In solid-state physics

Solid-state physics

Solid-state physics is the study of rigid matter, or solids, through methods such as quantum mechanics, crystallography, electromagnetism, and metallurgy. It is the largest branch of condensed matter physics. Solid-state physics studies how the large-scale properties of solid materials result from...

, Kondo

Jun Kondo

Jun Kondo is a theoretical physicist in Japan. His research is famous for the Kondo effect.- Honours and appointments :*1959 Dr...

 insulators (also referred as Kondo semiconductors and heavy fermion

Heavy Fermion

In solid-state physics, heavy fermion materials are a specific type of intermetallic compound, containing elements with 4f or 5f electrons. Electrons, a kind of fermion, found in such materials are sometimes referred to as heavy electrons...

 semiconductors) are understood as materials with strongly correlated electrons, that open up a narrow band gap

Band gap

In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...

 (in the order of 10 meV) at low temperatures with the chemical potential

Chemical potential

Chemical potential, symbolized by μ, is a measure first described by the American engineer, chemist and mathematical physicist Josiah Willard Gibbs. It is the potential that a substance has to produce in order to alter a system...

 lying in the gap, whereas in heavy fermions the chemical potential is located in the conduction band. The band gap opens up at low temperatures due to hybridization

Orbital hybridisation

In chemistry, hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part...

 of localized electrons (mostly f-electrons) with conduction electrons, a correlation effect known as Kondo effect

Kondo effect

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

. As a consequence, a transition from metallic behavior to insulating behavior is seen in resistivity measurements. The band gap could be either direct or indirect. Most studied Kondo insulators are FeSi, Ce₃Bi₄Pt₃, SmB₆, YbB₁₂, and CeNiSn.

Historical overview

In 1969, Menth et al. found no magnetic ordering in SmB₆ down to 3.5 K and a change from metallic to insulating behavior in the resistivity measurement with decreasing temperature. They interpreted this phenomenon as a change of the electronic configuration of Sm.

Gabriel Aeppli and Zachary Fisk found a descriptive way to explain the physical properties of Ce₃Bi₄Pt₃ and CeNiSn in 1992. They called the materials Kondo insulators, showing Kondo lattice behavior near room temperature, but becoming semiconducting with very small energy gaps (a few Kelvin to a few tens of Kelvin) when decreasing the temperature.

Transport properties

At high temperatures the localized f-electrons form independent local magnetic moments. According to the Kondo effect, the dc-resistivity of Kondo insulators shows a logarithmic temperature-dependence. At low temperatures, the local magnetic moments are screened by the sea of conduction electrons, forming a so-called Kondo resonance. The interaction of the conduction band with the f-orbitals results in a hybridization and an energy gap . If the chemical potential lies in the hybridization gap, an insulating behavior can be seen in the dc-resistivity at low temperatures.