View source for Physics:Kondo insulator

[[File:Unhybridized.png|thumb|Dispersion relation of conduction band and localized states.]]
[[File:Hybridization.png|thumb|Hybridization and forming of an indirect energy (hybridization) gap due to the coherent Kondo screening of the local moments by the sea of conduction electrons.]]
[[File:Kondo insulator.png|thumb|In case of Kondo insulators the Fermi level (chemical potential) is located in the hybridization gap.]]

In [[Physics:Solid-state physics|solid-state physics]], '''Kondo [[Physics:Insulator (electricity)|insulators]]''' (also referred as '''Kondo semiconductors''' and '''heavy fermion semiconductors''') are understood as materials with strongly correlated electrons, that open up a narrow [[Physics:Band gap|band gap]] (in the order of 10 meV) at low temperatures with the [[Chemistry:Chemical potential|chemical potential]] lying in the gap, whereas in heavy fermion materials the chemical potential is located in the conduction band. The band gap opens up at low temperatures due to [[Physics:Orbital hybridisation|hybridization]] of localized electrons (mostly f-electrons) with conduction electrons, a correlation effect known as the [[Physics:Kondo effect|Kondo effect]]. As a consequence, a transition from metallic behavior to insulating behavior is seen in resistivity measurements. The band gap could be either [[Physics:Direct and indirect band gaps|direct or indirect]]. Most studied Kondo insulators are FeSi, Ce<sub>3</sub>Bi<sub>4</sub>Pt<sub>3</sub>, SmB<sub>6</sub>, YbB<sub>12</sub>, and CeNiSn, although there are over a dozen known Kondo insulators.<ref>{{Cite journal |last1=Dzero |first1=Maxim |last2=Xia |first2=Jing |last3=Galitski |first3=Victor |last4=Coleman |first4=Piers |date=2016-03-10 |title=Topological Kondo Insulators |url=https://www.annualreviews.org/doi/10.1146/annurev-conmatphys-031214-014749 |journal=Annual Review of Condensed Matter Physics |volume=7 |issue=1 |pages=249–280 |doi=10.1146/annurev-conmatphys-031214-014749 |arxiv=1506.05635 |bibcode=2016ARCMP...7..249D |s2cid=15794370 |issn=1947-5454}}</ref>

==Historical overview==
In 1969, Menth ''et al.'' found no magnetic ordering in [[Chemistry:Samarium hexaboride|SmB<sub>6</sub>]] down to 0.35 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.<ref name="Menth Buehler Geballe pp. 295–297">{{cite journal | last1=Menth | first1=A. | last2=Buehler | first2=E. | last3=Geballe | first3=T. H. | title=Magnetic and Semiconducting Properties of SmB<sub>6</sub> | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=22 | issue=7 | date=17 February 1969 | issn=0031-9007 | doi=10.1103/physrevlett.22.295 | pages=295–297| bibcode=1969PhRvL..22..295M }}</ref>

Gabriel Aeppli and Zachary Fisk found a descriptive way to explain the physical properties of Ce<sub>3</sub>Bi<sub>4</sub>Pt<sub>3</sub> 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.<ref>''Kondo Insulators'', G. Aeppli, Z. Fisk, 1992, Comments Cond. Mat. Phys. 16, 155-170</ref>

==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 <math> \epsilon_{\mathrm{g}} </math>. If the chemical potential lies in the hybridization gap, an insulating behavior can be seen in the dc-resistivity at low temperatures.

In recent times, [[Physics:Angle-resolved photoemission spectroscopy|angle-resolved photoemission spectroscopy]] experiments provided direct imaging of band-structure, hybridization and flat band topology in Kondo insulators and related compounds.<ref>{{Citation|last1=Hasan|first1=M. Zahid|title=Topological Insulators, Topological Dirac semimetals, Topological Crystalline Insulators, and Topological Kondo Insulators|date=2015|work=Topological Insulators|pages=55–100|publisher=John Wiley & Sons, Ltd|language=en|doi=10.1002/9783527681594.ch4|isbn=978-3-527-68159-4|last2=Xu|first2=Su-Yang|last3=Neupane|first3=Madhab}}</ref>

==References==
{{Reflist}}
* {{cite book |title=Handbook of magnetism and advanced magnetic materials |arxiv=cond-mat/0612006 |last1=Coleman |first1=P. |publisher=John Wiley & Sons |year=2007 |isbn=9780470022177 |location=Chichester, West Sussex, England |chapter=Heavy Fermions: Electrons at the edge of magnetism |bibcode=2006cond.mat.12006C |oclc=124165851}}
* {{cite journal|last=Riseborough |first=Peter S. | title=Heavy fermion semiconductors |s2cid=119991477  | journal=Advances in Physics | publisher=Informa UK Limited | volume=49 | issue=3 | year=2000 | issn=0001-8732 | doi=10.1080/000187300243345 | pages=257–320| bibcode=2000AdPhy..49..257R }}

[[Category:Correlated electrons]]
[[Category:Condensed matter physics]]

{{Sourceattribution|Kondo insulator}}