We report time-resolved x-ray-scattering measurements of the transient structural response of the sliding ${\mathbf{Q}}_1$ charge-density wave (CDW) in ${\mathrm{NbSe}}_3$ to a reversal of the driving electric field. The observed time scale characterizing this response at 70 K varies from ∼15 msec for driving fields near threshold to ∼2 msec for fields well above threshold. The position and time-dependent strain of the CDW is analyzed in terms of a phenomenological equation of motion for the phase of the CDW order parameter. The value of the damping constant, γ=(3.2±0.7)×${10}^{\mathrm{-}19}$ eV sec A${\mathrm{̊}}^{\mathrm{-}3}$, is in excellent agreement with the value determined from transport measurements. As the driving field approaches threshold from above, the line shape becomes bimodal, suggesting that the CDW does not depin throughout the entire sample at one well-defined voltage.

Antiferromagnetic electronic-structure calculations of the perovskite fluorides KM${\mathrm{F}}_3$ (M=Mn,Fe,Co,Ni) carried out using the linear muffin-tin orbital method are reported here. The calculations have successfully predicted the insulating gaps of the systems ${\mathrm{KMnF}}_3$ and ${\mathrm{KNiF}}_3$. However, in the cases of ${\mathrm{KFeF}}_3$ and ${\mathrm{KCoF}}_3$ the insulating gap could not be accounted for by these calculations and this is found to be due to the fact that the spin-density-functional theory is inadequate to show the splitting of the M ${\mathit{t}}_{2\mathit{g}}$ bands. However, the magnetic moments obtained from the calculations are comparable to the experimental values, and also the calculated density of states show good agreement overall with the photoemission data.

We present a study of the local-field factors for the homogeneous, isotropic, three-dimensional interacting electron liquid as a function of momentum, imaginary frequency (iw), and density (${\mathit{r}}_{\mathit{s}}$). A variational approach is used to solve integral equations for the density-density and spin-spin response functions, and it provides approximations to the local-field factors which are exact in the high-density limit. We derive sum rules which show that for large iw, the local-field factors possess maxima (in both q and iw) whose magnitudes are related to the pair-distribution function evaluated at zero separation. We introduce a parametrization scheme which incorporates these sum rules with the known compressibility, susceptibility, and third-moment sum rules. The local-field factors are then used to calculate the effective electron-electron interaction using the Vignale-Singwi formalism [Phys. Rev. B 32, 2156 (1985)]. It is found to be significantly larger than that predicted by Hubbard-type local-field factors for small to intermediate values of the wave vector and imaginary frequency.

The conserving approximation includes a correct mix of the self-energy Σ(p) and the vertex function Γ(p,p+q). Only keeping the exchange self-energy ${\mathrm{\Sigma }}_{\mathit{x}}$(p), we obtain the self-consistent equation for the vertex function ${\mathrm{\Gamma }}_{\mathit{x}}$(p,p+q) in the closed form, which can be solved numerically. Using this vertex function, the polarization ${\mathit{P}}_{\mathit{x}}$(q) is obtained. We evaluate the long-wavelength limit of ${\mathit{P}}_{\mathit{x}}$(q) in the static case at both zero and nonzero temperatures. The latter case is compared with results obtained using the density-functional approach.

Conductivity measurements were made on lithium-intercalation complexes ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{NiPS}}_3$ both during the intercalation process and after the intercalation has ceased. The intercalation process was characterized by means of the conductivity measurements as a function of the intercalation time. Results for equilibrated samples are presented as a function of both the temperature and the lithium concentration. An overall and quick increase in the electrical conductivity, a contemporaneous decrease in the activation energy, and a degenerate semiconducting behavior at high lithium contents were observed. All these effects suggest that, with respect to the pure ${\mathrm{NiPS}}_3$, a new conduction mechanism occurs at a different energy level from the very beginning of the intercalation process. Taking the probable nickel reduction into account, a possible identification of the lithium 2s electron-accepting levels is proposed. In the early intercalation times a simple empirical model is formulated to correlate the data as a function of the intercalation time and as a function of temperature.

In this paper we present a comprehensive investigation of electrical resistivities [ρ(T)] of the γ-phase pseudobinary Fe alloy series ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Ni}}_{80\mathrm{-}\mathit{x}}$${\mathrm{Cr}}_{20}$ in the temperature range 0.4–300 K. These alloys exhibit exotic magnetic behavior at and near the critical composition for ferromagnetism ${\mathit{x}}_{\mathit{c}}$≃59–63. Special attention has been given to the critical region (x≃${\mathit{x}}_{\mathit{c}}$) where we find sharp changes in ${\mathrm{\rho }}_0$ (zero temperature resistivity) as well as in the temperature dependence of ρ(T). These observations have been explained as arising due to the absence of long-range magnetic order and a change in effective carrier concentration at x≃${\mathit{x}}_{\mathit{c}}$. For all x (≤66) these alloys exhibit a resistivity minimum at T=${\mathit{T}}_{\min }$. ${\mathit{T}}_{\min }$ continuously decreases from approximately 35 K for x≊12 to approximately 6 K for x≊59. ρ(T) for T<${\mathit{T}}_{\min }$ shows a rise (ρ∼- √T ) that does not depend on the magnetic state of the alloy. For ${\mathit{T}}_{\min }$<T<80 K, ρ(T) away from ${\mathit{x}}_{\mathit{c}}$ follows a predominantly ${\mathit{T}}^2$ dependence arising from electron–spin-wave scattering which shows a drastic reduction for x≊${\mathit{x}}_{\mathit{c}}$. We also compare our data with an α-phase ${\mathrm{Fe}}_{25}$${\mathrm{Cr}}_{75}$ alloy.

Magnetic susceptibility, resistivity, Seebeck, Hall, and powder x-ray and neutron-diffraction measurements were used to characterize single crystals of FeSi and polycrystalline samples of ${\mathrm{Fe}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ir}}_{\mathit{x}}$Si for x<0.2. The Rietveld refinement of low-temperature powder neutron-diffraction data on FeSi showed no change in the space group and no structural anomalies from 4 to 300 K. Magnetic and transport data from 4 to 700 K are consistent with the characterization of FeSi as a narrow-gap semiconductor (${\mathit{E}}_{\mathit{g}}$=1200 K) with strong intrasite correlations for the states just below and above the gap. Fits to the magnetic susceptibility and resistivity data suggest that the magnetic (or direct) gap may be larger than the transport (indirect) gap. Electron mobilities in FeSi are very low (3–5 ${\mathrm{cm}}^2$/V s). The thermopower of FeSi has a large positive peak (500 μV/K) at 50 K that is attributed to an unusually strong phonon-drag mechanism. Iridium acts as an electron donor in the ${\mathrm{Fe}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ir}}_{\mathit{x}}$Si alloys. As the iridium doping level is increased, there is a rapid decrease in the low-temperature resistivity and a large negative (-140 μV/K) phonon-drag contribution to the thermopower. For Peltier cooling applications, a maximum value for ZT of 0.07 was found for a ${\mathrm{Fe}}_{0.95}$${\mathrm{Ir}}_{0.05}$Si alloy at 100 K.

The electronic structure and equilibrium geometry of the pyrite form of ${\mathrm{FeS}}_2$ was studied using self-consistent density-functional theory comparing the local-density-approximation (LDA) and the generalized-gradient-approximation (GGA) forms of the exchange-correlation functional. The calculated contour map of the electron-deformation density is consistent with analysis of high-resolution x-ray-diffraction data, showing excess charge in nonbonding d states on the Fe sites. At the experimentally measured geometry, the cohesive energy is calculated to be 16.7 eV/${\mathrm{FeS}}_2$ and 9.4 eV/${\mathrm{FeS}}_2$ for the LDA and GGA forms, respectively; the GGA result comparing well with the experimental value of 10.7 eV/${\mathrm{FeS}}_2$. Optimizing the cohesive energy as a function of geometry, both the LDA and GGA calculations find the optimal structure to be expanded relative to the experimental geometry with the lattice constant expanded by 1% and the S-S bond length expanded by 6%, corresponding to a 0.1-eV increase in the cohesive energy for each ${\mathrm{FeS}}_2$ unit. Similar results were obtained for ${\mathrm{RuS}}_2$.

A phenomenological Ginzburg-Landau theory of charge-density waves is presented. It yields, in the absence of pinning impurities, a manifold of stable (equal-energy) states that are independent of the phase of the wave. The pinning of these waves by impurities (both weak and strong pinning) is examined in detail and spectra for various impurity distributions are analyzed in detail. Strong dependence on the wave phase, change in harmonic content, and phase shifts are common features, in addition to the appearance of a continuous background. There is also a plethora of metastable states, many of which have energies close to the ground state. Comparison between the proposed theory and experiment in ${\mathrm{NbSe}}_3$ is satisfactory.

We define a current-conserving approximation for the local conductivity tensor of a disordered system which includes the effects of weak localization. Using this approximation we show that the weak localization effect in conductance is not obtained simply from the diagram corresponding to the coherent backscattering peak observed in optical experiments. Other diagrams contribute to the effect at the same order and decrease its value. These diagrams appear to have no semiclassical analogues, a fact which may have implications for the semiclassical theory of chaotic systems. The effects of discrete symmetries on weak localization in disordered conductors are evaluated and compared to results from chaotic scatterers.

We use semiconductors as an example to show that quantum chaos manifests itself in the energy spectrum of crystals. We analyze the ab initio band structure of silicon and the tight-binding spectrum of the alloy ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As and show that some of their statistical properties obey the universal predictions of quantum chaos derived from the theory of random matrices. Also, the Bloch momenta are interpreted as external, tunable, parameters, acting on the reduced (unit cell) Hamiltonian, in close analogy to Aharonov-Bohm fluxes threading a torus. They are used in the investigation of the parametric autocorrelator of crystal velocities. We find that our results are in good agreement with the universal curves recently proposed by Simons and co-workers.

We consider spinless electrons in two dimensions with the bare spectrum ${\mathrm{\epsilon }}_{\mathbf{p}}$=${\mathit{v}}_{\mathit{F}}$(‖${\mathit{p}}_{\mathit{x}}$‖+‖${\mathit{p}}_{\mathit{y}}$‖). In momentum space, the interactions among electrons have a finite range ${\mathit{q}}_0$, which is small compared to the Fermi momentum. A golden-rule calculation of the electron lifetime indictes a breakdown of Landau’s Fermi-liquid theory in the model. At the one-loop level of perturbation theory, we show that the density wave and the superconducting instabilities cancel each other and there is no symmetry breaking. We solve the model via bosonization; the excitation spectrum is found to consist of gapless bosonic modes as in a one-dimensional Luttinger liquid.

A method for first-principles calculations of spectra of strongly correlated systems is proposed. The impurity-Anderson-model is solved with a many-electron trial wave function. The obtained eigenvalues and eigenfunctions are used for one-electron removal (or addition) Green’s-functions calculations. All parameters of the many-electron Hamiltonian are taken from band-structure calculations in the local density approximation with an on-site d-d Coulomb interaction (LDA+U). The method is applied to NiO and shows good agreement with experimental photoemission spectra and with the oxygen Kα x-ray-emission spectrum. Using MgO as an example, we show that the O Kα spectrum is equivalent to the oxygen partial density of states, and that there are no observable core-hole effects.

The generalized Lloyd formula and the trace of the Green’s function are equivalent expressions for the number of states of electrons in solids. The comparison of these two approaches enables one to estimate the accuracy of the number-of-states calculations. We study the accuracy of the Lloyd formula and of the trace of the Green’s function for the number of states of a binary random alloy as a function of the cutoff parameter ${\mathit{l}}_{\max }$ in the orbital momentum representation. We show that for transition metals truncation at ${\mathit{l}}_{\max }$=2 leads to an essential loss of accuracy. It is necessary to take into consideration the contributions of s, p, d, and f electrons in both approaches to obtain their numerical equivalence. All higher-l contributions can be neglected. We propose a quadratic extrapolation scheme to solve the coherent-potential-approximation equation in one iteration. We show that the use of the tetrahedron technique for the Brillouin-zone integration for calculation of the τ matrix of an alloy allows us to obtain high-precision results with minimum computational efforts. The proposed procedures reduce computational time at least by a factor of 100 in comparison with the standard approaches.

The effect of Cooperons on the metal-insulator transition in disordered interacting electronic systems is studied. We point out that a proper incorporation of Cooperons into the disordered electron problem must respect a Bethe-Salpeter equation for the effective Cooper interaction amplitude ${\mathrm{\Gamma }}^{\mathit{c}}$. This puts constraints on renormalization-group treatments of the problem. We discuss existing renormalization-group approaches, both of the field-theoretic and of the momentum-frequency shell variety, and show that none of them are technically satisfactory. A general analysis of the Bethe-Salpeter equation shows that all possible solutions fall into one of three classes which differ with respect to the scaling behavior of ${\mathrm{\Gamma }}^{\mathit{c}}$. We argue that both of the physically most plausible possibilities lead to logarithmic corrections to scaling, and discuss the experimental implications of this conjecture.

We have studied the structure of the ${\mathit{Q}}_1$ charge-density wave (CDW) in pure and Ta-doped ${\mathrm{NbSe}}_3$ as a function of temperature using high-resolution x-ray scattering. The CDW’s phase-phase correlation lengths are much greater than the impurity spacing, and in pure crystals can be comparable to typical sample thicknesses. The corrections decay exponentially in all directions. In a heavily Ta-doped crystal, transverse correlation lengths vary with temperature as the square of the CDW gap. These experimental results agree with the weak-pinning model of Fukuyama, Lee, and Rice, and with the weak-pinning interpretation of previous transport measurements.

Phonon imaging with frequency-selective detectors is used to probe the scattering from residual defects in semi-insulating GaAs. As in previous heat-pulse studies, we find that the transmission of non- equilibrium phonons with longitudinal and transverse polarizations can be modified by near-infrared excitation of the crystal at low temperatures. These effects have been attributed to metastable states of the ubiquitous EL2 defect in liquid-encapsulated-Czochralski-grown GaAs. In the present work we use the dispersive shifts in the phonon-focusing pattern to gain information about the frequency dependence of these optically induced changes. For particular polarizations and propagation directions, near-infrared excitation causes a huge decrease in the scattering of phonons with frequencies above 700 GHz. Our results show that, unlike the elastic scattering of simple mass defects, the residual defects are effective inelastic scatterers of phonons. An analysis of our data based on the theory of Nowick and Berry indicates that the change in the transmission of phonons following photoexcitation cannot be attributed to a unique symmetry change of a single defect.