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

The contribution that the technique of ferromagnetic resonance (FMR) has made to the understanding of the magnetic behaviour of ultrathin single films is reviewed. Experimental methods to measure FMR in situ in ultrahigh vacuum are presented. The temperature dependence of the magnetization, of the magnetic relaxation rate in the vicinity of the Curie temperature, and of the second- and fourth-order magnetic anisotropy energy (MAE) constants can be measured by FMR in situ for magnetic monolayers. Using the cases of Ni/Cu(001) and Gd/W(110) as examples, the role of the MAE for the quantitative description of temperature- and thickness-dependent reorientation transitions of the magnetization is discussed. Initial results for the anisotropy of the g-factor which is related to the anisotropy of the orbital moment (and the MAE) are presented.

https://iopscience.iop.org/article/10.1088/0034-4885/61/7/001/pdf

Ferromagnetic resonance of ultrathin metallic layers

Size dependence of nanostructures: Impact of bond order deficiency

State-of-the-art electronic-structure calculations based on the local-density approximation (LDA) to the density functional fail to reproduce the insulating antiferromagnetic ground state in the parent compounds of the high-temperature oxide superconductors. Similar problems have been observed earlier in classical transition-metal oxides such as FeO, CoO, and NiO. In this work we present the method which delivers the correct insulating antiferromagnetic ground state in the correlated oxides preserving other properties as well as the efficiency of the standard LDA. The method embeds the relevant (for a given system of electrons) part of the Hubbard Hamiltonian into the Kohn-Sham LDA equation. The resulting Hamiltonian attempts to fix two intrinsic problems of the LDA: the deficiency in forming localized (atomiclike) moments and the lack of discontinuity of the effective one-particle potential when going from occupied to unoccupied states. We present the detailed study of La2CuO4 and LaCuO3. In the case of La2CuO4 the energy gap and the value of the localized magnetic moment in the stable insulating antiferromagnetic solution are in good agreement with experiment. We compare our results with the standard local spin density approximation calculation and multiband Hubbard model calculations, as well as with results of spectroscopy: inverse photoemission, valence photoemission, and x-ray absorption at the K edge of oxygen. In the case of LaCuO3 such an extensive comparison is limited due to the limited data available for this compound. We discuss, however, the electric and magnetic properties and the insulator-metal-insulator transitions upon increase of oxygen deficiency.

Local-density functional and on-site correlations: The electronic structure of La2CuO4 and LaCuO3

Although the vast majority of high-$T_c$ cuprate superconductors are hole-doped, a small family of electron-doped compounds exists. Under investigated until recently, there has been tremendous recent progress in their characterization. A consistent view is being reached on a number of formerly contentious issues, such as their order parameter symmetry, phase diagram, and normal state electronic structure. Many other aspects have been revealed exhibiting both their similarities and differences with the hole-doped compounds. This review summarizes the current experimental status of these materials, with a goal to providing a snapshot of our current understanding of electron-doped cuprates. When possible we put our results in the context of the hole-doped compounds. We attempt to synthesize this information into a consistent view on a number of topics important to both this material class as well as the overall cuprate phenomenology including the phase diagram, the superconducting order parameter symmetry, phase separation, pseudogap effects, the role of competing orders, the spin-density wave mean-field description of the normal state, and electron-phonon coupling.

Progress and perspectives on electron-doped cuprates

We report on strong magnetic circular dichroism (MCD) in 4f photoemission (PE) from magnetized Gd(0001)/W(110) films. The shape of the 4${\mathit{f}}^{6}$${\mathrm{\ensuremath{-}}}^{7}$${\mathit{F}}_{\mathit{J}}$ final-state PE multiplet depends on the relative orientation between photon spin and sample magnetization and can be described within an atomic model. The spectra rule out antiferromagnetic alignment of the (0001) surface layer and the bulk of Gd. This MCD in 4f PE from rare-earth materials opens new perspectives in the analysis of surface and thin-film magnetism and as a sensor for circular polarization of soft x rays.

Strong magnetic circular dichroism in 4f photoemission from gadolinium metal.

Magnetic circular dichroism and orbital momentum coupling in 4d photoemission from Gd(0001)

For Tb(0001), a surface core-level shift of the 4${\mathit{f}}^{7}$ photoemission multiplet of ${\mathrm{\ensuremath{\delta}}}_{\mathit{s}}$=0.26\ifmmode\pm\else\textpm\fi{}0.03 eV is determined. This value is by more than a factor of 2 smaller than the result of a recent self-consistent SLAB calculation, but agrees with the prediction of a thermochemical model. It is demonstrated that the larger values of ${\mathrm{\ensuremath{\delta}}}_{\mathit{s}}$ reported previously for polycrystalline samples originate from contributions of low-coordinated surface sites. Implications of the present findings for an improved understanding of the surface electronic structure of rare-earth metals are briefly discussed.

Surface core-level shift of 4f states for Tb(0001).

A consistent description of magnetic circular dichroism (MCD) in resonant 4d\ensuremath{\rightarrow}4f photoemission (PE) and 4d photoabsorption (PA) from magnetized rare-earth metals is presented. In the case of ferromagnetic Gd metal it is found that the MCD of the resonant 4fPE spectra varies strongly when the photon energy is scanned across the resonance region. We show that the spectral shape of the 4fPE multiplet is determined by the total angular momentum of the intermediate ${4\mathrm{d}}^{9}$ ${4\mathrm{f}}^{8}$ PA state, which is reflected in the sign of the PA-MCD at the excitation energy.

Magnetic circular dichroism in 4d-->4f resonant photoemission and photoabsorption of Gd metal

Magnetic circular dichroism in core-level photoemission from Gd, Tb, and Dy in ferromagnetic materials.

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