Heavy fermion

In solid-state physics, heavy fermion materials are a specific type of intermetallic compound, containing elements with 4f or 5f electrons in unfilled electron bands. Electrons are one type of fermion, and when they are found in such materials, they are sometimes referred to as heavy electrons. Heavy fermion materials have a low-temperature specific heat whose linear term is up to 1000 times larger than the value expected from the free-electron theory. The properties of the heavy fermion compounds derive from the partly filled f-orbitals of rare earth or actinide ions which behave like localized magnetic moments. The name "heavy fermion" comes from the fact that the fermion behaves as if it has an effective mass greater than its rest mass. In the case of electrons, below a characteristic temperature (typically below 10 K), the conduction electrons in these metallic compounds behave as if they had an effective mass up to 1000 times the free-electron mass. This large effective mass is also reflected in a large contribution to the resistivity from electron-electron scattering via the Kadowaki Woods ratio. Heavy fermion behavior has been found in a broad variety of states including metallic, superconducting, insulating and magnetic states. Characteristic examples are CeCu₆, CeAl₃, CeCu₂Si₂, YbAl₃, UBe₁₃ and UPt₃.

Heavy fermion behavior was discovered by Andres, Graebner and Ott in 1975, who observed enormous magnitudes of the linear specific heat capacity in CeAl₃.

While investigations on doped superconductors led to the conclusion that the existence of localized magnetic moments and superconductivity in one material was incompatible, the opposite was shown, when in 1979 Steglich et al. discovered heavy fermion superconductivity in the material CeCu₂Si₂.

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