Emergent Quantum Phenomena

   Kondo Effect and Heavy Fermions

   The Kondo effect is the result of the interaction of magnetic atoms with conduction electrons and is a classic many-body issue. When this effect works, the resistance and thermodynamic properties of the system will be abnormal, and the resistance will be minimum at low temperature. Japanese scientists Kondo first pointed out that the emergence of resistance minimum was the consequence of magnetic atoms being gradually shielded by conductive electrons. Later, American physicist Wilson used the numerical renormalization group method to comprehensively solve some of the basic problems in the study of the Kondo effect, which made him win the Nobel Prize in Physics in 1982.


   Heavy fermion materials are a special type of strongly correlated electron system. In addition to conductive electrons, there are also a large number of regularly arranged magnetic ions, usually containing U, Ce or other heavy elements. Their specific heat is three orders of magnitude higher than that of high-purity metals. Therefore, it can be inferred that the effective mass of the electron is much larger than that of the free electron, so it is called heavy fermion. Some heavy fermion materials are superconductors, some have magnetic ordered structures, and some have nothing to do with both, but they are related to the Kondo effect. The electrons in these materials are neither propagated in the form of Bloch waves like electrons in metals, nor are they completely localized in insulators. They are in a new electronic state. How to describe this state still lacks a very clear physical image.

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