Strongly correlated material

Short description: Materials with electrical properties that cannot be explained by non-interacting entities

The perovskite structure of BSCCO, a high-temperature superconductor and a strongly correlated material.

Strongly correlated materials are a wide class of compounds that include insulators and electronic materials, and show unusual (often technologically useful) electronic and magnetic properties, such as metal-insulator transitions, heavy fermion behavior, half-metallicity, and spin-charge separation. The essential feature that defines these materials is that the behavior of their electrons or spinons cannot be described effectively in terms of non-interacting entities.[1] Theoretical models of the electronic (fermionic) structure of strongly correlated materials must include electronic (fermionic) correlation to be accurate. As of recently, the label quantum materials is also used to refer to strongly correlated materials, among others.

## Contents

* 1 Transition metal oxides
* 2 Electronic structures
* 3 Theories
* 4 Structural studies
* 5 Applications
* 6 See also
* 7 References
* 8 Further reading
* 9 External links

## Transition metal oxides

Many transition metal oxides belong to this class[2] which may be subdivided according to their behavior, e.g. high-Tc, spintronic materials, multiferroics, Mott insulators, spin Peierls materials, heavy fermion materials, quasi-low-dimensional materials, etc. The single most intensively studied effect is probably high-temperature superconductivity in doped cuprates, e.g. La2−xSrxCuO4. Other ordering or magnetic phenomena and temperature-induced phase transitions in many transition-metal oxides are also gathered under the term "strongly correlated materials."

## Electronic structures

Typically, strongly correlated materials have incompletely filled d\- or f-electron shells with narrow energy bands. One can no longer consider any electron in the material as being in a "sea" of the averaged motion of the others (also known as mean field theory). Each single electron has a complex influence on its neighbors.

The term strong correlation refers to behavior of electrons in solids that is not well-described (often not even in a qualitatively correct manner) by simple one-electron theories such as the local-density approximation (LDA) of density-functional theory or Hartree–Fock theory. For instance, the seemingly simple material NiO has a partially filled 3d band (the Ni atom has 8 of 10 possible 3d-electrons) and therefore would be expected to be a good conductor. However, strong Coulomb repulsion (a correlation effect) between d electrons makes NiO instead a wide-band gap insulator. Thus, strongly correlated materials have electronic structures that are neither simply free-electron-like nor completely ionic, but a mixture of both.

## Theories

Extensions to the LDA (LDA+U, GGA, SIC, GW, etc.) as well as simplified models Hamiltonians (e.g. Hubbard-like models) have been proposed and developed in order to describe phenomena that are due to strong electron correlation. Among them, dynamical mean field theory (DMFT) successfully captures the main features of correlated materials. Schemes that use both LDA and DMFT explain many experimental results in the field of correlated electrons.

## Structural studies

Experimentally, optical spectroscopy, high-energy electron spectroscopies, resonant photoemission, and more recently resonant inelastic (hard and soft) X-ray scattering (RIXS) and neutron spectroscopy have been used to study the electronic and magnetic structure of strongly correlated materials. Spectral signatures seen by these techniques that are not explained by one-electron density of states are often related to strong correlation effects. The experimentally obtained spectra can be compared to predictions of certain models or may be used to establish constraints on the parameter sets. One has for instance established a classification scheme of transition metal oxides within the so-called Zaanen–Sawatzky–Allen diagram.[3]

## Applications

The manipulation and use of correlated phenomena has applications like superconducting magnets and in magnetic storage (CMR) technologies. Other phenomena like the metal-insulator transition in VO2 have been explored as a means to make smart windows to reduce the heating/cooling requirements of a room.[4] Furthermore, metal-insulator transitions in Mott insulating materials like LaTiO3 can be tuned through adjustments in band filling to potentially be used to make transistors that would use conventional field effect transistor configurations to take advantage of the material's sharp change in conductivity.[5] Transistors using metal-insulator transitions in Mott insulators are often referred to as Mott transistors, and have been successfully fabricated using VO2 before, but they have required the larger electric fields induced by ionic liquids as a gate material to operate.[6]

## See also

* Electronic correlation
* Emergent behavior

## References

1. ↑ Quintanilla, Jorge; Hooley, Chris (2009). "The strong-correlations puzzle". Physics World (IOP Publishing) 22 (6): 32–37. doi:10.1088/2058-7058/22/06/38. ISSN 0953-8585. Bibcode: 2009PhyW...22f..32Q. http://purl.org/net/epubs/manifestation/3667/2009-Quintanilla-Hooley.pdf.
2. ↑ Millis, A. J.. "Lecture notes on "Strongly Correlated" Transition Metal Oxides". Columbia University. http://www.phys.columbia.edu/~millis/Cargeselecturenotes.pdf.
3. ↑ J. Zaanen; G. A. Sawatzky; J. W. Allen (1985). "Band Gaps and Electronic Structure of Transition-Metal Compounds". Physical Review Letters 55 (4): 418–421. doi:10.1103/PhysRevLett.55.418. PMID 10032345. Bibcode: 1985PhRvL..55..418Z. http://www.lorentz.leidenuniv.nl/~jan/zsa.pdf.
4. ↑ J. M. Tomczak; S. Biermann (2009). "Optical properties of correlated materials – Or why intelligent windows may look dirty". Physica Status Solidi B 246 (9): 1996–2005. doi:10.1002/pssb.200945231. Bibcode: 2009PSSBR.246.1996T.
5. ↑ Scheiderer, Philipp; Schmitt, Matthias; Gabel, Judith; Zapf, Michael; Stübinger, Martin; Schütz, Philipp; Dudy, Lenart; Schlueter, Christoph et al. (2018). "Tailoring Materials for Mottronics: Excess Oxygen Doping of a Prototypical Mott Insulator". Advanced Materials 30 (25). doi:10.1002/adma.201706708. PMID 29732633. Bibcode: 2018AdM....3006708S.
6. ↑ Nakano, M.; Shibuya, K.; Okuyama, D.; Hatano, T.; Ono, S.; Kawasaki, M.; Iwasa, Y.; Tokura, Y. (July 2012). "Collective bulk carrier delocalization driven by electrostatic surface charge accumulation". Nature 487 (7408): 459–462. doi:10.1038/nature11296. PMID 22837001. Bibcode: 2012Natur.487..459N.

## Further reading

* Anisimov, Vladimir; Yuri Izyumov (2010). Electronic Structure of Strongly Correlated Materials. Springer. ISBN 978-3-642-04825-8.
* Patrik Fazekas (1999). Lecture Notes on Electron Correlation and Magnetism. World Scientific. ISBN 978-981-02-2474-5.
* de Groot, Frank; Akio Kotani (2008). Core Level Spectroscopy of Solids. CRC Press. ISBN 978-0-8493-9071-5.
* Yamada, Kosaku (2004). Electron Correlations in Metals. Cambridge University Press. ISBN 978-0-521-57232-3.
* Robert Z. Bachrach, ed (1992). Synchrotron radiation research: advances in surface and interface science. Plenum Press. ISBN 978-0-306-43872-1.
* Pavarini, Eva; Koch Erik; Vollhardt, Dieter; Lichtenstein, Alexander; (eds.) (2011). The LDA+DMFT approach to strongly correlated materials. Forschungszentrum Jülich. ISBN 978-3-89336-734-4. http://www.cond-mat.de/events/correl11/manuscript.
* Amusia, M., Popov, K., Shaginyan, V., Stephanovich, V. (2014). Theory of Heavy-Fermion Compounds - Theory of Strongly Correlated Fermi-Systems. Springer Series in Solid-State Sciences. 182. Springer. doi:10.1007/978-3-319-10825-4. ISBN 978-3-319-10825-4.

## External links

* Quintanilla, Jorge; Hooley, Chris (June 2009). "The strong-correlations puzzle". Physics World 22 (6): 32–37. doi:10.1088/2058-7058/22/06/38. Bibcode: 2009PhyW...22f..32Q. http://purl.org/net/epubs/manifestation/3667/2009-Quintanilla-Hooley.pdf.

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