We report measurements down to 16 mK of the electron-spin resonance (ESR) of a sample of silicon doped with phosphorus, of doping density n/${\mathit{n}}_{\mathit{c}}$ (at ambient stress) of 0.85. Stress tuning to 0.28 GPa along the 〈100〉 direction is used to ‘‘tune’’ the critical doping density ${\mathit{n}}_{\mathit{c}}$ at the metal-nonmetal transition and thus vary n/${\mathit{n}}_{\mathit{c}}$ up to the value 0.91. The electron-spin susceptibility and ESR linewidth vary with temperature below 4.2 K in the same way, and the change in this variation with stress over most of the temperature range shows a strong magnetic-field and frequency dependence at the low fields of our experiments (∼10–20 G). This implies that, as the metal-nonmetal transition approaches, the effect of magnetic field in polarizing the electron-spin pairs becomes rapidly weaker. However, at the lowest temperatures, with or without stress, a cusp develops in the susceptibility versus temperature behavior; we speculate that, so close to the metal-nonmetal transition, the exchange interactions are becoming sufficiently long range for spin-glass behavior to develop. Alternatively, the ‘‘rare-spins’’ scenario, coupled by Ruderman-Kittel-Kasuya-Yosida interactions, of Bhatt and Fisher may be driving the spin-glass behavior. At the low end of the temperature range only about 7% of the electron spins remain magnetically active. The rate of variation of the susceptibility with stress (which tunes ${\mathit{n}}_{\mathit{c}}$) at the lowest temperatures is rather fast, with χ varying as ${\mathit{n}}_{\mathit{c}}^4$.

We present a method for electronic band-structure calculations based on the density-functional formalism using, instead of the exchange potential in the local-density approximation (LDA), the exact Kohn-Sham density-functional exchange potential. The method is formulated within the linear muffin-tin-orbital method in the atomic-sphere approximation. We apply the method to the alkaline-earth oxides MgO andCaO. The obtained band gaps, 7.77 eV for MgO and 7.72 eV for CaO, are closer to the experimental values of 7.83 and 7.09 eV, respectively, while the corresponding LDA band gaps of 4.64 and 4.30 eV are too small. To our knowledge, the exact exchange potential is first applied to realistic solids. The exact exchange potentials thus calculated show structures well reflecting the atomic shells.

Electron spin resonance experiments, at low temperature (1.6–4.2 K), of ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Yb}}^{3+}$ in the small-gap semiconductor ${\mathrm{CeFe}}_4$${\mathrm{P}}_{12}$ show cubic-crystal-field ${\mathrm{\Gamma }}_8^{\mathrm{}\left(2\right)\mathrm{}}$ and ${\mathrm{\Gamma }}_6$ ground states, respectively. The sign and ratio between the fourth and sixth order cubic-crystal-field parameters were found to be in agreement with those previously determined for ${\mathrm{Dy}}^{3+}$ and ${\mathrm{Er}}^{3+}$ in the same compound. Dysonian resonance line shapes, characteristic of metallic hosts, were observed in all our experiments. However, no exchange interaction effects between the localized rare-earth magnetic moment and conduction electrons were detected. This result supports the idea that in ${\mathrm{CeFe}}_4$${\mathrm{P}}_{12}$ there is a strong 4f-conduction-electron hybridization with an open gap in the density of states at the Fermi level.

We present as a direct generalization of the recently developed full-potential photoemission theory a relativistic theory for space-filling cell potentials. This theory is based on the well-known one-step model of photoemission. The Dirac equation is solved for space-filling cell potentials, using the relativistic phase functional approach. A practical scheme to calculate the generalized spin-density matrix of the photocurrent is described. The relationship between the new model and the widely used relativistic muffin-tin-like theory, as well as the connection to nonrelativistic full-potential photoemission is discussed. Moreover, a detailed analysis of the angular matrix elements and the selection rules for a single-cell potential in the relativistic and nonrelativistic case are given.

We determine the static dielectric function of the electron gas at selected densities by using variational and fixed-node diffusion quantum Monte Carlo to find the response to a sinusoidal electric field. The dielectric function is then calculated by relating the induced charge density to the change in ground-state energy as a function of the amplitude of the applied field. A released-node Green’s-function Monte Carlo calculation shows that the diffusion results are exact within the determined error bars, apart from the size dependence correction for extrapolation to the thermodynamic limit. The dielectric response is consistent at long wavelengths with the compressibility sum rule derived from a fit to Ceperley and Alder’s Green’s-function Monte Carlo results, but exhibits dfferences from accepted dielectric models at intermediate wave vectors.

Based on ab initio electronic-structure calculations by the linear muffin-tin orbital method and the combined analysis of x-ray photoelectron and x-ray B Kα, O Kα, and Fe Lα emission spectra in a common energy scale, the aspects of hybridization within the valence band of ${\mathrm{FeBO}}_3$ are discussed in detail. X-ray photoelectron spectra of ${\mathrm{B}}_2$${\mathrm{O}}_3$ and ${\mathrm{H}}_3$${\mathrm{BO}}_3$ have also been obtained and are discussed in comparison with that of ${\mathrm{FeBO}}_3$.

Electronic energy bands of metallic ${\mathrm{TiTe}}_2$ are calculated, and the transport masses (n/m${)}_{\mathrm{\alpha }\mathrm{\beta }}$ are evaluated. Comparison with the measured resistivity shows that this is an ordinary metal with a moderate electron-phonon interaction in the usual quasiparticle gas description. However, photoemission line shapes as measured by Claessen et al. are not consistent with this picture, and are used to predict a resistivity that disagrees with experiment. Similarities to high-${\mathit{T}}_{\mathit{c}}$ superconductors are pointed out. Possible resolutions are discussed.

Local distortions and optical properties induced by the substitutional ${\mathrm{Cr}}^{3+}$ impurity in the MgO host lattice are computed using the embedded-cluster approach implemented in the icecap code. Within this methodology, the impurity centered cluster is described quantum mechanically and is self-consistently coupled with the lattice environment by means of the shell-model treatment of the lattice polarization and distortion. Our calculations predict inward relaxations of the six ${\mathrm{O}}^{2\mathrm{-}}$ nearest neighbors surrounding the ${\mathrm{Cr}}^{3+}$ ion for both cubic and noncubic (tetragonal and orthorhombic) configurations in MgO. For the cubic configuration, selected low-lying excited states, including the 10Dq generator ${}_{}{}^4{}_{}{}^{}$${\mathit{T}}_2$, are calculated at several Cr-O separations. After taking into account lattice relaxations and correlation corrections, the computed 10Dq value lies 0.19 eV lower than the experimental one. Finally, the response of the Dq parameter and the Cr-O separation to hydrostatic pressure is obtained by performing analogous cluster-in-the-lattice calculations with the lattice parameter of MgO varying according to its equation of state. Analysis of these results reveals that the compressibility at the host cation site is greater than that at the impurity site.

Porous silicon has been studied by combined photoluminescence and photomodulated infrared spectroscopy, either in situ, during a postanodization etching treatment in HF solution, or under gaseous ambient, after the etching treatment. It has been found that during the etching treatment, porous silicon exhibits a weak yellow-green luminescence, whereas it becomes strongly red luminescent as soon as it is removed from the etching solution. This change of behavior is associated with the wetting-nonwetting of the porous silicon surface by the etching solution. When porous silicon is in contact with either HF solution or ambient, no Si-O vibration is present in the infrared spectra, which rules out the presence of siloxenelike compounds at the surface of our samples. Also, upon sample aging, no correlation is found between SiH absorption and photoluminescence intensity, which dismisses the role of polysilanelike compounds in luminescence. The electronic states occupied by photoexcited carriers in porous silicon exhibit different infrared absorption in etching solution and in ambient. In wet porous silicon, most of the photocarriers thermalize in delocalized, long-lived, nonradiative states. On the contrary, photocarriers in dry porous silicon may be efficiently captured by localized states involved in the red-luminescence process. These states exhibit a broad infrared absorption whose maximum energy lies in the 150–200-meV energy range and shifts similarly to that of photoluminescence. Finally, an unambiguous evidence of the diffusion of photocarriers in wet porous silicon is obtained. A diffusion length of ∼1 μm is determined.

The scalar relativistic version of an accurate first-principles full-potential self-consistent linearized muffin-tin orbital (LMTO) method has been employed for describing the physical properties of the II-VI semiconducting compounds, ZnS, ZnSe, and ZnTe. The presently employed modified version of the LMTO method is quite fast and goes beyond the usual LMTO-ASA (atomic-sphere approximation) method in the sense that it permits a completely general shape of the potential and the charge density. Also, in contrast to LMTO-ASA, the present method is capable of treating distorted lattice structures accurately. The calculated values of the lattice parameters are within 1.5% of the experimental values. The calculated values of the bulk modulus and the elastic constants are in good agreement with the available experimental data. The values of the phonon frequencies at some symmetry points are also in close agreement with the experimental data wherever available.

Electronically conductive polymers can be synthesized within the pores of microporous membranes to yield microscopic tubules of the polymer. We have investigated the temperature dependence of resistance of thin films prepared from microtubules composed of polyaniline and polypyrrole. All resistance data obtained followed the Mott variable-range hopping (MVRH) model. However, films prepared from large-diameter tubules (400 nm) showed three-dimensional (3D) MVRH, while films prepared from small-diameter tubules (≤100 nm) showed 2D MVRH. Hence, we have demonstrated the ability to reproducibly vary the dimensionality of conduction in a conductive polymer film.

We consider a kinetic semiphenomenological approach to (a) the diffusion processes in Si and Ge, (b) the compensation effect (CEF) associated with the linear dependence ln${\mathit{D}}_0$=aΔE+C between the Arrhenius activation energy ΔE and preexponential factor ${\mathit{D}}_0$, and (c) the diffusion-melting correlation in Si and Ge linking the diffusion and melting parameters. The proposed model is based on the kinetic many-body theory of thermally activated rate processes in solids. The model considers picosecond atomic and electronic phenomena occurring in the nanometer vicinity of the diffusing atom, which are synchronized with the diffusion event of duration Δ${\mathrm{\tau }}_{\mathit{d}}$=${10}^{\mathrm{-}13}$–${10}^{\mathrm{-}12}$ s. The following main results are obtained: (i) The Arrhenius-like equation for the diffusion coefficient D is derived from the kinetic consideration of the diffusion event without invoking the equilibrium rate theory. (ii) ΔE and ${\mathit{D}}_0$ are expressed in terms of local parameters that characterize the picosecond atomic and electronic processes induced by short-lived large energy fluctuations of diffusing atoms that occur in their vicinity during Δ${\mathrm{\tau }}_{\mathit{d}}$. (iii) The large range of observed variations in the prefactor ${\mathit{D}}_0$ (about five orders of magnitude) is explained and related to the observed variations in ΔE. (iv) The observed linear CEF dependence between ln${\mathit{D}}_0$ and ΔE is explained and the coefficients a and C in the CEF equation are expressed in terms of the melting temperature ${\mathit{T}}_{\mathit{m}}$ and entropy Δ${\mathit{S}}_{\mathit{m}}$, in good agreement with experimental data.

We use a Green’s-function technique to calculate the position of deep defects in narrow-gap semiconductors. In order to predict chemical trends, we examine the effects of several different chemical elements. Substitutional (including antisite), (ideal) vacancy, and interstitial (self and foreign) deep defects are considered. The compounds considered are mercury cadmium telluride (MCT), mercury zinc telluride (MZT), and mercury zinc selenide (MZS). The effect of relaxation of neighbors is considered for the substitutional and interstitial cases. Relaxation effects can be greater for the interstitial case than for the substitutional one. For all cases we find deep defects in the energy gap only for cation-site s-like orbitals or anion-site p-like orbitals, and for the substitutional case only the latter are appreciably effected by relaxation. For substitutional impurities in MCT, MZT, and MZS, we consider x (the concentration of Cd or Zn) in the range 0.1<x<0.3 and also for both the substitutional and interstitial cases we do extensive calculations for x values appropriate to a band gap of 0.1 eV. Specific results are given in figures and tables and comparison to experiment and other calculations is made in a limited number of cases. For the substitutional case we find that I, Se, S, Rn, and N are possible defect candidates to form cation-site, s-like levels in MCT, and Zn and Mg are for anion-site, p-like levels.

Similarly, in MCT for the interstitial case we find deep defect levels in the band gap for Au, Ag, Hg, Cd, Cu, and Zn for the cation site, and N, Ar, O, and F for the anion site. For the substitutional cases we have some examples where relaxation moves the levels into the band gap, whereas for interstitial cases we have examples where relaxation moves them out of the band gap. We find that the chemical trends of defect levels in MZT are similar to that in MCT. However, the same conclusion does not hold for MZS. We have also used perturbation theory (see the Appendix) to look at the effect of nonparabolicity on shallow donor levels, and find it can increase the binding by 10% or so. Although the absolute accuracy of our results is limited, the precision is good, and hence chemical trends are accurately predicted. Further work involves calculating the effect of charged-state interactions and the effect of relaxation on vacancy levels.

Upon irradiating poly(p-phenylene vinylene) thin films with white light in air (total exposure ∼5 J/${\mathrm{cm}}^2$), an enhancement of about forty times in photocarrier generation and a similar reduction in the photoluminescence intensity are observed. The wavelength and electric-field dependence of the photoconductivity are, however, essentially unchanged. These results demonstrate the sensitivity of the conjugated polymers to photo-oxidation and the importance of extrinsic photogeneration processes in these materials. The microscopic physics of carrier generation and luminescence quenching are established, and we discuss the implications for various conjugated polymer devices.

The planarity of the aromatic stage of two-dimensional Si and Ge layers are theoretically investigated by first-principles total-energy calculations. While a C atom prefers to form the flat aromatic stage, i.e., graphite, Si and Ge prefer to form the corrugated aromatic stage. Si can be said to be the critical element by which the corrugated stage is formed.

We present the results of a direct comparison between experiment and theory of the temperature dependencies of the integrated intensities and spectral linewidths in the quasielastic light-scattering spectra from the electron gas in III-V semiconductors. An explicit expression for the inelastic-electronic light-scattering cross sections in semiconductors with nonparabolic dispersion of energy bands is given for a realistic hydrodynamic free-electron-gas model. We find rather different effects of single-particle electronic excitations on polarized and depolarized scattering in n-InP in the temperature range 27–300 K with near infrared excitation of the spectra. The results provide clear evidence for the existence of strong temperature-dependent free-electron-gas fluctuations and prove unambiguously the observation of light scattering from energy- and momentum-density fluctuations.

We have measured the ac susceptibility, specific heat, resistivity, and thermal conductivity of single-crystalline samples of FeSi in various temperature ranges between 0.04 and 300 K. The unequivocal result emerges from the ac susceptibility data, and is that there is no evidence for any magnetic ordering, at least down to 40 mK. The specific heat contains a linear contribution visible between 1.5 and 3.5 K, and a slow upturn below 1.5 K. This latter feature, and the observed Curie behavior of the ac susceptibility can both be quantitatively accounted for within a model of Anderson-localized states associated with impurities, at the level ${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$. However, the electrical conductivity, which is nowhere activated, appears to saturate below about 5 K, which would suggest a metallic state. We draw attention to numerous similarities between our data and previously published data for the rare-earth compound ${\mathrm{SmB}}_6$.

The energy spectrum of a narrow-gap ordered alloy is calculated using the three-band Kane model. It is shown that when the energy gap, reduced by ordering, becomes negative, the transition to a gapless, rather than a semimetallic, state occurs. In order to study the effects beyond the Kane model, the exact equation for the energy spectrum including the finite hole mass and the band warp is derived. The stability of a gapless state with respect to these higher-band corrections is demonstrated.

The lattice constant, bulk modulus, and single-particle eigenvalues of Si, Ge, and GaAs are evaluated by using the local-density approximation (LDA) and the generalized-gradient approximation (GGA) for the exchange-correlation energy. We consider two different generalized-gradient functionals, the exchange correlation recently proposed by Perdew and Wang and the Becke exchange combined with a previous version of the Perdew-Wang correlation. We perform an all-electron calculation with the linear-augmented-plane-wave method to test the capability of these generalized-gradient exchange-correlation functionals to describe structural properties of semiconductor systems. Our results differ from previous calculations of the same quantities within the pseudopotential scheme although the qualitative trends are the same. The LDA yields lattice constants that are smaller by less than 0.6% than the experimental values. Both GGA’s overcorrect, giving lattice constants that are larger than the experimental ones by about 2% for the earlier GGA and by about 1.5% for the later GGA. The LDA predicts bulk moduli that are within 3% of the experimental value whereas the GGA’s underestimate the bulk moduli by 14–22 %. The underestimate of the LDA band gaps is not improved by either of the GGA’s.

The structural response of diamond-cubic silicon under hydrostatic compression is analyzed using both an empirical interatomic potential proposed by Tersoff and ab initio total-energy calculations. The stability analysis based on elastic stiffness coefficients (which are generalized elastic constants at finite strain) gives a critical pressure of 64 GPa at which the ideal lattice becomes unstable against homogeneous tetragonal shear deformation. This prediction is explicitly verified by direct molecular-dynamics simulation, which shows that the instability causes the lattice to transform to the β-Sn structure. It is demonstrated that the large value of the transition pressure implies an intrinsic activation barrier, for which the Tersoff potential results agree well with ab initio calculations. This barrier exists for a transformation involving ideal crystal lattices that, while uniformly deformed from the equilibrium diamond-cubic structure, do not contain any point or extended defects. We suggest that, due to the neglect of lattice defects, the critical pressure corresponding to this transformation represents the upper bound on the limit of stability. A lower bound, corresponding to the barrierless transition, is obtained by equating free energies of different phases. Lower bounds calculated with the Tersoff potential and present ab initio results are 12.7 and 7.8 GPa, respectively, close to the range of experimental values for the diamond-cubic to β-Sn transition under compression. To support our conjecture on the role of defects we invoke certain analogies between the studied structural transitions in crystalline solids and melting.