Metal-insulator transitions are accompanied by huge resistivity changes, even over tens of orders of magnitude, and are widely observed in condensed-matter systems. This article presents the observations and current understanding of the metal-insulator transition with a pedagogical introduction to the subject. Especially important are the transitions driven by correlation effects associated with the electron-electron interaction. The insulating phase caused by the correlation effects is categorized as the Mott Insulator. Near the transition point the metallic state shows fluctuations and orderings in the spin, charge, and orbital degrees of freedom. The properties of these metals are frequently quite different from those of ordinary metals, as measured by transport, optical, and magnetic probes. The review first describes theoretical approaches to the unusual metallic states and to the metal-insulator transition. The Fermi-liquid theory treats the correlations that can be adiabatically connected with the noninteracting picture. Strong-coupling models that do not require Fermi-liquid behavior have also been developed. Much work has also been done on the scaling theory of the transition. A central issue for this review is the evaluation of these approaches in simple theoretical systems such as the Hubbard model and $t\ensuremath{-}J$ models. Another key issue is strong competition among various orderings as in the interplay of spin and orbital fluctuations. Experimentally, the unusual properties of the metallic state near the insulating transition have been most extensively studied in $d$-electron systems. In particular, there is revived interest in transition-metal oxides, motivated by the epoch-making findings of high-temperature superconductivity in cuprates and colossal magnetoresistance in manganites. The article reviews the rich phenomena of anomalous metallicity, taking as examples Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Ru compounds. The diverse phenomena include strong spin and orbital fluctuations, mass renormalization effects, incoherence of charge dynamics, and phase transitions under control of key parameters such as band filling, bandwidth, and dimensionality. These parameters are experimentally varied by doping, pressure, chemical composition, and magnetic fields. Much of the observed behavior can be described by the current theory. Open questions and future problems are also extracted from comparison between experimental results and theoretical achievements.

Metal-insulator transitions

Al environment in tectosilicate and peraluminous glasses: A 27Al MQ-MAS NMR, Raman, and XANES investigation

The evaluation of magnetic hyperfine field distributions from 57Fe Mossbauer spectra in systems with both magnetic dipole and electric quadrupole interactions is considered. This problem is treated in higher order perturbation theory, and the case of general magnetic texture and isotropic electric quadrupole texture is treated in full. Simple expressions are given enabling the calculation of all spectrum line positions, intensities and widths. It is shown that in cases where first order perturbation treatments yield low-field artifacts in the magnetic hyperfine distribution, this procedure yields much more reliable results. It is also shown that the calculated magnetic spin texture is much more reliable. The asymmetry effects from the electric quadrupole interaction and from a correlation between this and a dipole magnetic field are considered.

Improving the validity of hyperfine field distributions from magnetic alloys: Part I: Unpolarized source☆

Mott insulators are identified here with ordinary magnetic insulators. The insulating gap, local moment, and effective spin hamiltonian aspects are qualitatively explained by means of a novel set of solutions of the Hartree-Fock equations. The apparent conflict between Bloch's theorem and localized-electron phenomenology is thereby resolved in an elementary manner. This Hartree-Fock approach also sheds considerable light on the physical mechanisms responsible for the associated metal-insulator (Mott) and other related phase transitions, as observed in V2O3 and several other materials. With some generalizations and refinements, this theoretical picture is shown to also account semiquantitatively for a number of detailed properties of NiO and CoO, two of the most extensively studied Mott insulator materials. A wide variety of experimental data for NiO is surveyed in order to determine reasonable values for its effective Hubbard hamiltonian parameters, suitably generalized for the 3d electrons. The ...

Electronic structure of Mott insulators

A review is given of the physical properties, composition and crystal structure of intermetallic compounds formed between rare-earth elements and non-magnetic metals, with emphasis on the magnetic properties. Included are the properties of compounds in which the non-rare-earth component is a 4d or 5d transition element. Special consideration is given to the properties of pseudo-binary compounds. Results of magnetisation measurements, neutron diffraction and neutron scattering are discussed together with results derived from NMR, ESR and Mossbauer effect spectroscopy. An evaluation is given of the relevance of the experimental results with respect to different types of exchange interactions in this class of intermetallics. Special consideration is also given to the influence of crystal field effects on the magnetic properties and, furthermore, to the occurrence of intermediate valences in several of the compounds of Ce, Sm, Eu and Yb.

https://iopscience.iop.org/article/10.1088/0034-4885/42/8/003/pdf

Intermetallic compounds of rare earths and non-magnetic metals

Hyperfine interactions in intermetallic compounds between Gd and 3d transition metals

The compounds NiSx (1.91 or approximately=2.0. The transition to the paramagnetic state occurs at a temperature which increases roughly proportional to x. The presence of at least two sites characterized by different hyperfine fields indicates the existence of some inhomogeneity in the antiferromagnetic phase for T>31K.

Electronic and magnetic properties of the pyrite-structure compound NiS2: influence of vacancies and copper impurities

Hyperfine interactions at 155Gd nuclei in alloys GdxCe1−xRu2 with 0.05 ≦ x ≦ 0.5 are investigated by Mossbauer spectroscopy. For x ≧ 0.11 a linear variation of the magnetic ordering temperature TM with x is found. The temperature dependence of the magnetic hyperfine fields below TM indicates a gradual reduction of the extension of ferromagnetic correlations with decreasing Gd concentration. For dilute alloys (0.11 ≦ x ≦ 0.18), the temperature and field dependence of the hyperfine field is characterized by nearly critical behaviour over a large range of temperatures.



Hyperfeinwechselwirkungen an 155Gd-Kernen werden in Legierungen GdxCe1−xRu2 mit 0,05 ≦ x ≦ 0,5 durch Mossbauerspektroskopie gemessen. Fur x ≧ 0,11 wird eine lineare Zunahme der magnetischen Ordnungstemperatur mit x gefunden. Die Temperaturabhangigkeit der magnetischen Hyperfeinfelder fur T < TM weist auf eine graduelle Abnahme der Ausdehnung ferromagnetischer Korrelationen mit abnehmender Gd-Konzentration hin. Fur verdunnte Legierungen (0,11 ≦ x ≦ 0,18) ist die Temperatur- und Feldabhangigkeit des Hyperfeinfeldes durch fast kritisches Verhalten uber einen grosen Temperaturbereich charakterisiert.

An investigation of magnetic ordering in GdxCe1-xRu2 by155Gd mössbauer spectroscopy

/pdf/investigation-of-magnetic-structures-and-phase-transitions-2tlv2574wq.pdf

INVESTIGATION OF MAGNETIC STRUCTURES AND PHASE TRANSITIONS IN NiS2 BY 61Ni-MÖSSBAUER SPECTROSCOPY

We discuss the physical properties of mixed compounds NiS2-xSex. In this system for x ~ 0.45 an insulator-metal transition occurs which is interpreted in the sense of the model of Mott and Hubbard. Our experimental results (susceptibility, N.M.R. with 77Se, Mossbauer spectroscopy with 61Ni and neutron diffraction) demonstrate the existence of an antiferromagnetic metallic phase (AFM) in the range of concentrations 0.45 ⩽ x ⩽ 1. The magnetic properties of the AFM phase are discussed in terms of a model which implies a strong influence of the local environment on the magnetic state of nickel atoms.

/pdf/physical-properties-of-compounds-nis2-xsex-with-pyrite-296h8lgyp0.pdf

PHYSICAL PROPERTIES OF COMPOUNDS NiS2-xSex WITH PYRITE STRUCTURE : METAL-NON METAL TRANSITION, EVIDENCE FOR THE EXISTENCE OF AN ANTIFERROMAGNETIC METALLIC PHASE

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