Achievements of Professor Jun Kondo

Professor Kondo began his research in the late 1950ls, when the band theory was already established as the basis of semiconductor electronics and the long-standing mystery of superconductivity was about to be resolved by BCS theory. The success of the BCS theory accelerated a trend in solid-state physics for tackling difficult many-body problems. In his thesis Professor Kondo studied a prominent problem in magnetism. He succeeded in clarifying the microscopic mechanism of super-exchange interaction, and opened the way to systematic study of magnetism in metallic oxides. In 1963 he moved to the Electrotechnical Laboratory, one of ancestors of the present AIST, and within a year he announced his solution of the resistance minimum problem. The resistivity of metals normally decreases as the temperature is lowered, but for certain metals it increases at low temperatures. This further low-temperature problem had eluded explanation since the 1930ls. Professor Kondo subsequently explained various anomalies accompanying the resistance minimum. These phenomena are together known as the Kondo effect. The Kondo effect proved to be a basic property not only of magnetic alloys but also of diverse many-body systems; it triggered considerable progress in the study of many-body problems in a wide range of research fields. It has recently re-emerged as an important concept in present research. It will remain a genuine landmark in the physical science of many-body systems.

1. Discovery of the Kondo effect

Professor Kondo found that the scattering probability of an electron from a localized magnetic spin within the metal exhibits anomalous behavior. It increases logarithmically with the inverse temperature, due to contributions from third-order perturbation theory in which the dynamical nature of the spin was first taken into account. In combination with the scattering probability due to phonons, which decreases with lowering temperature, this behavior gives rise to a minimum in the total scattering rate as the temperature varies. The low-temperature variation of the resistivity is accurately reproduced. This work also explained the anomalously large thermopower of materials having a minimum resistance, and predicted anomalies in the spin susceptibility and specific heat. These were then verified experimentally. All of these anomalies are now called the Kondo effect.

The Kondo theory triggered many theoretical and experimental studies, which indicated that the Kondo effect is not simply a property of dilute magnetic alloys but a fundamental property of many-body systems, relevant to many areas of physical science including particle physics. The first issue was how to reconcile the logarithmic divergence as absolute zero is approached with the finite value observed. This problem was tackled by leading theorists, who thereby generated powerful theoretical methods and discovered interesting new physics. They found that at very low temperatures a bound state is formed between the localized spin and conduction electrons, which causes the spin to vanish and suppresses the logarithmic divergence. It also means that the resulting theory clarifies the origin of the localized spin in the metal at the microscopic level. In a broad sense the whole phenomena accompanying the vanishing of the localized spin with decreasing temperature is called the Kondo effect. New physics that emerged from this field of research includes infra-red divergence of the X-ray absorption edge, Andersonls orthogonality theorem, and low-energy excitation properties of many-body systems. Confinement of quarks by gluons in particle physics is now known to take place through a Kondo effect. Professor Kondo himself predicted that a logarithmic anomaly exists in the diffusion constant of muons in metals and in the resistivity of amorphous metals. He showed that these anomalies are all due to a diverging response to low-energy excitations of an electronic system having a Fermi surface. He therefore refers to all such anomalies, including the Kondo effect, as a Fermi-surface effect.

The Kondo effect is central to the study of a group of materials called high-density Kondo systems, or the Kondo lattice. In these materials the Kondo effect appears in the high temperature region; at low temperatures the electronic system behaves like heavy electrons taking ordered states, giving rise to anisotropic superconductivity, antiferromagnetism, ferromagnetism, electric multipole state and other phenomena. These are now research priority topics. Kondo materials are also studied for their potential as electronic cooling materials, because of their high thermopower values. It was a great surprise when the Kondo effect was recently observed very clearly in tunneling conductivity in quantum-dot systems. This work is in the forefront of nanotechnology, so that the Kondo effect is important in this important new research front.

According to ISI the name KONDO has been cited in about 4300 papers since 1980, even though the Kondo paper was published in 1964 and cited most often in the immediately following years.

2. Microscopic Mechanism of the Super-Exchange Interaction

In halides and oxides of transition metals, such as manganese oxide, a magnetic interaction occurs between two localized spins residing in those metallic atoms which straddle the oxygen or halogen atom. This interaction was first observed in the 1930ls and is called the super-exchange interaction. Several microscopic mechanisms were proposed. Professor Kondo proposed a theory by which virtual electronic excitation processes give rise to the interaction; this was finally proved to be the correct mechanism. It is often called Andersonls super-exchange interaction since Andersonls review of it is well known, but in that review Anderson states that Kondols mechanism is the appropriate one. Diverse magnetic phenomena in transition metal oxides have been clarified through this interaction. The super-exchange interaction determines the basic magnetic properties of the undoped states of high-temperature cuprate superconductors and manganese oxides having colossal magnetoresistance. This idea is also indispensable in the microscopic theory of high-temperature superconductivity and colossal magnetoresistance.

The Kondo effect and super-exchange interaction are both standard fundamental concepts familiar to every condensed-matter researcher. Professor Kondo has made valuable contributions in other areas including the theory of the anomalous Hall effect, two-band superconductors, the doped hole distribution in cuprate high-temperature superconductors and so on. He has made continued fundamental advances in the physics of many-body systems, and especially in the study of solid-state magnetic phenomena.