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

We present a review of the basic ideas and techniques of the spectral density functional theory which are currently used in electronic structure calculations of strongly{correlated materials where the one{electron description breaks down. We illustrate the method with several examples where interactions play a dominant role: systems near metal{insulator transition, systems near volume collapse transition, and systems with local moments.

/pdf/electronic-structure-calculations-with-dynamical-mean-field-244tl5lhyg.pdf

Electronic structure calculations with dynamical mean-field theory

The development of efficient and robust earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) is an ongoing challenge. We report metallic cobalt pyrite (cobalt disulfide, CoS2) as one such high-activity candidate material and demonstrate that its specific morphology—film, microwire, or nanowire, made available through controlled synthesis—plays a crucial role in determining its overall catalytic efficacy. The increase in effective electrode surface area that accompanies CoS2 micro- and nanostructuring substantially boosts its HER catalytic performance, with CoS2 nanowire electrodes achieving geometric current densities of −10 mA cm–2 at overpotentials as low as −145 mV vs the reversible hydrogen electrode. Moreover, micro- and nanostructuring of the CoS2 material has the synergistic effect of increasing its operational stability, cyclability, and maximum achievable rate of hydrogen generation by promoting the release of evolved gas bubbles from the electrode surface. The benefits of ca...

High-Performance Electrocatalysis Using Metallic Cobalt Pyrite (CoS2) Micro- and Nanostructures

Materials with correlated electrons exhibit some of the most intriguing phenomena in condensed matter physics. A new theoretical framework is now allowing theorists to calculate the electronic structure of these materials, which can exist in a rich variety of phases.

Strongly Correlated Materials: Insights From Dynamical Mean-Field Theory

Using first-principles band structure methods we studied the general chemical trends of defect formation in II-VI semiconductors. We systematically calculated the formation energies and transition energy levels of intrinsic and extrinsic defects and defect complexes in the prototype CdTe and investigated the limiting factors for p-type and n-type doping in this material. Possible approaches to significantly increase the doping limits are discussed. Our general understanding of the chemical trends of defect formation energies and transition energy levels in CdTe is expected to be applicable also to other II-VI semiconductors.

Chemical trends of defect formation and doping limit in II-VI semiconductors: The case of CdTe

We report Hall-effect measurements on single crystals of antiferromagnetic pyrite-type ${\mathrm{NiS}}_{2}$ (${\mathit{T}}_{\mathit{N}}$=40 K), in which the resistivity appears to show a metal-insulator transition at T\ensuremath{\approxeq}100 K. Our transport measurements demonstrate that this transition is not intrinsic to the bulk: At low temperatures the transport is dominated by a metalliclike conduction at the surface, with carrier density ${\mathit{n}}_{\mathit{s}}$\ensuremath{\approxeq}5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, and mobility ${\mathrm{\ensuremath{\mu}}}_{\mathit{s}}$\ensuremath{\approxeq}1.3 ${\mathrm{cm}}^{2}$/Vs at T=0. For Tg100 K the sample conductance is dominated by the bulk, which behaves like a doped semiconductor; the acceptorlike impurity states have binding energy ${\mathit{E}}_{\mathit{B}}$=80 meV. The valence-band mobility is \ensuremath{\mu}\ensuremath{\approxeq}0.002 ${\mathrm{cm}}^{2}$/Vs.

Hall effect and conductivity in pyrite NiS2.

In pyrite-structure antiferromagnetic NiS{sub 2}, the magnetic susceptibility {chi}({ital T}) shows downward curvature above the Neel transition ({ital T}{sub {ital N}1}=39.2 K), suggesting that magnetic correlations persist to {ital T}=400 K. However, neutron-scattering measurements indicate that the bulk magnetic behavior above {ital T}{sub {ital N}1} is conventional. We present evidence that the unusual temperature dependence of {chi}({ital T}) arises from a surface contribution. At {ital T}{sub {ital N}2}=29.75 K a transition occurs to a weakly ferromagnetic state, accompanied by a small structural distortion. We show that this distortion is not tetragonal as suggested previously, but is most likely rhombohedral. Below {ital T}{sub {ital N}2}, {chi}({ital T}) is anisotropic; our data suggest that in this range {chi}({ital T}) is dominated by next-nearest-neighbor exchange interactions.

Surface and bulk magnetic properties of pyrite NiS2: Magnetization and neutron-scattering studies.

Thin, horizontal-plane Hall sensors for read-head in magnetic recordings and methods for fabricating the sensor provide a novel sensor exhibiting high sensitivity and high spatial resolution.

Thin, horizontal-plane hall sensors for read-heads in magnetic recording

The x-ray-photoemission spectra of the core levels and valence-band region of ${\mathrm{NiS}}_{2}$ is interpreted in terms of a cluster model in the impurity limit. The d-d and p-d charge-fluctuation parameters are evaluated and the nature of the lowest-lying ionization state is investigated.

Electronic-correlation effects in the x-ray-photoemission spectra of NiS 2

Giant magnetoresistance in Hg1−xCdxTe and applications for high density magnetic recording

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