Kondo effect

(metallurgy) The large anomalous increase in the resistance of certain dilute alloys of magnetic materials in nonmagnetic hosts as the temperature is lowered.

An unusual, temperature-dependent effect displayed in the thermal, electrical, and magnetic properties of nonmagnetic metals containing very small quantities of magnetic impurities. A striking example is the anomalous, logarithmic increase in the electrical resistivity with decreasing temperature. Other properties, such as heat capacity, magnetic susceptibility, and thermoelectric power, also display anomalous behavior because of the Kondo effect. For these properties, the temperature dependence of a typical dilute magnetic metal (Kondo alloy) differs greatly from the behavior expected of an ordinary metal containing no magnetic impurities.

The Kondo effect has been observed in a wide variety of dilute magnetic alloys. Usually these alloys are made from a nonmagnetic host such as copper, silver, gold, magnesium, or zinc and a small amount of a magnetic metal impurity such as chromium, manganese, iron, cobalt, nickel, vanadium, or titanium. Typical concentrations range from about one to a few hundred magnetic atoms per million host atoms. At higher concentrations, the dilute magnetic alloys may display spin-glass behavior. See also Spin glass.

The Kondo effect is used in thermometry applications, especially thermocouple thermometers at very low temperatures (that is, millikelvin temperatures). In other applications where the properties of pure metals are studied, the Kondo effect serves as a useful indicator of the metal's magnetic-impurity level.

The problem of understanding the Kondo effect is considered important since it is recognized to be a simpler version of the more complex problem of understanding ferromagnetism in magnetic materials, which is one of the great challenges in physics. Basically the Kondo effect is an example of the most simple possible magnetic system—a single magnetic atom in a nonmagnetic environment. (The alloys used are so dilute that the interaction between different magnetic impurities can be safely ignored.) Although this involves a simple physical model, the problem has required some of the most sophisticated mathematical techniques known to advance its understanding.

An important step in this direction was the development of a partial mathematical solution of the Kondo problem using renormalization field theory techniques. Information gained in this step helped with the final development of a mathematically exact solution of the Kondo problem. The exact solution permits a systematic calculation of all properties (resistivity, thermal conductivity, thermopower, specific heat, magnetic susceptibility, neutron scattering behavior, and so forth) and provides a physical understanding of these properties. The theoretical work on the Kondo problem has been connected with new understanding in a variety of other scientific disciplines such as condensed-matter physics, surface physics, critical phenomena, elementary particle physics, magnetism, molecular physics, and chemistry, where parallels and analogs to the Kondo problem can be identified and utilized. See also Critical phenomena; Ferromagnetism; Renormalization.

Kondo effect: How gold with a small amount of what were probably iron impurities behaves at low temperatures

In physics, the Kondo effect is a scattering mechanism of conduction electrons in a metal due to magnetic impurities. It is a measure of how electrical resistivity changes with temperature.^[1]

The effect was described by Jun Kondo who showed that electrical resistance will diverge as the temperature approaches 0 K. The temperature dependence of the resistance including the Kondo effect is written as:

where ρ₀ is the residual resistance, aT² shows the contribution from the Fermi liquid properties, and the term bT⁵ is from the lattice vibrations. Jun Kondo 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.

Schematic of the weakly coupled high temperature situation in which the magnetic moments of conduction electrons in the metal host pass by the impurity magnetic moment at speeds of v_F, the Fermi velocity, experiencing only a mild antiferromagnetic correlation in the vicinity of the impurity. In contrast, as the temperature tends to zero the impurity magnetic moment and one conduction electron moment bind very strongly to form an overall non-magnetic state.

The Kondo effect is the first known example of asymptotic freedom in physics, in which the coupling becomes non-perturbatively strong at low temperatures and 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 constituents of nuclear matter, interact weakly at high energies and strongly at low energies, preventing the unbinding of baryons (fermions like protons or neutrons with three quarks) or mesons (bosons like pions with two quarks), the composite particles of nuclear matter. For this discovery, Frank Wilczek, David Gross, and David Politzer shared the 2004 Nobel Prize in physics.

Extended to a lattice of magnetic impurities, the Kondo effect is believed to underlie the formation of heavy fermions in intermetallic compounds based especially upon rare earth elements like cerium, praseodymium, and ytterbium, and actinide elements like uranium. In these materials, the nonperturbative growth of the interaction leads to quasi-electrons with masses up to thousands of times the free electron mass, ie, the electrons are dramatically slowed by the interactions. In a number of instances they actually are superconductors. More recently, it is believed that a manifestation of the Kondo effect is necessary for understanding the unusual metallic delta-phase of plutonium.

Citations

1. ^ Hewson, Alex C; Jun Kondo (2009). "Kondo effect". Scholarpedia. p. 7529. doi:10.4249/scholarpedia.7529. http://www.scholarpedia.org/article/Kondo_effect. Retrieved 2009-12-18. 

External links

• Jun Kondo's web page
• Kondo Effect - 40 Years after the Discovery - special issue of the Journal of the Physical Society of Japan
• The Kondo Problem to Heavy Fermions - Monograph on the Kondo effect by A.C. Hewson (ISBN 0-521-59947-4)
• Exotic Kondo Effects in Metals - Monograph on newer versions of the Kondo effect in non-magnetic contexts especially (ISBN 0-7484-0889-4)
• Correlated electrons in δ-plutonium within a dynamical mean-field picture, Nature 410, 793 (2001). Nature article exploring the links of the Kondo effect and plutonium

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)