Von Klitzing constant h/e² — the resistance plateau of the quantum Hall effect, now the resistance standard.The von Klitzing constant, denoted R_K, is the fundamental constant equal to h/e², the ratio of Planck's constant to the square of the elementary charge. It appears as the precise value of the Hall resistance plateaus in the integer quantum Hall effect. When a two-dimensional electron gas is subjected to a strong perpendicular magnetic field at low temperatures, the Hall resistance exhibits quantized plateaus at values R_K / i, where i is an integer. This quantization is extraordinarily precise, with an accuracy of about one part in a billion, making it a cornerstone of metrology.
The effect was discovered by Klaus von Klitzing in 1980, for which he received the Nobel Prize in Physics in 1985. The constant was subsequently named in his honor. The discovery revolutionized electrical metrology, leading to the redefinition of the ohm in terms of R_K.
In the quantum Hall effect, electrons in a two-dimensional system form Landau levels in a magnetic field. When the Fermi level lies between Landau levels, the longitudinal resistance drops to zero, and the Hall resistance becomes quantized. The quantization arises from the topological properties of the electron wavefunctions, which lead to a robust, material-independent value of the Hall conductance, exactly i e²/h. This robustness is why the effect can serve as a primary standard for electrical resistance.
The von Klitzing constant is used as the international standard for electrical resistance, defining the ohm in the International System of Units (SI) since 2019. It is also used in precision measurements of fundamental constants and in quantum metrology.
The von Klitzing constant is now exactly 25812.80745... ohms by definition, and its value is used to define the ohm in the SI system, making it a fixed constant rather than a measured quantity.









