A vortex structure consisting of vortex chains embedded into the flux-line lattice (FLL) has been observed in ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$${)}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals using the Bitter decoration technique. This structure formed in magnetic fields applied at a high angle, between 50° and 80°, with respect to the c^ axis. Although the vortex chains–FLL mixture looked similar to the vortex structure observed earlier in the highly anisotropic ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$, its evolution with the tilt angle and the applied field demonstrated that it represents an essentially different vortex state which, unlike ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$, originates from the attractive vortex-vortex interaction. The coexistence of the vortex chains with the FLL is attributed to an incomplete transition from the isotropic vortex lattice to the pure chain state during the sample cooling, owing to pinning.

The electrical resistivity and the Hall effect have been measured on oriented single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ in magnetic fields up to 10 T and temperatures from 2 to 300 K. In the normal state the anisotropic, transverse relaxation rates determined from the Hall angle for I∥a and b agree with those obtained from NQR for Cu(2) and Cu(1), respectively, and indicate the formation of a spin gap. In the mixed phase the Hall effect changes its sign twice, but only when I is parallel to the chains. An interpretation in terms of a two-band model is given.

An orthorhombic-to-monoclinic phase transition with associated (100) twinning has been observed by transmission electron microscopy in superconducting ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{y}}$ single crystals after heating to between 350 and 400 °C. A model describing the monoclinic phase structure and the twin boundary geometrical configuration is presented. We propose that the monoclinic phase transition results from oxygen ordering in the ${\mathrm{CuO}}_2$ planes associated with oxygen diffusion out of the crystal at high temperature and high vacuum. This transition is accompanied by (100) twinning to reduce the strain energy.

A sequence of doped fcc or doped rhombohedral, bct, bcc, and K-saturated fcc phases was observed in ${\mathrm{C}}_{70}$${\mathrm{K}}_{\mathit{x}}$ with increasing K concentration using x-ray-diffraction measurements. A model for the structure of the K-saturated fcc phase ${\mathrm{C}}_{70}$${\mathrm{K}}_9$ is proposed. Superconductivity in each phase was checked using ac magnetic-susceptibility measurements. A small diamagnetic transition was observed below 12 K in the doped rhombohedral phase. All other phases were nonsuperconducting down to 1.3 K.

The Raman spectra of pristine ${\mathrm{C}}_{70}$ films and ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{70}$ doped to the maximum-conductivity phase (nominally ${\mathrm{K}}_4$${\mathrm{C}}_{70}$) are studied. All the Raman modes for ${\mathrm{C}}_{70}$ are only moderately broadened upon K doping, distinctively different from the superconducting ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ where several ${\mathit{H}}_{\mathit{g}}$ modes are broadened to the extent that they have almost disappeared from the spectrum. Assuming that the electron-phonon interaction is responsible for the broadening, the electron-phonon coupling strength is found to be comparable to the Coulomb repulsion. These results are important for understanding the absence of superconductivity in ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{70}$.

In the nominally hole-doped Mott insulator system ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{TiO}}_3$, the metal-insulator transition takes place near x=0.4. A thermal hysteretic behavior of the resistivity is observed in the immediate vicinity of the metal-insulator phase boundary. Application of pressure tends to suppress the quasireentrant (metal-insulator-metal) transition and to stabilize the metallic phase. Measurements of the Hall coefficient and the electronic specific heat indicate that the barely metallic phase at about x=0.4 is characterized by an enhancement of carrier mass rather than by a reduction of carrier density.

According to the theory for the thermal shift of the crystal-field spectra of rare-earth or transition-metal ions, the thermal shift includes four contributions of different characteristics. By use of this theory, the R-line thermal shift of MgO:${\mathrm{V}}^{2+}$ has been calculated by fitting the experimental data. The results show very good agreement with the thermal-shift experimental data and a series of other experimental data of MgO:${\mathrm{V}}^{2+}$. Furthermore, the values of the most important parameters have been calculated theoretically with wave functions obtained from diagonalization of complete ${\mathit{d}}^3$ energy matrices. They are in very good agreement with the values acquired from fitting the experimental data. For MgO:${\mathrm{V}}^{2+}$, the results similar to those of MgO:${\mathrm{Cr}}^{3+}$ are obtained: the ${\mathit{t}}_2^{32}$${\mathit{T}}_1$ term (resulting in redshift) is the most important one; the Raman term (resulting in blueshift) eliminates part of the ${\mathit{t}}_2^{32}$${\mathit{T}}_1$ term, their algebraic sum gives the contribution of the electron-phonon interaction of the acoustic branches; the contribution of the optical branches (resulting in redshift) increases rapidly with temperature, but it is still slightly less than that of the acoustic branches at 333.5 K; the contribution by thermal expansion (resulting in blueshift) is about 1/3 of the total thermal shift in absolute value. Some differences between the results of MgO:${\mathrm{V}}^{2+}$ and those of MgO:${\mathrm{Cr}}^{3+}$ are briefly discussed.

We perform a thorough analysis of the critical behavior of the density of states at the metal-insulator transition within the framework of the field-theoretic renormalization group. Of the previous, conflicting results obtained by resumming the perturbation theory, and by frequency-momentum-shell renormalization-group methods, we agree with the one yielding conventional power-law behavior. Emphasis is placed on a detailed technical derivation of this result, and on peculiarities in the renormalization-group description that arises from the long-ranged nature of the Coulomb interaction, and from the presence of multiple time scales in the problem.

The temperature dependence of the thermoelectric power was investigated for the emeraldine salt form of polyaniline protonated with camphor sulfonic acid (PANI-CSA) and for blends of PANI-CSA with poly(methyl methacrylate) (PANI-CSA/PMMA). The thermoelectric power (S) of PANI-CSA increases linearly in proportion to the temperature with S≊+10 μV/K at room temperature. This characteristic behavior persists when the fraction of PANI-CSA is diluted as a blend in insulating PMMA, even at volume fractions of PANI-CSA as low as about 1%. We discuss the PANI-CSA results in the context of transport at the metal-insulator boundary. The data from the blends are consistent with charge transport along the self-organized interconnected pathways of PANI-CSA in PMMA.

There is a great interest in the properties of electrons in nanostructures with superposed potentials varying slowly in space, or with slowly graded composition, or both. The quantum mechanics of these electrons, moving in nearly periodic potentials, is conveniently described in a basis of localized generalized Wannier functions, similar to the conventional Wannier functions for strictly periodic potentials. This has led us to consider generalized Wannier functions for (i) weakly perturbed and (ii) compositionally graded crystals. We first construct generalized Wannier functions for uniform crystals with an everywhere weak perturbation. The corrections to an unperturbed Wannier function of a given band may be chosen to involve only the unperturbed Wannier functions from other bands. We then consider crystals with slowly varying composition, and construct generalized Wannier functions from the Wannier functions of uniform crystals.

Starting with an exact expression for the quasiparticle decay rate due to the phonon-mediated electron-electron interaction, we derive a simple approximate formula for the magnitude of its ${\mathit{T}}^2$ coefficient at low temperatures. Comparison with measured decay rates in Cd and several nonmagnetic transition metals yields agreement to within at worst a factor of 2. Similar agreement holds for estimated electrical resistivities.

Weighted-density-approximation calculations of ground-state properties of diamond, Si, GaAs, V, and Mo, using two different forms of the coupling-constant averaged pair-correlation function, g¯, are reported. Substantial improvements over the local-density approximation are found for all materials when a form of g¯ that accurately models the uniform electron gas is used.

We present the results of transient photoconductivity measurements on highly oriented trans-polyacetylene. Using a photoconductive switching method utilizing picosecond optical pulses we have improved the temporal resolution of the photoresponse by one order of magnitude. We observe a fast temperature-independent photocurrent, which decays within a few picoseconds and reflects the intrinsic motion of charge carriers along the chains. Estimation of the mobility yields values of the order 1 ${\mathrm{cm}}^2$/V s, in good agreement with theoretical calculations. By comparison of the data to that obtained from recent transient photoinduced midgap absorption experiments, we conclude that the fast photocurrent is mainly due to polarons.

The existence of nonscattered states in random systems composed of trimers in their generalized form and a second element is studied in the context of a tight-binding Hamiltonian. Many conditions for the existence of such states in this generalized random trimer model are obtained by introducing appropriate structural correlations between the host element and the trimer. This aspect of the correlated-disorder systems has not been studied before to our knowledge. The condition for tuning the positions of the two sets of nonscattered states is obtained. We further show that the two resonances of a single trimer in the host lattice can be merged at a single energy. For this case, when the resonance energy is inside the host band, the width of the nonscattered states is found to decay as ∼${\mathit{N}}^{\mathrm{-}1/4}$. $N— is the number of sites in the system. Experiments to observe the broadening of the width are proposed. In all other cases width is shown to decay as ∼${\mathit{N}}^{\mathrm{-}1/2}$.

Using the transfer-matrix technique for the propagation of electromagnetic waves in dielectric structures, we calculate the transmission coefficient versus the frequency of the incident wave for different polarizations in two-dimensional periodic and/or random arrangements of dielectric cylinders. This technique has been applied to cases where the plane-wave method fails or becomes too time consuming, such as when the dielectric constant is frequency dependent or has a nonzero imaginary part, and when defects are present in an otherwise periodic system. For all the cases studied, the results compared well with experiment.

We present a study of the Van Vleck paramagnets ${\mathit{A}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Cr}}_{\mathit{x}}$B and ${\mathit{A}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$B, where AB is a II-VI semiconductor having the zinc-blende or the wurtzite structure. In cubic zinc-blende hosts the nonlinear magnetization M depends on the orientation of the external magnetic field B with respect to the cubic axes of the crystal. We show that, while for iron M is largest when B is along a cubic axis and a minimum for B∥[111], for chromium the anisotropy is reversed and the largest value of M occurs for B∥[111]. Expansions of the angular variation of the magnetization in terms of combinations of spherical harmonics invariant under the operations of the symmetry group (${\mathit{T}}_{\mathit{d}}$ or ${\mathit{C}}_{3\mathit{v}}$) of the site of the impurity are used to analyze the anisotropy. The magnetic-field dependence of the coefficients in the expansion is investigated. The magnetic susceptibility of ${\mathrm{Cr}}^{2+}$ in wurtzite-structure hosts is expected to be particularly large.

The electronic band structure of the semiconducting β and γ polytypes of InSe is calculated from first principles, with spin-orbit effects included. The two polytypes differ by the lateral stacking arrangement of four-atomic-plane building blocks, composed of Se-In-In-Se planes. These building blocks are terminated by Se lone-pair orbitals, which form the electronic states at the valence-band maximum. There is very little bonding between these four-atomic-plane units, and so the band structures of the γ and β polytypes are nearly identical, being related simply by zone folding. We find that both materials have direct band gaps (which occur at the Γ and Z points in the Brillouin zone for the β and γ polytypes, respectively), and that there is a large dipole matrix element (i.e., a strong oscillator strength) between the valence maximum and conduction minimum states. These materials are thus potentially significant for light emitting and absorbing devices. In order to study the optical absorption at energies above the band gap, the imaginary part of the dielectric function ${\mathrm{\epsilon }}_2$(ω) is calculated for ω≤12 eV. The effective masses are also calculated: the out-of-plane masses are extremely light, and are equal in magnitude at the valence maximum and conduction minimum states (${\mathit{m}}_{\mathrm{\perp }}^{\mathit{c}}$=${\mathit{m}}_{\mathrm{\perp }}^{\mathit{v}}$≊0.03${\mathit{m}}_0$), while the in-plane masses are light for the electrons (${\mathit{m}}_{\mathrm{\parallel }}^{\mathit{c}}$≊0.1${\mathit{m}}_0$) and heavy for the holes (${\mathit{m}}_{\mathrm{\parallel }}^{\mathit{v}}$≊3${\mathit{m}}_0$). The ‘‘anomalous’’ mass character (${\mathit{m}}_{\mathrm{\perp }}$<${\mathit{m}}_{\mathrm{\parallel }}$) is explained in terms of intralayer bonding and the lack of lateral bonding between the Se lone-pair orbitals that consitute the valence maximum states.

High-resolution measurements show that the KLV Auger spectra of Si have a three-peaked structure. The experimental results are compared with calculations of the Auger profiles based on results for the local screened density of states (DOS) on a core-ionized site obtained from two theoretical approaches; self-consistent calculations within the linear-muffin-tin-orbital–atomic-spheres-approximation scheme and a parametrization of the ground-state DOS. Both approaches reproduce the three-peaked structure of the KLV spectral profiles but give results which are less bound than experiment. It is argued that the discrepancies between theory and experiment point to the need for a more sophisticated theory of electron correlation in Si.

The electronic properties of substitutional 3d transition-metal impurities in II-VI semiconductors have been studied using the cluster and Anderson impurity models with configuration interaction. It is shown that the photoemission and inverse-photoemission spectra, d-d optical-absorption spectra, exchange interaction between the 3d magnetic moment and the host band states, and donor and acceptor ionization energies can be reproduced with the same set of parameters, which show systematic variation with expected chemical trends. The importance of multiplet effects in the formation of donor and acceptor levels within the band gap is demonstrated.

Optically detected magnetic resonance (ODMR) of Zn-doped InP under uniaxial stress reveals the spectrum of the Zn acceptors. The zero-stress spectrum consists of a strong ODMR from native ${\mathrm{P}}_{\mathrm{In}}$ antisites. Uniaxial stress was applied in the [11¯0] direction perpendicular to the magnetic field. With the magnetic field in the [110], a new resonance appears for a stress of 160 MPa, which sharpens and shifts as the stress is increased to 400 MPa. The anisotropy of the signal was measured by performing the experiment with the field in the [001] direction. An analysis in terms of a Hamiltonian for a spin-3/2 acceptor gives ${\mathit{g}}_1$=0.869(3) and ${\mathit{g}}_2$=0.031(2) at 400 MPa. These values are compared to those for Zn in other III-V semiconductors and to a measurement of the Luttinger parameter for InP. This observation confirms the assignment of the 0.80-eV emission in InP:Zn to ${\mathrm{P}}_{\mathrm{In}}$-Zn pairs. When this result is combined with the published result from excitation spectroscopy, the ${\mathrm{P}}_{\mathrm{In}}^{+}$/${\mathrm{P}}_{\mathrm{In}}^{2+}$ energy level is (1.1±0.1) eV above the valence band.