Mott insulator

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Mott insulators are a class of materials that are expected to conduct electricity as predicted by conventional band theories, but these compounds turn out to be insulators (particularly at low temperatures). These insulators failed to be correctly described by band theories of solids due to strong electron–electron interactions, which are not considered in conventional band theory. Mott transition is a transition from a metal to an insulator, driven by the strong interactions between electrons.[1] One of the simplest models that can capture Mott transition is Hubbard model.

The bandgap in a Mott insulator exists between bands of like character, such as 3d character, whereas the bandgap in charge-transfer insulators exists between anion and cation states,[2] such as between O 2p and Ni 3d bands in NiO.[3]

History

Although the band theory of solids had been very successful in describing various electrical properties of materials, in 1937 Jan Hendrik de Boer and Evert Johannes Willem Verwey pointed out that a variety of transition metal oxides predicted to be conductors by band theory are insulators.[4] With an odd number of electrons per unit cell, the valence band is only partially filled, so the Fermi level lies within the band. From the band theory, this implies that such a material has to be a metal. This conclusion fails for several cases, such as, e.g., CoO, one of the strongest insulators known.[1]

Nevill Mott and Rudolf Peierls also in 1937 predicted the failing of band theory can be explained by including interactions between electrons.[5]

In 1949, in particular, Mott proposed a model for NiO as an insulator, where conduction is based on the formula[6]

(Ni2+O2−)2 → Ni3+O2− + Ni1+O2−.

In this situation, the formation of an energy gap preventing conduction can be understood as the competition between the Coulomb potential U between 3d electrons and the transfer integral t of 3d electrons between neighboring atoms (the transfer integral is a part of the tight binding approximation). The total energy gap is then

Egap = U − 2zt,

where z is the number of nearest-neighbor atoms.

In general, Mott insulators occur when the repulsive Coulomb potential U is large enough to create an energy gap. One of the simplest theories of Mott insulators is the 1963 Hubbard model. The crossover from a metal to a Mott insulator as U is increased, can be predicted within the so-called dynamical mean field theory.

Mottness

Mottism denotes the additional ingredient, aside from antiferromagnetic ordering, which is necessary to fully describe a Mott Insulator. In other words, we might write: antiferromagnetic order + mottism = Mott insulator.

Thus, mottism accounts for all of the properties of Mott insulators that cannot be attributed simply to antiferromagnetism.

There are a number of properties of Mott insulators, derived from both experimental and theoretical observations, which cannot be attributed to antiferromagnetic ordering and thus constitute mottism. These properties include:

• Spectral weight transfer on the Mott scale[7][8]
• Vanishing of the single particle Green function along a connected surface in momentum space in the first Brillouin zone[9]
• Two sign changes of the Hall coefficient as electron doping goes from ${\displaystyle n=0}$ to ${\displaystyle n=2}$ (band insulators have only one sign change at ${\displaystyle n=1}$)
• The presence of a charge ${\displaystyle 2e}$ (with ${\displaystyle e<0}$ the charge of an electron) boson at low energies[10][11]
• A pseudogap away from half-filling (${\displaystyle n=1}$)[12]

Applications

Mott insulators are of growing interest in advanced physics research, and are not yet fully understood. They have applications in thin-film magnetic heterostructures and the strong correlated phenomena in high-temperature superconductivity, for example.[13][14][15][16]

This kind of insulator can become a conductor by changing some parameters, which may be composition, pressure, strain, voltage, or magnetic field. The effect is known as a Mott transition and can be used to build smaller field-effect transistors, switches and memory devices than possible with conventional materials.[17][18][19]

Notes

1. ^ a b Fazekas, Patrik (2008). Lecture notes on electron correlation and magnetism. World Scientific. pp. 147–150. ISBN 978-981-02-2474-5. OCLC 633481726.
2. ^ lecture slides
3. ^ P. Kuiper; G. Gruizinga; J. Ghijsen; G.A. Sawatzky; H. Verweij (1987). "Character of Holes in LixNi1−xO2". Physical Review Letters. 62 (2): 221–224. Bibcode:1989PhRvL..62..221K. doi:10.1103/PhysRevLett.62.221. PMID 10039954.
4. ^ de Boer, J. H.; Verwey, E. J. W. (1937). "Semi-conductors with partially and with completely filled 3d-lattice bands". Proceedings of the Physical Society. 49 (4S): 59. Bibcode:1937PPS....49...59B. doi:10.1088/0959-5309/49/4S/307.
5. ^ Mott, N. F.; Peierls, R. (1937). "Discussion of the paper by de Boer and Verwey". Proceedings of the Physical Society. 49 (4S): 72. Bibcode:1937PPS....49...72M. doi:10.1088/0959-5309/49/4S/308.
6. ^ Mott, N. F. (1949). "The basis of the electron theory of metals, with special reference to the transition metals". Proceedings of the Physical Society. Series A. 62 (7): 416–422. Bibcode:1949PPSA...62..416M. doi:10.1088/0370-1298/62/7/303.
7. ^ Phillips, Philip (2006). "Mottness". Annals of Physics. Elsevier BV. 321 (7): 1634–1650. arXiv:cond-mat/0702348. Bibcode:2006AnPhy.321.1634P. doi:10.1016/j.aop.2006.04.003. ISSN 0003-4916.
8. ^ Meinders, M. B. J.; Eskes, H.; Sawatzky, G. A. (1993-08-01). "Spectral-weight transfer: Breakdown of low-energy-scale sum rules in correlated systems". Physical Review B. American Physical Society (APS). 48 (6): 3916–3926. Bibcode:1993PhRvB..48.3916M. doi:10.1103/physrevb.48.3916. ISSN 0163-1829. PMID 10008840.
9. ^ Stanescu, Tudor D.; Phillips, Philip; Choy, Ting-Pong (2007-03-06). "Theory of the Luttinger surface in doped Mott insulators". Physical Review B. American Physical Society (APS). 75 (10): 104503. arXiv:cond-mat/0602280. Bibcode:2007PhRvB..75j4503S. doi:10.1103/physrevb.75.104503. ISSN 1098-0121.
10. ^ Leigh, Robert G.; Phillips, Philip; Choy, Ting-Pong (2007-07-25). "Hidden Charge 2e Boson in Doped Mott Insulators". Physical Review Letters. 99 (4): 046404. arXiv:cond-mat/0612130v3. Bibcode:2007PhRvL..99d6404L. doi:10.1103/physrevlett.99.046404. ISSN 0031-9007. PMID 17678382.
11. ^ Choy, Ting-Pong; Leigh, Robert G.; Phillips, Philip; Powell, Philip D. (2008-01-17). "Exact integration of the high energy scale in doped Mott insulators". Physical Review B. American Physical Society (APS). 77 (1): 014512. arXiv:0707.1554. Bibcode:2008PhRvB..77a4512C. doi:10.1103/physrevb.77.014512. ISSN 1098-0121.
12. ^ Stanescu, Tudor D.; Phillips, Philip (2003-07-02). "Pseudogap in Doped Mott Insulators is the Near-Neighbor Analogue of the Mott Gap". Physical Review Letters. 91 (1): 017002. arXiv:cond-mat/0209118. Bibcode:2003PhRvL..91a7002S. doi:10.1103/physrevlett.91.017002. ISSN 0031-9007. PMID 12906566.
13. ^ Kohsaka, Y.; Taylor, C.; Wahl, P.; et al. (August 28, 2008). "How Cooper pairs vanish approaching the Mott insulator in Bi2Sr2CaCu2O8+δ". Nature. 454 (7208): 1072–1078. arXiv:0808.3816. Bibcode:2008Natur.454.1072K. doi:10.1038/nature07243. PMID 18756248. S2CID 205214473.
14. ^ Markiewicz, R. S.; Hasan, M. Z.; Bansil, A. (2008-03-25). "Acoustic plasmons and doping evolution of Mott physics in resonant inelastic x-ray scattering from cuprate superconductors". Physical Review B. 77 (9): 094518. Bibcode:2008PhRvB..77i4518M. doi:10.1103/PhysRevB.77.094518.
15. ^ Hasan, M. Z.; Isaacs, E. D.; Shen, Z.-X.; Miller, L. L.; Tsutsui, K.; Tohyama, T.; Maekawa, S. (2000-06-09). "Electronic Structure of Mott Insulators Studied by Inelastic X-ray Scattering". Science. 288 (5472): 1811–1814. arXiv:cond-mat/0102489. Bibcode:2000Sci...288.1811H. doi:10.1126/science.288.5472.1811. ISSN 0036-8075. PMID 10846160. S2CID 2581764.
16. ^ Hasan, M. Z.; Montano, P. A.; Isaacs, E. D.; Shen, Z.-X.; Eisaki, H.; Sinha, S. K.; Islam, Z.; Motoyama, N.; Uchida, S. (2002-04-16). "Momentum-Resolved Charge Excitations in a Prototype One-Dimensional Mott Insulator". Physical Review Letters. 88 (17): 177403. arXiv:cond-mat/0102485. Bibcode:2002PhRvL..88q7403H. doi:10.1103/PhysRevLett.88.177403. PMID 12005784. S2CID 30809135.
17. ^ Newns, Dennis (2000). "Junction mott transition field effect transistor (JMTFET) and switch for logic and memory applications". http://www.google.com/patents/US6121642
18. ^ Zhou, You; Ramanathan, Shriram (2013-01-01). "Correlated Electron Materials and Field Effect Transistors for Logic: A Review". Critical Reviews in Solid State and Materials Sciences. 38 (4): 286–317. arXiv:1212.2684. Bibcode:2013CRSSM..38..286Z. doi:10.1080/10408436.2012.719131. ISSN 1040-8436. S2CID 93921400.
19. ^ Son, Junwoo; et al. (2011-10-18). "A heterojunction modulation-doped Mott transistor". Applied Physics Letters. 110 (8): 084503–084503–4. arXiv:1109.5299. Bibcode:2011JAP...110h4503S. doi:10.1063/1.3651612. S2CID 27583830.