The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

It is shown that the convergence of the configuration expansion of a many‐electron wavefunction may be drastically improved without significantly complicating the energy matrix elements by using partially non‐orthogonal orbitals. The wavefunction thus obtained is the many‐electron analog to the natural orbital expansion of a two‐electron wavefunction. The orbitals involved may be approximated by pseudonatural orbitals (``PNO–CI''), and an efficient way of calculating these orbitals is presented. An approximate treatment of multiple substitutions is discussed yielding a coupled electron pair theory formulated within the CI scheme. These methods are applied to the ground state and three ionized states of methane. Including only double substitutions, the computed upper bound to the energy of the ground state is −40.4584 hartree, i.e. 0.057 above the experimental nonrelativistic energy. About 83% of the total correlation energy is accounted for variationally, whereas the coupled‐pair approximation yields 89%....

PNO–CI Studies of electron correlation effects. I. Configuration expansion by means of nonorthogonal orbitals, and application to the ground state and ionized states of methane

Advances in the accuracy and efficiency of first-principles electronic structure calculations have allowed detailed studies of the energetics of materials under high pressures. At the same time, improvements in the resolution of powder x-ray diffraction experiments and more sophisticated methods of data analysis have revealed the existence of many new and unexpected high-pressure phases. The most complete set of theoretical and experimental data obtained to date is for the group-IVA elements and the group-IIIA--VA and IIB--VIA compounds. Here the authors review the currently known structures and high-pressure behavior of these materials and the theoretical work that has been done on them. The capabilities of modern first-principles methods are illustrated by a full comparison with the experimental data.

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High-pressure phases of group-IV, III–V, and II–VI compounds

The measurements of the hyperfine structure of free, naturally occurring, alkali atoms are reviewed. The experimental methods are discussed, as are the relationships between hyperfine structure data and other atomic constants.

Experimental determinations of the hyperfine structure in the alkali atoms

A Calculation of Atomic Structures. By D. R. Hartree Pp. xiii, 181. 40s. 1957. (Wiley, New York)

Nuclear Quadrupole Resonance Spectroscopy

The II-VI semiconductors ZnO and ZnSe have been investigated by x-ray and $^{67}\mathrm{ssbauer}$ spectroscopy at high external pressures. In ZnSe, the recoilfree fraction f increases from f=0.50% at ambient pressure to 1.19% at 6.1 GPa. It then decreases to f=0.92% as the pressure is further raised to 8.2 GPa. This decrease of f is caused by softening of phonon modes which occurs far below the crystallographic phase transition (13.5 GPa). In the high-pressure phase of ZnO (NaCl structure), low-frequency acoustic-phonon modes become harder and high-frequency optic modes become softer as compared to ZnO (wurtzite structure). Modern theoretical Hartree-Fock cluster and full potential scalar-relativistic linearized-augmented plane-wave calculations have been performed. These calculations reveal that in both systems covalent contributions to the chemical bond determine the change of the s electron density \ensuremath{\rho}(0) at the Zn nucleus between the different crystallographic phases as well as the electric-field-gradient tensor in ZnO (wurtzite). In particular, \ensuremath{\rho}(0) in ZnO (NaCl phase) is reduced compared to \ensuremath{\rho}(0) in ZnO (wurtzite phase) by -1.15e/${\mathit{a}}_{0}^{3}$. Thus, contrary to observation for ZnSe, the electrical conductivity in ZnO (NaCl phase) is not expected to increase in comparison with the low-pressure wurtzite structure. \textcopyright{} 1996 The American Physical Society.

Lattice dynamics and hyperfine interactions in ZnO and ZnSe at high external pressures.

With use of an unrestricted Hartree-Fock self-consistent-field cluster approach, the potential experienced by a muon in diamond has been investigated. The results strongly indicate that normal muonium is localized in the tetrahedral interstitial space. By using the calculated spin density and averaging it explicitly over the vibrational motion of the muon a quantitative explanation is obtained for the observed reduction of the hyperfine field as compared to free muonium.

Self-consistent-field investigation of location and hyperfine interaction of muonium in diamond

Relativistic quantum mechanics of electrons

A new determination of the /sup 57/Fe/sup m/ quadrupole moment has yielded the value 0.082 b, which is only half of the currently accepted value. The procedure combined first-principles calculation of the field gradients at the Fe nucleus in FeCl/sub 2/ and FeBr/sub 2/ molecules with published quadrupole splittings for those molecules trapped in rare-gas matrices. The field gradients are strongly influenced by iron 3p covalency.

T. P. Das

Papers

Nuclear Quadrupole Resonance Spectroscopy

Lattice dynamics and hyperfine interactions in ZnO and ZnSe at high external pressures.

Self-consistent-field investigation of location and hyperfine interaction of muonium in diamond

Relativistic quantum mechanics of electrons

Ab initio determination of /sup 57/Fe/sup m/ quadrupole moment from Moessbauer data