The distribution of the hopping probabilities in the strong localization regime is calculated via the independent-directed-path formalism, allowing for level broadening of the intermediate site energies of the hopping process. The distribution belongs to the Gaussian unitary ensemble even in the absence of a magnetic field, whereas in the absence of level broadening it belongs to the orthogonal ensemble. In the presence of the field it leads to an orbital magnetoconductance quadratic in the field, changing from positive to negative as the temperature is lowered and the number of paths increases.

It has been shown recently that the dc conductivity (along the highly conducting axis) of fluoranthene radical cation salts exhibits the behavior of a quasi-one-dimensional conductor with a Peierls transition at about 180 K to a charge-density-wave ground state. In the high-temperature range fluctuations of the order parameter lead to a pseudogap in the electronic density of states, whereas below the phase transition the energy gap is BCS-like. To analyze the conductivity data, a simple band-structure model is used and related to optical data. In spite of the occurrence of the Peierls transition, it is assumed that three-dimensional effects and fluctuations are strong enough for the conduction to be essentially due to electron-hole transport in bands and not polaronlike. Therefore the conductivity is determined simply by the Boltzmann equation with deformation-potential scattering. (a) This model yields a good fit of experimental data using the BCS-like gap below the transition and the pseudogap as determined from paramagnetic spin susceptibility above the transition. (b) It is possible to obtain the temperature dependence of the concentration, the mobility, the relaxation time, and the mean free path of carriers. (c) One can check a fundamental criterion for the applicability of the model. (d) We obtain from the fit parameters reasonable estimates of the quantities determining the deformation-potential scattering. Moreover, it is demonstrated that the theory can be used to determine the full temperature dependence of the gap/pseudogap directly from conductivity data.

The validity of the wide-band approximation is discussed and, in particular, the effects of the speed of the ion and the position of ion orbital energy relative to the band are considered. It is shown that care is needed in interpreting the integrals involved using generalized functions and that, for slow ions and a wide band, the wide-band approximation is valid for orbital energies well within the band, partially valid at the band edges, and invalid outside the band. Equations are also derived for mixed bands including the situation where the band can be divided into two parts, a narrow band and a wide band.

Fluorescence line-narrowing experiments have been made for the ${\mathrm{Sm}}^{2+}$ ion in fluoride glass at 77 K under cw dye-laser excitation at various energies within the inhomogeneously broadened ${}_{}{}^7{}_{}{}^{}$${\mathit{F}}_{0\mathrm{-}}^5$${\mathit{D}}_0$ and ${}_{}{}^7{}_{}{}^{}$${\mathit{F}}_{0\mathrm{-}}^5$${\mathit{D}}_1$ absorption lines. The energies of the three Stark levels of the ${}_{}{}^7{}_{}{}^{}$${\mathit{F}}_1$ and ${}_{}{}^5{}_{}{}^{}$${\mathit{D}}_1$ manifolds have been obtained as a function of the ${}_{}{}^5{}_{}{}^{}$${\mathit{D}}_{0\mathrm{-}}^7$${\mathit{F}}_0$ energy separation. From the analysis of this result, it has been found that the site-to-site variations of the energies of these Stark levels can be explained well only by taking into account the J-mixing effect. The low-energy levels of the 4${\mathit{f}}^5$5d configuration lie in the vicinity of the ${}_{}{}^5{}_{}{}^{}$${\mathit{D}}_{\mathit{J}}$ states in this material. However, no effect of the crystal-field mixing of the 4${\mathit{f}}^5$5d states on the inhomogeneous distribution of the energies of the ${}_{}{}^7{}_{}{}^{}$${\mathit{F}}_1$ and ${}_{}{}^5{}_{}{}^{}$${\mathit{D}}_1$ states has been observed. The origin of the unusually intense ${}_{}{}^5{}_{}{}^{}$${\mathit{D}}_{0\mathrm{-}}^7$${\mathit{F}}_0$ transition in ${\mathrm{Sm}}^{2+}$ is also discussed.

In this paper we reexamine the problem of the separation of spin and charge degrees of freedom in two-dimensional strongly correlated systems. We establish a set of sufficient conditions for the occurrence of spin and charge separation. Specifically, we discuss this issue in the context of the Heisenberg model for spin-1/2 on a square lattice with nearest (${\mathit{J}}_1$) and next-nearest (${\mathit{J}}_2$) neighbor antiferromagnetic couplings. Our formulation makes explicit the existence of a local SU(2) gauge symmetry once the spin-1/2 operators are replaced by bound states of spinons. The mean-field theory for the spinons is solved numerically as a function of the ratio ${\mathit{J}}_2$/${\mathit{J}}_1$ for the so-called s-RVB ansatz (s-RVB stands for short range resonating valence bond). A second-order phase transition exists in a flux state for ${\mathit{J}}_2$/${\mathit{J}}_1$ larger than the critical value (${\mathit{J}}_{2/}$${\mathit{J}}_1$${)}_{\mathrm{cr}}$ identify the range 0<${\mathit{J}}_2$/${\mathit{J}}_1$<(${\mathit{J}}_2$/${\mathit{J}}_1$${)}_{\mathrm{cr}}$ as the s-RVB phase. It is characterized by the existence of a finite gap to the elementary excitations (spinons) and the breakdown of all the continuous gauge symmetries. An effective continuum theory for the spinons and the gauge degrees of freedom is constructed just below the onset of the flux phase. We argue that this effective theory is consistent with the deconfinement of the spinons carrying the fundamental charge of the gauge group. We contrast this result with the study of the one-dimensional quantum antiferromagnet within the same approach. We show that in the one-dimensional model, the spinons of the gauge picture are always confined and thus cannot be identified with the gapless spin-1/2 excitations of the quantum antiferromagnet Heisenberg model.

We study a class of kinetic-energy functionals suitable for orbital-free first-principles molecular dynamics and assess their validity for the study of simple metals. With a simple functional which guarantees correct behavior to second order in perturbation theory and approximates higher-order terms, we find that for Na accuracy is already achieved, while Al is reasonably well described. The simple functional can be readily generalized to include higher-order terms.

We study the behavior of response functions of two-dimensional (2D) fermions interacting via a long-range transverse gauge field in the eikonal approximation. We observe that an exponentially vanishing wave-function renormalization prevents divergences in the density-density correlation function and the pairing susceptibility. The wave-function renormalization also shows up in a suppression of the dHvA effect. Elaborating on an observation recently made by Ioffe, Lidsky, and Altschuler, we also infer an effective bosonic description which makes it possible to reproduce our eikonal results in terms of free 2D bosons.

We propose a model for porous silicon, an irregular structure obtained by removing some silicon atoms randomly from a perfect silicon crystal. It is shown by using two-dimensional clusters with a tight-binding model that this model exhibits the energy-gap widening and the nonexponential decay of the photoluminescence (PL), which are in good agreement with the observed properties of porous silicon. In this model, the energy-gap widening is due to the localization of eigenstates caused by the randomness of the structure. The distributions in both size and position of the localized eigenstates, i.e., a statistical effect, yield nonexponential PL decay which is describable by the stretched exponential function. A dynamical effect due to electron and hole hopping between the eigenstates explains the observed strong energy dependence of the PL lifetimes.

An analytical investigation has been made for the nonlinear refractive index and absorption coefficient of the Brillouin-scattered Stoke’s mode resulting from the nonlinear interaction of an intense pumping light beam with acoustic perturbations internally generated due to the electrostrictive property of the doped semiconductor plasma. The origin of this nonlinear interaction lies in the third-order optical susceptibility arising from the nonlinear current density and electrostriction of the medium. The magnitude of third-order optical susceptibility determined from our analysis is found to agree with the theoretically and experimentally quoted values. The total refractive index and absorption coefficient are determined through the effective susceptibility derived with the help of the coupled-mode theory of plasmas. The magnetic field and the obliquity of the pump are found to augment the refractive index. The absorption coefficient decreases with increasing scattering angle and achieves a minimum value for the backscattered mode. The analysis reveals that the large refractive index and small absorption coefficient can easily be obtained in a magnetized centrosymmetric semiconducting crystal, and establishes their potentials as candidate materials for fabrication of cubic nonlinear devices.

We present an extensive study of the structure and energetics of monovacancies and antisites in InP. Using a first-principles approach, the different charge states of indium and phosphorus vacancies and antisites are examined. The lattice distortions around the defects are derived fully self-consistently with respect to both electronic and ionic degrees of freedom. Jahn-Teller relaxations, defect-induced one-electron energy levels, and ionization potentials in the band gap are discussed. From the formation energies we predict the favored vacancies and antisites under different stoichiometry conditions.

Measurements of the space-charge grating buildup in the dark are performed in potassium niobate using short pulse excitation at a wavelength of 532 nm. The experiments allow the separate determination of the lifetime of the photoexcited electrons and of their mobility in a number of ${\mathrm{KNbO}}_3$ crystals. The lifetime of (3.9±0.2) ns is shown to be an intrinsic property of ${\mathrm{KNbO}}_3$. Estimates of the electron mobility are refined to (0.5±0.1) ${\mathrm{cm}}^2$ ${\mathrm{V}}^{\mathrm{-}1}$ ${\mathrm{s}}^{\mathrm{-}1}$ in reduced ${\mathrm{KNbO}}_3$. From saturation effects we derive an electron donor photoexcitation cross section of (2.6±1.0)×${10}^{\mathrm{-}17}$ ${\mathrm{cm}}^2$.

Electron paramagnetic resonance (EPR), photo-EPR, optical absorption, and photoluminescence measurements have been made on photorefractive vanadium-doped CdTe. This EPR observation of the ${\mathrm{V}}^{3+}$ ion in CdTe, along with the observed features assigned to ${\mathrm{V}}^{2+}$ in the optical-absorption spectrum, provide direct evidence for the substitutional incorporation of vanadium. Photo-EPR measurements were also conducted and provide direct evidence for the optical activity of ${\mathrm{V}}^{3+}$ under sub-band-gap illumination. A qualitative discussion linking these observations to photoionization channels is presented.

Deuteration of Zn-doped InP was performed by indirect exposure of the InP surface to a remote ${\mathrm{D}}_2$ glow-discharge plasma. The dopant passivation and plasma-induced defects were investigated by photoluminescence (PL) and spectroscopic ellipsometry as a function of temperature, ${\mathit{T}}_{\mathrm{pt}}$, during plasma treatment in the range 30–220 °C. It is found that low ${\mathit{T}}_{\mathrm{pt}}$ causes weak surface alteration whereas high ${\mathit{T}}_{\mathrm{pt}}$ leads to preferential etching of P and the formation of In islands. At an intermediate deuteration temperature ${\mathit{T}}_{\mathrm{pt}}$=90 °C, the surface is less affected by the plasma treatment compared to any other ${\mathit{T}}_{\mathrm{pt}}$. In addition, a broad, low-energy PL band in the range 1.24–1.34 eV shows up. These results are explained by the in-diffusion of P vacancies (${\mathrm{V}}_{\mathrm{P}}$) created at the surface and the subsequent formation of P vacancy-related defects in the bulk of InP. Complexes such as (${\mathrm{V}}_{\mathrm{P}}^{+}$${\mathrm{Zn}}_{\mathrm{In}}^{\mathrm{-}}$) are believed to form and yield the PL band, in analogy with earlier PL studies on GaAs:Zn. The results of secondary-ion mass spectrometry and effusion measurements indicate that the diffusion of D proceeds by trapping on the Zn atoms, thus confirming the dopant passivation seen by PL measurements. In addition, the sample treated at 90 °C contains a significant fraction of excess deuterium near the surface, which passivates the plasma-induced recombination centers. Additional centers are observed in InP deuterated between 90 °C and 170 °C. These centers can be partly annealed by laser irradiation at 77 K. The resulting strong enhancement of the low-energy PL band is interpreted as being due to the rearrangement of ${\mathrm{V}}_{\mathrm{P}}$-related recombination centers into ${\mathrm{V}}_{\mathrm{P}}$-related radiative centers assumed to be ${\mathrm{V}}_{\mathrm{P}}^{+}$ ${\mathrm{Zn}}_{\mathrm{In}}^{\mathrm{-}}$.

Electrical properties of high-dose (1.6×${10}^{16}$ at./${\mathrm{cm}}^2$) ${\mathrm{H}}^{+}$-implanted B-doped silicon have been investigated using transient capacitance spectroscopy, capacitance-voltage, and spreading resistance profiling. The role of hydrogen is twofold: to interact with the defects created by ion implantation, modifying their electrical properties, and to neutralize the shallow-acceptor dopants. The evolution of the defects responsible for the deep levels and the depth of the neutralized region have been investigated after iso- chronal annealing at various temperatures up to 800 °C. Deep-level transient spectroscopy spectra show three hole traps; two of them, H(0.67), H(0.33), have been tentatively identified as vacancy-hydrogen complexes (${\mathrm{VH}}_2$, ${\mathrm{VH}}_3$) while the attribution of the third, H(0.23), detected in the samples annealed at 400–450 °C, is uncertain. As a function of the heat treatment, the total number of defects is strongly reduced at 300 °C, it increases for T>300 °C and at the highest temperatures, namely 800 °C, the defects disappear. The first decrease is attributed to the formation of neutral ${\mathrm{VH}}_4$ complexes and the disappearance to complete decoration of point defects or their agglomeration. The thickness of the passivated region has a minimum at 300 °C, which corresponds to the formation of the stable ${\mathrm{VH}}_4$. At lower temperatures, the hydrogen necessary for the passivation is the unbonded one and presumably comes from the implantation process itself. At higher temperature, H comes from the breaking of the ${\mathrm{VH}}_4$ or ${\mathrm{H}}_{\mathit{n}}$ complexes. These results are in good agreement with our previous studies concerning the role played by hydrogen in affecting the crystal properties of silicon.

We report the experimental determination of the wave-function mixture and composition for hybridized Zeeman states of P in Si and give a related theoretical discussion. The intensity evolution of the Zeeman transitions near the anticrossing region for the isolated P donors in highly pure Si has been measured quantitatively by means of high-sensitivity and high-resolution photothermal ionization spectroscopy. The wave-function compositions and their evolutions with magnetic field for the hybridized Zeeman states of bounded electrons of impurities have been deduced from the measurement. A variational calculation in the framework of the effective-mass approach has been performed to estimate theoretically the wave-function compositions, and a comparison has been made between the theoretical results and experimental ones.

The structural and dynamic properties of carbon defects in aluminum arsenide are investigated using first-principles local-density-functional cluster theory. The method accounts satisfactorily for the structure and phonon modes of AlAs. The carbon acceptor and donor possess triplet modes in the band gap between the acoustic and optic branches as well as localized triplet modes. The local mode of the acceptor lies within 40 ${\mathrm{cm}}^{\mathrm{-}1}$ of the observed mode. Four modes of a close-by acceptor pair lie within 40 ${\mathrm{cm}}^{\mathrm{-}1}$ of the local mode of the isolated acceptor and this supports a previous assignment of four satellite lines seen in heavily doped material to this defect. The modes of a donor-acceptor pair are investigated but there is no evidence of their existence.

Time-resolved luminescence measurements of CdS:In cyrstals in the near-band-gap region are presented for a wide range of indium doping levels, i.e., for concentrations between ${10}^{17}$ and ${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$. Interactions of the indium dopants lead to a broad luminescence band near the band-gap energy and alter the luminescence dynamics. A decrease of the lifetimes of both donor and acceptor bound exciton complexes is observed at medium doping levels (${10}^{17}$–${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$). This is explained by the reduction of the effective binding energies leading to shorter decay constants of these complexes. The decay of the donor-acceptor-pair recombination luminescence is also investigated for indium concentrations below the Mott density. A statistical model proposed by Thomas and Hopfield is used to determine the electronically active impurity concentration. At higher doping levels (up to ${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$) the degeneracy of the conduction band leads to a luminescence band attributed to radiative non-k-conserving band-to-band transitions. The measured lifetimes reflect the number of occupied states in the conduction band and the number of free states in the valence band. A nonexponential fit reveals the concentration of electronically active impurity centers. A correlation between photogenerated and doping-generated carriers is established for high excitation densities.

We study the screening of a macroscopic electric field in a crystalline dielectric. Density-functional perturbation theory provides the static dielectric constant (or tensor) as a bulk property; we give a formulation which extends the local-density approximation, and specifically we discuss its implementation within gradient-corrected schemes. We briefly consider the relevance (if any) of the so-called ‘‘gap problem’’ to static linear response. As a case study, we perform an an initio calculation of the dielectric constant in silicon within a popular gradient-corrected local-density scheme. We find that the gradient corrections reduce the discrepancy found so far between local-density predictions and experiments in covalently bonded materials. The amount of this reduction is sizable if the calculations are performed at the experimental equilibrium lattice constant of the crystal, while, however, it is only marginal when the calculations are carried out, at the calculated lattice constants, consistently within each given theoretical scheme.

We present detailed calculations using the total-energy pseudopotential method in the local-density approximation of the relative stability and pressure-induced behavior of complex tetrahedrally bonded structures formed metastably in silicon and germanium by depressurization from their metallic phases. The corresponding structures in carbon are also investigated. These calculations present the first direct atomistic relaxation of BC8 under the influence of Hellmann-Feynman forces, and the first calculations on the ST12 structure using any form of relaxation. We also present evidence to show that in both Si and Ge the BC8 and ST12 structures are covalently bonded, while the equivalent structures in carbon cannot support such covalent bonding.

We show that a simple pairwise model for covalent materials provides a good description of the high-density fourfold-coordinated metastable phases BC8 and ST12, in agreement with experimental results and ab initio calculations. We use this potential to study the high-pressure behavior and the free energy. We find that both phases are metastable at all temperatures. The relative energetic stability depends on the parametrization, but the ST12 phase has higher entropy. We therefore conclude that with correct pressure and temperature treatment it may be possible to synthesize ST12 as a new high-density metastable phase in Si.