The localized-moment contribution to the specific heat of Si:P in the vicinity of the metal-insulator transition has been calculated as a function of temperature and magnetic field. The magnetic susceptibility has also been obtained and compared with available experimental data. Small clusters of spins are used in the calculations. The specific-heat predictions are in fairly good agreement with experimental data in fields up to 5.7 T for n/${\mathit{n}}_{\mathit{c}}$<0.2. At higher concentrations a reduced number of effective spins must be used in order to obtain reasonable agreement with experiment.

The electronic structure of the nonmagnetic impurities Ag, Au, Zn, Cd, Ga, and In in Cu is investigated. The effective charge on the impurities is determined with the help of the Friedel criterion for bound states and lattice dilation. The spherical-solid model is used to include the effect of the discrete lattice. Self-consistent nonlinear screening calculations are carried out and the host-impurity potential, impurity-induced charge density, charge transfer, and electric-field gradients (EFG’s) are calculated. The host-impurity potentials are found to be nearly proportional to the impurity charge in the vicinity of impurities; however, Friedel oscillations exist at large distances. The same characteristics are found in the impurity-induced charge densities. The impurities transfer from 25% to 80% of their charges to the host. The calculated EFG’s at the first nearest neighbors and second nearest neighbors of impurities are found to be in agreement with the experimental values.

The conduction bands of cuprous halides are studied through bremsstrahlung-isochromat-spectroscopy (BIS) experiments. These results are compared with the densities of states calculated by the linear-muffin-tin-orbital (LMTO) method. The good agreement in the low-energy range permits us to determine the contributions of the different symmetries on the two sites of the three halides. On the higher-energy side BIS, as well as x-ray-absorption spectra, show that the LMTO gives better agreement when they are compared with the p states on copper in CuI than in CuCl.

Within the framework of density-functional theory, first-principles pseudopotential methods have been highly successful in modeling the valence-electron properties of solids in their ground states. In this paper, we introduce ‘‘core-cancellation functions’’ which are designed to improve the accuracy of the treatment of the exchange-correlation interaction. This formalism, expected to be especially effective for transition metals, is tested for bulk tungsten and niobium, comparing results obtained using the local-density approximation to those obtained using the generalized gradient approximation.

We investigate the optical properties and the dielectric response of Al through self-consistent ab initio pseudopotential calculations. The frequency and wave-vector dependent dielectric function of Al is calculated for both the real and imaginary parts within the random-phase approximation. The accuracy of the calculations is tested using Kramers-Krönig relations and sum rules. In the limit q→0, we find major optical peaks at 0.5 and 1.5 eV, which originate from the electronic structure, in good agreement with experimental data. For the wave vectors along the [100] direction, the electron-energy-loss function is calculated to examine the plasmon mode. Considering the exchange-correlation and core polarization effects, we find good agreement of the calculated plasma frequencies with experiments. The anisotropy of the plasmon dispersion is also investigated.

A fermion-spin transformation is used to implement the charge-spin separation, and is developed to study the low-dimensional t-J model. In this approach, the charge and spin degrees of freedom of the physical electron are separated, and the charge degree of freedom is represented by a spinless fermion while the spin degree of freedom is representd by a hard-core boson. The on-site local constraint for single occupancy is satisfied even in the mean-field approximation and the sum rule for the physical electron is obeyed. This approach can be applied to both one and two-dimensional systems. In the one-dimensional case, the spinon as well as the physical electron behave like Luttinger liquids. We have obtained a gapless charge and spin excitation spectrum, a good ground state energy, and a reasonable electron-momentum distribution within the mean-field approximation. The correct exponents of the correlation functions and momentum distribution are also obtained if the squeezing effect and rearrangement of the spin configurations are taken into account. In the two-dimensional case, within the mean-field approximation the magnetized flux state with a gap in the spinon spectrum has the lowest energy at half filling. The antiferromagnetic long-range order is destroyed by hole doping of the order ∼10-15 % for t/J=3-5 and a disordered flux state with gapless spinon spectrum becomes stable. The calculated specific heat is roughly consistent with observed results on copper oxide superconductors. The possible phase separation is also discussed at the mean-field level.

Point defects were studied by positron-lifetime spectroscopy in highly Te-doped GaAs, in the as-grown state, and after plastic deformation. In as-grown GaAs:Te, positron trapping to vacancylike defects, supposed to be the compensating centers ${\mathrm{Te}}_{\mathrm{As}}$${\mathit{V}}_{\mathrm{Ga}}^{\mathrm{-}}$, was observed. At low temperatures (T<200 K), additional trapping is caused by shallow positron traps, probably by ${\mathrm{Ga}}_{\mathrm{As}}^{\mathrm{-}}$ antisite defects. In two differently deformed samples, additional defects were identified as microvoids, consisting of not less than ten vacancies. The dominating vacancy supply mechanism for vacancy agglomeration is the dragging of jogs on screw dislocations. The microvoids almost completely anneal after thermal treatment at 1075 K. The density of shallow traps increased after deformation by about one order of magnitude as a result of dislocation climb. These shallow traps additionally formed during plastic deformation exhibit the same positron binding energy, ${\mathit{E}}_{\mathit{b}}$=43 meV, as found in as-grown material. This part could be annealed at 1000 K, whereas the part of shallow traps originally present is determined by the nonstoichiometry and is therefore thermally stable. The density of the compensating centers remains almost unchanged after deformation. They are thermally stable up to 1075 K.

We report the results of transient-photoconductivity measurements on films of poly(p-phenylenevinylene) (PPV) in the subnanosecond-time regime. The initial fast transient photocurrent decays exponentially with a decay time of about 100 ps, followed by a slower component with a decay time of about 600 ps. The magnitude of the fast component is proportional to the light intensity and independent of temperature, while the magnitude of the slower component is proportional to the square root of the light intensity and decreases as temperature decreases with a thermal-activation energy of about 100 meV. We attribute the initial response to photogenerated electrons and holes (polarons) and the following slower decay to bipolarons in the nondegenerate ground-state conducting polymer. Measurements of the spectral response of both transient and steady-state photoconductivity demonstrate that the onset of the photoconductivity coincides with that of photoabsorption, consistent with the photogeneration of free carriers via an interband transition. The interpretation of the initial temperature-independent transient photocurrent as the displacement current from the field-induced polarization of neutral excitons is ruled out by careful analysis of the transient-photocurrent data and by direct measurements on polydiacetylene-(toluene sulfonate), a conjugated polymer with a known exciton binding energy (${\mathit{E}}_{\mathrm{exc}}$), ${\mathit{E}}_{\mathrm{exc}}$≊0.4 eV. A variety of measurements has enabled us to set an upper limit on ${\mathit{E}}_{\mathrm{exc}}$ in PPV and several of its alkoxy derivatives; ${\mathit{E}}_{\mathrm{exc}}$ is comparable to, or less than, ${\mathit{k}}_{\mathit{B}}$T at room temperature.

We present a study of the effect of strong electron-phonon coupling on the levels of the ${}_{}{}^5{}_{}{}^{}\mathrm{D}$ term of ${\mathrm{Fe}}^{2+}$ in II-VI zinc-blende semiconductors. The nonperturbative approach leads to a dramatic decrease of the level spacings of the ${}_{}{}^5{}_{}{}^{}\mathrm{\Gamma }_5^{}{}_{}{}^{}$ manifold of ${\mathrm{Fe}}^{2+}$ in a tetrahedral environment. The results allow us to propose a mechanism for those absorption lines in the excitation spectrum of ${\mathrm{Fe}}^{2+}$ in CdTe which are not attributed to zero-phonon lines deduced from crystal-field theory. It is also shown that transitions between the crystal-field split states of ${\mathrm{Fe}}^{2+}$ can be explained on the basis of mixing of 3${\mathit{d}}^6$ states with odd-parity states of higher configurations.

The influence of the higher ${\mathrm{\Gamma }}_{15}^{\mathit{c}}$ conduction-band set on the strength of the two-photon-absorption coefficient, the two-photon linear/circular dichroism, and the two-photon-absorption anisotropy is calculated across the spectral region below the fundamental absorption edge, for the cubic materials GaAs and InSb. The anisotropy is entirely due to the higher band and is predicted to produce up to a 70% variation in the two-photon-absorption coefficient of GaAs as the relative orientation of the optical polarization to the crystal axes is altered. This is in good agreement with recent experiments.

A model for the equilibrium and metastable defect structure in a-Si:H is presented. Allowing chemical equilibration between dangling bonds and weak bonds with a broadened band of available defect energy levels, and requiring electronic occupancy according to occupation statistics during the chemical equilibration process, an expression for the steady-state defect structure under general nonequilibrium conditions is derived. A single reaction involving e-h pair recombination is found to account for the vast majority of experimental data, such as the dependence of equilibrated and saturated defect densities on temperature, generation rate, and band gap. The model predicts an excess of charged defects in as-grown undoped a-Si:H and a change in the energetic distribution of defect states by light soaking with an associated decrease of the ratio of charged-to-neutral defects, in agreement with measurements.

Many-body renormalization of phonon frequencies, due to their interaction with a high-density electron-hole plasma, such as the one created by an intense laser pulse applied to GaAs, is calculated. Results for the change in phonon-dispersion curves are obtained, both for a distribution characterized by zero carrier temperature and for the electron-hole plasma equilibrium temperature. We use a many-valley model for the conduction and valence bands of GaAs, and the deformation potentials available from the literature for the carrier-acoustic-phonon interaction. We do not find complete softening of the phonons below the experimental electron-hole pair density (<${10}^{22}$ ${\mathrm{cm}}^{\mathrm{-}3}$), although zone-boundary phonon softening is observed at (1–5)×${10}^{22}$ ${\mathrm{cm}}^{\mathrm{-}3}$. From the increase in the mean squared amplitude of the atomic vibrations caused by the phonon softening, we obtain an effective lattice melting temperature which decreases with increasing carrier temperature. Thus, a nonthermal electronic mechanism could at best play only a partial role in semiconductor laser annealing experiments even on ultrafast sub- picosecond time scales.

Rate equations are derived, in the linear approximation with respect to the electric field, for hopping transport in disordered systems in the presence of a magnetic field H. The resulting system of linear equations is in general not equivalent to a random resistor network. By means of an effective-medium approximation, an analytical expression for the magnetoresistance Δσ(H) has been obtained for R hopping. In the case of strong coupling with phonons, Δσ(H) is independent of the temperature T and it is proportional to ${\mathit{H}}^2$ in the region of weak magnetic fields. For weak coupling with phonons, we find that Δσ(H)∼${\mathit{H}}^2$/T. $DELTA sigma (H)— shows a monotonic behavior in a wide range of magnitude of the magnetic field. It does not exhibit an oscillating contribution. The sign of Δσ(H) depends on the character of the conductivity (electronlike or holelike). Δσ(H) approaches zero at some point if one varies the chemical potential.

Relaxation of coherent LO phonons is investigated in InP, GaAs, and GaP by use of an infrared time-resolved coherent anti-Stokes Raman-scattering technique. Measurements were performed as a function of crystal temperature in the range 6–320 K. The measured LO-phonon dephasing times are about four-times longer in InP than in GaAs. This difference is attributed to the closing of the cubic combination relaxation channel which dominates the decay in GaAs but is energetically forbidden in InP. In this compound, as in GaP, both a less efficient combination relaxation channel and the overtone decay route where the initial phonon splits into two isoenergetic longitudinal-acoustic phonons significantly contribute to the LO-phonon decay.

A hydrogen-stretching vibration at 2688 ${\mathrm{cm}}^{\mathrm{-}1}$, previously observed in heavily-carbon-doped GaAs grown by metal-organic molecular-beam epitaxy and assigned to a defect complex that contains carbon and hydrogen, has been studied by infrared-absorption spectroscopy. The absorption at 2688 ${\mathrm{cm}}^{\mathrm{-}1}$ has been found to be preferentially polarized along a particular 〈110〉 axis in the (001) growth plane. We propose that this band is due to a complex that is aligned at the growth surface and then maintains its alignment as the crystal is grown. The annealing of the 2688-${\mathrm{cm}}^{\mathrm{-}1}$ band and its alignment have been examined. We also find that the 2688-${\mathrm{cm}}^{\mathrm{-}1}$ complex can be formed by annealing heavily-carbon-doped GaAs between 500 and 650 °C in the presence of hydrogen. These results provide insight into the interactions of carbon and hydrogen in heavily-carbon-doped GaAs and lead us to tentatively assign the 2688-${\mathrm{cm}}^{\mathrm{-}1}$ band to a (${\mathrm{C}}_{\mathrm{As}}$${)}_2$H complex. We suggest that the annealing of this center and of its alignment is due to the dissociation and reassociation of ${\mathrm{C}}_{\mathrm{As}}$ pairs that are controlled by the presence of hydrogen.

The precipitation and dissolution of monoclinic SiAs, in Si samples implanted with 1 and 1.5×${10}^{17}$ ${\mathrm{As}}^{+}$/${\mathrm{cm}}^2$, was followed at 800, 900, and 1050 °C by transmission electron microscopy (TEM) and secondary neutral mass spectrometry. It has been found that the concentration ${\mathit{C}}_{\mathrm{sat}}$ of mobile As, which diffuses after equilibration with the monoclinic SiAs precipitates, is given by ${\mathit{C}}_{\mathrm{sat}}$=1.3×${10}^{23}$ exp(-0.42/kT) ${\mathrm{cm}}^{\mathrm{-}3}$, where kT is in eV. This saturation concentration includes different states of As in equilibrium with the monoclinic phase at the annealing temperatures, namely, (i) the electrically active and (ii) the inactive mobile dopant. The results presented in this paper indicate that the inactive As is in the form of clusters. The link between the clustering phenomenon and the presence of small particles detected by TEM observations is also discussed.

Near-ultraviolet light induces band-to-band electron transitions and is used to record interband photorefractive gratings in undoped ${\mathrm{KNbO}}_3$. The high absorption constant in the range from 500 to 6000 ${\mathrm{cm}}^{\mathrm{-}1}$ dictates the use of thin crystals or of the crystal volume near the surface. The probing of the grating is usually performed in the visible. We measured a fast grating response of 10 μs at 1 W/${\mathrm{cm}}^2$ for refractive index changes of 2×${10}^{\mathrm{-}5}$. The hologram diffraction efficiency reaches η=1.4% in a longitudinal recording geometry with a grating approximately 200 μm thick. In a transverse geometry we measured η=60% in 7.5-mm-long crystals. The photorefractive sensitivity of ${\mathrm{KNbO}}_3$ is similar to that of multiple-quantum-well structures while the resolution is better due to smaller carrier mobilities and because of shorter optical wavelengths used. Interband photorefractive gratings are highly robust with respect to intensive illumination in the visible. A theoretical model that describes interband electron transitions predicts the important role of free carriers in both bands. It explains the transition from a linear to a sublinear dependence of photoconductivity on light intensity which is observed already at intensities of a few mW/${\mathrm{cm}}^2$. The predictions are consistent with dynamic and steady-state holographic investigations performed under variation of grating spacing, applied electric field, ultraviolet and visible light intensities, and grating readout depth.

We present a first-principles study of the electronic and magnetic properties of the chromium spinel ${\mathrm{CdCr}}_2$${\mathrm{Se}}_4$, a ferromagnetic semiconductor, which, for its interesting magnetic and electric properties has been the object of many experimental studies. Using calculations based on the full-potential linearized augmented plane wave relativistic scheme we are able to reproduce the structural equilibrium properties and to explain many of the features measured by resonant photoemission spectroscopy. In particular, we analyze the contribution of the Cr 3d electrons in a density-of-states study including the two spin orientations and a comparison with the binary analog CdSe.

We present the analytical theory for the conduction-band spectrum in partially ordered semiconductor alloys. It is assumed that the ordering causes the coupling of the electronic states of Γ minimum with the states of the closest-in-energy L minimum only. We analyze the reduction of the band gap, change of the effective mass, nonparabolicity, and anisotropy of the electronic spectrum, caused by ordering. Using the spectrum obtained we study the size quantization in an ordered domain sandwiched between two disordered regions. We also calculate the electronic spectrum in the region of interpenetration of two variants of ordering along the [1¯11] and [11¯1] directions. The localization of an electron in such a region is studied.

Angular correlation of positron annihilation radiations has been measured along the [100], [110], [111], [211], and [221] crystallographic axes in GaAs using the long-slit geometry. A pseudopotential calculation is carried out to understand the experimental data. There is good agreement between experimental and theoretical data. The structures seen in the low-momentum region of the data are interpreted in terms of the σ and π bonds of the ${\mathit{sp}}^3$ hybridized bonding orbitals. The experimental data when compared with the calculated data reveal the presence of electron-positron many-body correlation effects.