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+/

Cr3+:LiSrGaF6 is a new promising candidate for flash-lamp pumped laser due to very low scattering loss. We will discuss our work on the material preparation and crystal growth of large transparent boules of Cr3+:LiSrGaF6.

Synthesis and Crystal Growth of Cr3+:LiSrGaF6

Nd:BaY2F8 has been successfully grown and operated under flashlamp pumping conditions at both 1.0488 and 1.3176 μm. Nd:BaY2F8 is an interesting laser material because of its long upper laser level lifetime, 456 μsec, and its birefringent nature. Because of the short length of the available laser rod, an uncommon pulse forming network was employed. Under this experimental configuration, Nd:BaY2F8 displayed a higher slope efficiency than a Nd:YAG laser rod with the same diameter.

Flashlamp Pumped Nd:BaY2F8

We report epitaxial growth of the simple cubic perovskites Ba1-xKxBiO3 and Ba1-xRbxBiO3 on Ba3LaNb3O12, LiBaF3, BaF2, and Al2O3. Films with excellent crystallinity are obtained on the perovskite substrates, LiBaF3, and Ba3LaNb3O12. (111) or (110) films can be obtained on {111} BaF2, depending on the nucleation temperature. Non-epitaxial superconducting films have been grown on Si and on GaAs. At the low growth temperatures used (200-450°C), reactions with the substrates do not pose problems. The choice of substrate not only determines the microstructure of the film, but also plays a role in allowing the film to change its oxygen stoichiometry. Stoichiometry shifting is a pervasive necessity for the synthesis of high Tc thin films.

Molecular Beam Epitaxy of Superconducting Bismuthates on Various Substrates

Magnetoresistance (MR) measurements are reported for pure single crystals of La2CuO4+y at magnetic fields up to 23T With the magnetic field →H‖→c, parallel to the staggered moment, the MR shows evidence of a spin flop transition, which provides a measure of both the antisymmetric and the symmetric anisotropies of the nearest-neighbour exchange The MR, which arises from the interlayer hopping conductivity, is proportional to M†+, the order parameter of the antiferromagnetism (AF) with propagation vector →gt‖→c

Magnetoresistance and the spin flop transition in pure single - Crystal La2CuO4+y

Summary form only given. We report cascade laser action at 1.4 /spl mu/m and 3.9 /spl mu/m in 10% Ho:BaY/sub 2/F/sub 8/ (Ho:BYF) pumped by a Q-switched, frequency-doubled Nd:YAG laser at 532 nm. At this high concentration we measured an absorption coefficient of 2.42 cm/sup -1/ for polarization parallel to the b-axis, which is adequate for end pumping.

Hans P. Jenssen

Papers

Synthesis and Crystal Growth of Cr3+:LiSrGaF6

Flashlamp Pumped Nd:BaY2F8

Molecular Beam Epitaxy of Superconducting Bismuthates on Various Substrates

Magnetoresistance and the spin flop transition in pure single - Crystal La2CuO4+y

Efficient, room temperature cascade laser action at 1.4 /spl mu/m and 3.9 /spl mu/m in Ho:BaY/sub 2/F/sub 8/