In the light of the tremendous progress that has been made in raising the transition temperature of the copper oxide superconductors (for a review, see ref. 1), it is natural to wonder how high the transition temperature, Tc, can be pushed in other classes of materials. At present, the highest reported values of Tc for non-copper-oxide bulk superconductivity are 33 K in electron-doped CsxRbyC60 (ref. 2), and 30 K in Ba1-xKxBiO3 (ref. 3). (Hole-doped C60 was recently found4 to be superconducting with a Tc as high as 52 K, although the nature of the experiment meant that the supercurrents were confined to the surface of the C60 crystal, rather than probing the bulk.) Here we report the discovery of bulk superconductivity in magnesium diboride, MgB2. Magnetization and resistivity measurements establish a transition temperature of 39 K, which we believe to be the highest yet determined for a non-copper-oxide bulk superconductor.

Superconductivity at 39 K in magnesium diboride

This review provides numerical and graphical information about many (but by no means all) of the physical and electronic properties of GaAs that are useful to those engaged in experimental research and development on this material. The emphasis is on properties of GaAs itself, and the host of effects associated with the presence of specific impurities and defects is excluded from coverage. The geometry of the sphalerite lattice and of the first Brillouin zone of reciprocal space are used to pave the way for material concerning elastic moduli, speeds of sound, and phonon dispersion curves. A section on thermal properties includes material on the phase diagram and liquidus curve, thermal expansion coefficient as a function of temperature, specific heat and equivalent Debye temperature behavior, and thermal conduction. The discussion of optical properties focusses on dispersion of the dielectric constant from low frequencies [κ0(300)=12.85] through the reststrahlen range to the intrinsic edge, and on the ass...

Semiconducting and other major properties of gallium arsenide

Molecular beam epitaxy

Quantum levels associated with the confinement of carriers in very thin, molecular-beam—grown AlxGa1−xAs−GaAs-Alx Ga1−xAs heterostructures result in pronounced structure in the GaAs optical absorption spectrum Up to eight resolved exciton transitions, associated with different bound-electron and bound-hole states, have been observed The heterostructure behaves as a simple rectangular potential well with a depth of ≈088ΔE g , for confining electrons and ≈012ΔE g for confining holes, where ΔE g is the difference in the semiconductor energy gaps

Quantum States of Confined Carriers in Very Thin Al x Ga 1 − x As -GaAs- Al x Ga 1 − x As Heterostructures

Phonon scattering limits charge-carrier mobilities and governs emission line broadening in hybrid metal halide perovskites. Establishing how charge carriers interact with phonons in these materials is therefore essential for the development of high-efficiency perovskite photovoltaics and low-cost lasers. Here we investigate the temperature dependence of emission line broadening in the four commonly studied formamidinium and methylammonium perovskites, HC(NH2)2PbI3, HC(NH2)2PbBr3, CH3NH3PbI3 and CH3NH3PbBr3, and discover that scattering from longitudinal optical phonons via the Frohlich interaction is the dominant source of electron–phonon coupling near room temperature, with scattering off acoustic phonons negligible. We determine energies for the interacting longitudinal optical phonon modes to be 11.5 and 15.3 meV, and Frohlich coupling constants of ∼40 and 60 meV for the lead iodide and bromide perovskites, respectively. Our findings correlate well with first-principles calculations based on many-body perturbation theory, which underlines the suitability of an electronic band-structure picture for describing charge carriers in hybrid perovskites. Phonon scattering limits charge transport in perovskite solar cells, yet the interactions involved are still poorly understood. Here, Wright et al. show by photoluminescence measurements and first-principles calculations that longitudinal optical phonons dominate the electron-phonon coupling at room temperature.

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Electron–phonon coupling in hybrid lead halide perovskites

Hall coefficient factor for polar mode scattering in n-type GaAs☆

Magnetospectroscopy of shallow donors in GaAs

Total ionized impurity densities (ND+NA) from 7×1013 to 3×1017 cm−3 are determined for epitaxial samples of n‐type GaAs by analyzing mobility and carrier concentration data as a function of temperature with the Brooks‐Herring formula for ionized impurity scattering. This procedure results in the determination of a temperature range within which the effects of other scattering mechanisms are minimal and gives values of ND and NA which are in good agreement with impurity densities obtained from analyses of the temperature variation of the Hall constant. These results are then used to determine empirical curves relating the impurity density to the 77°K Hall mobility. With these data a good estimate of the total ionized impurity concentration in a sample can be determined from Hall constant and resistivity measurements at 77°K.

Ionized Impurity Density in n‐Type GaAs

The electroabsorption coefficient of GaAs has been measured in uniform electric fields at wavelengths from 0.91 to 0.93 μm. These measurements were made using Schottky barrier contacts on low‐loss GaAs waveguides consisting of high‐purity epitaxial GaAs grown on heavily doped GaAs substrates. The experimental results are in good agreement with theoretical calculations of the Franz‐Keldysh effect. Electroabsorption detectors with subnanosecond response time and 100% internal quantum efficiency have been integrated into these waveguides. Small values of avalanche gain have been obtained without any intentional guard‐ring structure. Integrated electroabsorption modulators with greater than 20‐dB depth of modulation were also fabricated.

C.M. Wolfe

Papers

Hall coefficient factor for polar mode scattering in n-type GaAs☆

Magnetospectroscopy of shallow donors in GaAs

Ionized Impurity Density in n‐Type GaAs

Electroabsorption in GaAs and its application to waveguide detectors and modulators

Precision verification of effective mass theory for shallow donors in GaAs