Kondo effect

The Kondo effect refers to the non-trivial physics associated with the presence of a magnetic impurity in a solid (generally, a metal). It was found that the resistance of a metal with these impurities does not decrease to a constant at low temperatures (as for a metal with non-magnetic impurities), but actually hits a shallow minimum at a temperature of order 10 K, then increases at lower temperatures.

Jun Kondo (近藤 淳) did the first proper calculation regarding this effect, which showed that in higher orders of perturbation theory, the resistance will diverge as the temperature approaches 0 K. The temperature dependence of the electronic resistance including the Kondo effect is written as:

$\rho (T)=\rho _{0}+aT^{2}+c_{m}\ln {\frac {\mu }{T}}+bT^{5},$

where $\rho _{0}$ is the residual resistance, $aT^{2}$ shows the contribution from the Fermi liquid properties, and the term $bT^{5}$ is from the lattice vibrations. Jun Kondo has derived the third term of the logarithmic dependence. Later calculations refined this result to produce a finite resistivity but retained the feature of a resistance minimum at a non-zero temperature. One defines the Kondo temperature as the energy scale, limiting the validity of the Kondo results. The Anderson model and accompanying renormalization theory was an important contribution to understanding the underlying physics of the problem.

The Kondo effect is the first known example of asymptotic freedom in physics, in which the coupling becomes non-perturbatively strong at low temperatures/low energies. In the Kondo problem, this refers to the interaction between the localized magnetic impurities and the itinerant electrons. In a more complex form, asymptotic freedom underlies the theory of quantum chromodynamics, or the so-called strong nuclear force so that quarks, the fundamental consituents of nuclear matter, interact weakly at high energies and strongly at low energies, preventing the unbinding of hadrons or mesons, the composite particles of nuclear matter. For this discovery, Frank Wilczek, David Gross, and David Politzer shared the 2004 Nobel Prize in physics.

External links

• Jun Kondo's web page - follow links in English, other parts are written in Japanese
• Kondo Effect - 40 Years after the Discovery - special issue of the Journal of the Physical Society of Japan