1985 Nobel Prize in Physics
Reason for Award
for the discovery of the quantized Hall effect (Phys. Rev. Lett. 45, 494–497, 1980; Metrologia 21, 11–19, 1985)
Laureates
West Germany
Explanation
When a magnet is placed near some materials, the way electricity flows usually changes smoothly. Klaus von Klitzing found that if a very thin sheet is cooled and a strong magnet is applied, the difficulty for electricity to flow—its resistance—jumps like steps to exact values. Those values depend only on nature’s basic numbers, the Planck constant and the electron’s charge, so everyone measures the same number.
Related Keywords
quantum Hall effect
The quantum Hall effect occurs when a strong magnetic field and very low temperature are applied to a two-dimensional electron system, causing the Hall resistance to lock at integer or fractional multiples of h/e^2. The longitudinal resistance vanishes, so current flows without dissipation, allowing metrological links between the Planck constant h and the electron charge e. There are integer and fractional forms, the latter dominated by electron correlations. The phenomenon is underpinned by topologically protected edge states.
Hall resistance
Hall resistance is the ratio V_H/I of the transverse voltage that appears when a magnetic field is applied to a current-carrying conductor. Under normal conditions it varies smoothly with carrier density and temperature, but in the quantum Hall regime it forms step-like plateaus that are precisely quantized. This property makes it the basis of the international resistance standard and a tool for assessing semiconductor device quality.
Landau level
Landau levels are discrete energy states that electrons occupy when they undergo cyclotron motion in a magnetic field. In two dimensions these levels are highly degenerate, and when the Fermi level lies in the gap between them, transport becomes scattering-free. Quantum Hall plateaus correspond to ranges in which an integer number of Landau levels is completely filled and the filling factor stays constant.
two-dimensional electron gas
A two-dimensional electron gas (2DEG) is formed when electrons are confined to a layer only a few nanometers thick, such as in semiconductor heterojunctions, so their motion is effectively two-dimensional. Very high mobilities can be achieved, making quantum effects under strong magnetic fields prominent. GaAs/AlGaAs heterostructure 2DEGs became the standard platform for quantum Hall experiments.
Planck constant
The Planck constant h is a fundamental constant of quantum mechanics, appearing in relations such as E = hν for photons and p = h/λ linking momentum to wavelength. It enters directly into the quantum Hall resistance R_K = h/e^2 and is embedded in the definition of electrical resistance. In the 2019 SI revision, h was given an exact value, contributing to the redefinitions of the kilogram and the ampere.
elementary charge
The elementary charge e is the amount of charge carried by a single electron and is a cornerstone constant of electromagnetism. Through the quantum Hall relation R_K = h/e^2 it is tied to the international resistance standard. In the new SI system, e also has an exact value, supporting high-precision measurements of current and amount of substance.
topological invariant
A topological invariant is a mathematical quantity that remains unchanged under smooth deformations of a system; in quantum Hall systems the Chern number is the prime example. The integer value of the Chern number guarantees the quantization of Hall conductance and provides robustness against scattering and defects. The concept has been extended to other quantum phases such as topological insulators and semimetals.