. Instead of seeing electrons pulled sideways in curved continuous paths, the electrons made sudden jumps. There were sharp peaks in the resistance to flow at specific energy levels as the magnetic field strength was changed. In between peaks, the resistance dropped to a value near zero, a characteristic of low-temperature superconductors.

The physicists also realized that the energy level necessary to cause a spike in resistance was not a function of the conductor’s composition. The resistance peaks occurred at whole-number multiples of each other. These peaks are so predictable and consistent that instruments based on the quantum Hall effect can be used to create standards of resistance. Such standards are essential to testing electronics and ensuring reliable performance.

The quantum theory of atomic structure, which is the concept that energy is available in discrete, whole packets at the subatomic level, had predicted the quantum Hall effect as early as 1975. In 1980, Klaus von Klitzing received the Nobel Prize in Physics for his discovery that the quantum Hall effect was indeed exactly discrete, meaning that the electrons could exist only in sharply defined levels of energy. The quantum Hall effect has become another argument in support of the quantum nature of matter.