The transport properties of disordered solids have been the subject of much work since at least the 1950s, but with a new burst of activity during the 1980s which has survived up to the present day. There have been numerous reviews of a more or less specialized nature. The present review aims to fill the niche for a non-specialized review of this very active area of research. The basic concepts behind the theory are introduced with more detailed sections covering experimental results, one-dimensional localization, scaling theory, weak localization, magnetic field effects and fluctuations.

https://iopscience.iop.org/article/10.1088/0034-4885/56/12/001/pdf

Localization: theory and experiment

Since the discovery of superconductivity above 30 K by Bednorz and M\"uller in the La copper oxide system, the critical temperature has been raised to 90 K in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and to 110 and 125 K in Bi-based and Tl-based copper oxides, respectively. In the two years since this Nobel-prize-winning discovery, a large number of electronic structure calculations have been carried out as a first step in understanding the electronic properties of these materials. In this paper these calculations (mostly of the density-functional type) are gathered and reviewed, and their results are compared with the relevant experimental data. The picture that emerges is one in which the important electronic states are dominated by the copper $d$ and oxygen $p$ orbitals, with strong hybridization between them. Photon, electron, and positron spectroscopies provide important information about the electronic states, and comparison with electronic structure calculations indicates that, while many features can be interpreted in terms of existing calculations, self-energy corrections ("correlations") are important for a more detailed understanding. The antiferromagnetism that occurs in some regions of the phase diagram poses a particularly challenging problem for any detailed theory. The study of structural stability, lattice dynamics, and electron-phonon coupling in the copper oxides is also discussed. Finally, a brief review is given of the attempts so far to identify interaction constants appropriate for a model Hamiltonian treatment of many-body interactions in these materials.

Electronic structure of the high-temperature oxide superconductors

White light-emitting diodes (WLEDs) as new solid-state light sources have a greatly promising application in the field of lighting and display. So far much effort has been devoted to exploring novel luminescent materials for WLEDs. Currently the major challenges in WLEDs are to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. In recent years, numerous efforts have been made to develop single-phase white-light-emitting phosphors for near-ultraviolet or ultraviolet excitation to solve the above challenges with certain achievements. This review article highlights the current methods to realize the white light emission in a single-phase host, including: (1) doping a single rare earth ion (Eu3+, Eu2+ or Dy3+) into appropriate single-phase hosts; (2) co-doping various luminescent ions with different emissions into a single matrix simultaneously, such as Tm3+/Tb3+/Eu3+, Tm3+/Dy3+, Yb3+/Er3+/Tm3+etc.; (3) codoping different ions in one host to control emission color via energy transfer processes; and (4) controlling the concentration of the defect and reaction conditions of defect-related luminescent materials.

How to produce white light in a single-phase host?

The 5d level positions of the trivalent lanthanides in inorganic compounds

The absorption and emission cross sections of the transition between the ground spin-orbit multiplet and the lowest excited multiplet were measured for Er/sup 3+/, Tm/sup 3+/, and Ho/sup 3+/ ions in a variety of crystalline hosts. The materials that were investigated include LiYF/sub 4/, BaY/sub 2/F/sub 8/, Y/sub 3/Al/sub 5/O/sub 12/, LaF/sub 3/, KCaF/sub 3/, YAlO/sub 3/, and La/sub 2/Be/sub 2/O/sub 5/. The absolute magnitudes of the emission cross sections were determined from the absorption spectra, with the aid of the principle of reciprocity. The calculated radiative emission lifetimes derived from these measured cross sections agree well with the measured emission decay times for most materials. The potential use of these rare-earth-doped materials in pulsed laser applications requires that the ground state exhibit adequate splitting to minimize the detrimental effects of the ground state thermal population, and also that the emission cross section be sufficiently large to permit efficient extraction energy. The systems based on Ho/sup 3+/ in the eightfold coordinated sites of LiYF/sub 4/, BaY/sub 2/F/sub 8/, and Y/sub 3/Al/sub 5/O/sub 12/ appear to be the most promising. >

Infrared cross-section measurements for crystals doped with Er/sup 3+/, Tm/sup 3+/, and Ho/sup 3+/

Measurements are reported of the conductivity and dielectric constant at frequencies up to 10 MHz for a single crystal of La{sub 2}CuO{sub 4+{ital y}} with {ital y}{similar to}0.001 to 0.01. The reported behavior is typical of conventional doped semiconductors, with dielectric constant (at {ital y}=0) and effective mass that are not exceptionally large. The dielectric constant grows as the insulator-to-metal transition is approached, but only for the electric field in the CuO{sub 2} layers, indicating that the transition is two dimensional. The crossover to weak localization occurs close to the antiferromagnetic phase boundary.

Frequency dependence of the conductivity and dielectric constant of La2CuO4+y near the insulator-metal transition.

Room-temperature tunable-laser action has been observed in Cr-doped SrAlF5. The measured tuning range is 852–947 nm in a quasi-cw laser mode. The effect of nonequivalent Cr sites in SrAlF5 is studied from the absorption and the excitation spectra. A broad absorption band peaked near 750 nm does not contribute to the Cr emission and appears to be associated with a nonradiative site.

Tunable-laser characteristics and spectroscopic properties of SrAlF 5 :Cr

Variably valent iron impurities have been suggested as the source of photorefractive charge carriers in BaTiO3. High-purity BaTiO3 crystals were grown with transition-metal impurity levels below the 1016 cm−3 baseline estimated for photorefractivity. Iron-doped crystals were grown with concentrations of 5, 50, 250, 500, 750, and 1000 parts in 106 of iron. Changes in iron valence monitored by optical absorption in the PO2 range 1–10−4 atm were found to be consistent with a defect-chemical model indicating Fe3+ and Fe4+ to be the stable valence states in this range. Photorefractive characterization of high-purity BaTiO3 suggests that variably valent iron ions are not the dominant photorefractive species in these crystals, whereas the role of iron centers in doped crystals is complicated by large absorption losses.

Hans P. Jenssen

Papers

Frequency dependence of the conductivity and dielectric constant of La2CuO4+y near the insulator-metal transition.

Tunable-laser characteristics and spectroscopic properties of SrAlF 5 :Cr

Role of iron centers in the photorefractive effect in barium titanate

Phase diagram and single crystal growth of pure and Sr doped La2CuO4

Top seeded solution growth of La4CuO4