Professor Kondo started research activities later in 1950's, when the band theory has already established the basis of semiconductor electronics
and a long-standing mystery of superconductivity was being solved by the BCS theory. The success of the latter theory was encouraging a trend
for tackling difficult many-body problems in solid-state physics. In his thesis work Professor Kondo challenged a remaining problem in magnetism.
He succeeded in clarifying the microscopic mechanism of super-exchange interaction and opened a mainstream path for studying the magnetism
in metallic oxides. In 1964, i. e., one year after he moved to Electrotechnical Laboratory, which is an institute which flowed into the present-day AIST,
he published a further prominent achievement, i. e., solution of the resistance minimum problem. Normally the resistivity of metals decreases with
lowering temperature but that of some turns to increase at low temperatures. Its explanation had been a difficult but fascinating problem in
low-temperature physics for a long time, together with superconductivity, since 1930fs. Subsequently he clarified various anomalies accompanying
the resistance minimum. All these effects as a whole were nicknamed gKondo effect.h The gKondo effecth proved to be a basic property not only
of magnetic alloys but also of various many-body systems; it triggered a far-reaching progress in the study of many-body problems in wide-ranging
research fields. Further, in the present-day frontier of research it has reemerged as an important fundamental concept. In the future as well it will be
a brilliant landmark in the physical science of many-body problems.
1. Discovery of the gKondo effecth
Professor Kondo found that the scattering probability of an electron from a localized magnetic spin in the metal takes an extremely anomalous behavior
increasing with lowering temperature as a logarithmic function of inverse temperature. The addition of it with the scattering probability due to phonons
which simply decreases with lowering temperature was shown to lead to a minimum in the total scattering rate as a function of temperature and to
marvelously reproduce the temperature dependence of the low-temperature resistivity. In this way he solved the long-standing mystery. Immediately
he explained the anomalously large thermopower in the resistance-minimum metals and further predicted anomalies of spin susceptibility and specific
heat, which were verified subsequently. Since then all the above-mentioned anomalies are called gKondo effect.h
The Kondo theory triggered at once extremely numerous both theoretical and experimental studies, which revealed that the gKondo effecth is not
simply a property of dilute magnetic alloys but a most fundamental property shared generally by many-body systems including even elementary particle
systems, thus exerting a remarkable impact in the whole physics community. The first pressing problem was how the logarithmic divergence at absolute
zero of temperature is finally converted to a finite value in the close vicinity of absolute zero in various properties to be consistent with the observation.
This problem was attacked by many forefront theorists in the world, fruitfully resulting in the development of powerful new theoretical methods and the
discovery of many very exciting new physics as well as the finding that at very low temperatures a bound state is formed between the localized spin and
conduction electrons which makes the spin vanish and arrests the logarithmic divergence. Such a theory clarified the microscopic mechanism emergence
of the localized spin in the metal. In the wide definition the whole phenomenon accompanying the vanishing of the magnetic spin with decreasing
temperature is named gKondo effect.h Among above-mentioned many exciting new physics are the infra-red divergence of the X-ray absorption edge,
Andersonfs orthogonality theorem, subtle low-energy excitation properties of many-body systems and so on. Confinement of quarks by gluons in
particle physics was clarified to be a similar phenomenon as the gKondo effect.h Professor Kondo himself theoretically found that a logarithmic
anomaly appears in the diffusion constant of muons in metals and the resistivity in amorphous metals as well. He clarified that all these anomalies are
due to a divergent response to low-energy excitations of the electronic system having the Fermi surface. In this sense he calls the whole above-mentioned
anomalies, including the gKondo effect,h as gFermi-surface effect.h
The gKondo effecth is an absolutely necessary fundamental concept in the study of a group of materials called as high-density Kondo systems, or
Kondo lattice. In these materials the gKondo effecth appears in the high temperature region and in the low temperature region the electronic system
presents itself as heavy electrons which take such various exciting ordered states as anisotropic superconductivity, antiferromagnetism, ferromagnetism,
electric multipole state and so on and are giving rise to hot research activities. The Kondo materials are paid attention also due their high values of
thermopower for a potential electronic cooling material. It was a recent great surprise that the gKondo effecth was very clearly observed in tunneling
conductivity in quantum dot systems in the forefront of nanotechnology, indicating that the gKondo effecth is an indispensable concept also in this
developing research front.
According to ISI, the name KONDO has been cited in about 4300 papers since 1980, although the Kondo paper was published in 1964 and cited most
frequently in subsequent years.
2. Microscopic Mechanism of the Super-Exchange Interaction
In oxygenized or halogenized transition metals such as manganese oxide a magnetic interaction had been observed since 1930fs to work between
two localized spins residing in metallic atoms which straddles oxygen or halogen atom. This interaction is called super-exchange interaction. Several
theories on its microscopic mechanism were proposed. Professor Kondo proposed a theory in which virtual electronic excitation processes gives rise
to this interaction, which finally proved to be the right one. This mechanism is often called Andersonfs super-exchange interaction since his review
paper on this interaction is well known, but it is in this review that he judged that the Kondo mechanism is the right one. Colorful magnetic phenomena
in transition metal oxides were clarified on the basis of this interaction. Since this interaction determines the basic magnetic properties of the undoped
states of high-temperature cuprate superconductors and manganese oxides showing colossal magnetoreistance, it is an indispensable basic concept
for microscopically clarifying high-temperature superconductivity and colossal magnetoresitance.
Both Kondo effect and super-exchange interaction are standard fundamental concepts that any condensed-matter researcher fails to know. Besides
these two, Professor Kondo presented results of high reputation such as theory of anomalous Hall effect, theory of two-band superconductors,
theory of doped hole distribution in cuprate high-temperature superconductors and so on. Thus, Professor Kondo discovered several monumental
fundamental concepts in many-body systems. In particular, he performed outstanding contributions in constructing the basis of condensed-matter
physics related with magnetism.