The conductivity σ of a series of AlCuFe and AlPdMn quasicrystals and approximant phases can be written σ(T)=${\mathrm{\sigma }}_4$ K+δσ(T). The ${\mathrm{\sigma }}_{4\mathrm{}\mathrm{K}}$ term is very low, decreases with improvement of structural quality, and varies strongly with composition. δσ(T) is independent of the sample and increases anomalously with temperature up to 1000 K, with ${\mathrm{\sigma }}_{1000\mathrm{}\mathrm{K}}$/${\mathrm{\sigma }}_{4\mathrm{}\mathrm{K}}$ greater than 10. The results are interpreted by electronic hopping between structural entities separated by about 30 Å. Within a band picture, this process would correspond to interband transitions.

Studies of icosahedral quasicrystals have revealed a conductivity pseudogap much narrower than the density-of-states pseudogap, resulting in a rapid suppression of electron diffusivity inside the latter. In contrast, measurements of the new metallic compounds ${\mathrm{Al}}_2$Ru show the formation of a semiconductinglike gap with localized gap states. The imminence of Anderson localized and absence of a real gap in quasicrystals are compared with current calculations of electronic properties on metallic quasiperiodic, as well as periodic, systems.

The statistical analysis of the acoustic signals, emitted by hydrogenated Nb samples during hydrogen precipitation from solid solution, reveals that the distribution of their amplitudes follows the Gutenberg-Richter power law, N(A’>A)∼${\mathit{A}}^{\mathrm{-}\mathit{c}}$ with c=0.9 over more than 2 orders of magnitude. This behavior can be considered as evidence that fracture processes due to hydride precipitation in the Nb-H system determine a self-orignized critical state.

Recent experimental and theoretical studies report that quenched pressure-amorphized α-quartz is elastically anisotropic and displays a reversal of anisotropy with respect to the original crystallographic orientation. We demonstrate that samples recovered from static compression experiments of α-quartz exhibit an unusual pressure-induced microstructure, which arises from a new phase transformation in α-quartz at 21 GPa. Upon decompression, the high-pressure phase reverts to a quartzlike structure in a twinned state, accounting for the previously reported elastic behavior.

The temperature of shock compressed iron has been measured to 340 GPa, using well characterized iron films sputtered on transparent diamond substrates and a 1 ns time-resolved optical method. We find a knee on the (P,T) iron Hugoniot indicating melting at 6350 K and 235 GPa and at 6720 K and 300 GPa. An extrapolation yields an iron melting temperature of 6830 (± 500) K at 330 GPa, the pressure of the Earth inner-outer core boundary. Implication of the melting data for the iron phase diagram is also discussed.

We discuss the process by which energy, initially evenly distributed in a nonlinear lattice, can localize itself into large amplitude excitations. We show that the standard modulational instability mechanism, which can initiate the process by the formation of small amplitude breathers, is completed efficiently, in the presence of discreteness, by energy exchange mechanisms between the nonlinear excitations which favor systematically the growth of the large excitations. The process is, however, self-regulated because the large amplitude excitations are finally trapped by the Peierls-Nabarro potential.

The origin of fractal properties in diffusion limited aggregation is related to the persistence of screening in the scale invariant growth regime. This effect is described by the effective noise reduction parameter S spontaneously generated by the scale invariant dynamics. The renormalization of this parameter under scale transformation shows the following: (i) The fixed point is attractive, implying the self-critical nature of the process. (ii) The fixed point value ${\mathit{S}}^{\mathrm{*}}$ is of the order of unity, showing that the small scale growth rules are already close to the scale invariant ones and that screening effects persisit in the asympotic regime.

An inversion of the triangular shape of the two-dimensional islands observed in a narrow temperature range by scanning tunneling microscopy during growth of Pt on Pt(111) is reported. This phenomenon is due to the inversion of the growth speed ratio of the two types of close packed 〈110〉 step edges. It is tentatively traced back to an inversion in the hierarchy of the atom diffusion speeds along the two types of step edges in this temperature range.

We report the development of an ab initio constant pressure extended molecular dynamics method with variable cell shape. This is a symmetry conserving method which allows for efficient structural searches and optimizations in spaces with preselected symmetry groups. We have used it to investigate ${\mathrm{MgSiO}}_3$, a perovskite, the marjor Earth-forming mineral phase which exists particularly in the lower mantle. We predict its structural behavior up to pressures which exceed the highest values reached in this region.

A marked reduction in the velocity dispersion of excitons propagating through a thick ${\mathrm{Cu}}_2$O crystal at low temperatures and high densities is attributed to the onset of a superfluid excitonic phase.

We report an ab initio evaluation of the dynamical density-response function for real Al and Na. The method we employ is a generalization of the Dalgamo-Lewis scheme of perturbation theory to systems with a continuous spectrum and to dynamical problems. The crystal lattice is found to lower the plasmon frequency of Al for large wave vectors by as much as 4 eV. For this metal, agreement with experiment is excellent in the time-dependent local-density approximation for exchange correlation. The ‘‘anomalous’’ dispersion observed for Na for large wave vectors is assigned to subtler exchange-correlation effects.

Binding energies calculated in the local density approximation for small transition metal clusters establish a trend with atomic number which accurately represents the trend in cohesive energies for their crystalline counterparts. This correlation in trends means that small clusters differ from each other, as far as binding energies are concerned, in the same way the corresponding crystalline solids differ. Local near-neighbor bonding determines the variations in cohesive properties that distinguish one metal from another.

Qualitative differences in the behavior of H in Si and Ge have been reported. In Si, shallow acceptors and donors are easily passivated by exposure to atomic H. In Ge, only one unsuccessful attempt at acceptor passivation has been reported and donor passivation was never realized. Further, the amount of D that penetrates into a Si sample exposed to a D plasma increases with the temperature of the plasma. The opposite holds for Ge. Other qualitative differences are apparent from muon spin rotation studies. Our systematic calculations of properties of H in diamond, Si, Ge, and α-Sn show that there may be a very simple explanation for all these differences.

A resonant-level impurity Hamiltonian with finite range interactions is considered. It is shown that for repulsive interactions above a critical threshold, the model displays scale invariant properties unlike those of a Fermi liquid. The various correlation functions behave as power laws with nonuniversal exponents depending on the interactions. At the transition between the Fermi-liquid and non-Fermi-liquid regimes the resistivity behaves as A+B/ln(T${)}^2$, whereas it behaves as A+${\mathit{BT}}^{\nu }$ with ν>0 in the latter.

The barrier for dissociative adsorption of ${\mathrm{H}}_2$ on Al(110) has been calculated within the generalized gradient approximation. A pronounced increase of the barrier height is found compared with what is calculated in the local density approximation (LDA). The apparent LDA underestimation of the barrier height is shown to be intimately linked with the LDA underbinding of core electrons and we suggest it to be a general phenomenon not limited to the particular nonlocal exchange-correlation approximation used or the particular system studied.

The magnetoresistance ${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$ of two-dimensional electrons, density n, on liquid helium was measured up to the quantum limit, near 1 K where the scattering is due to ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ vapor atoms. Temperature- and density-dependent postive magnetoresistance was observed. Many-electron theory is presented that ‘‘restores’’ the Drude formalism for ħ${\mathrm{\omega }}_{\mathit{p}}$≪kT≪${\mathit{e}}^2$${\mathit{n}}^{1/2}$/${\mathrm{\varepsilon }}_0$ [${\mathrm{\omega }}_{\mathit{p}}$=(${\mathit{e}}^2$${\mathit{n}}^{3/2}$/2${\mathrm{\varepsilon }}_0$m${)}^{1/2}$ is the plasma frequency] for classically strong magnetic fields, and shows large magnetoresistance in higher fields, in agreement with the data. The crossover to single-electron kinetics at the highest fields is discussed.

The first direct observation of the As derived empty surface state on the Ge(111):As1×1 surface is reported. The state is first populated with a subpicosecond pulse of 2.03 eV light and then probed with equally short pulses of light at 10, 14, 18, and 22 eV which photoemit electrons from the sample. The surface band gap at 300 K is determined to be 0.83±0.04 eV and compared well with a many-body calculation for this system. Photoexcited electrons are observed to rapidly scatter into the empty As derived state and are trapped for times in excess of 200 ps. The long lifetime is due to a paucity of electrically active defect states within the surface band gap.

A long wavelength, low-energy excitation of the fractional quantum Hall state at ν=1/3 has been observed by inelastic light scattering. The mode appears as a very sharp peak with marked temperature and magnetic field dependence. Its energy is consistent with theoretical predictions for the collective gap excitations of the incompressible quantum fluid. Spectra interpreted as q=0 collective spin-wave excitations also display the strong dependence on field and temperature associated with the fractional quantum Hall state.

Muon spin relaxation resulting from spin exchange scattering of endohedral muonium (${\mathrm{\mu }}^{+}$${\mathit{e}}^{\mathrm{-}}$) with thermal electronic excitations has been observed in the fullerene superconductor ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$. The temperature dependence of ${\mathit{T}}_1^{\mathrm{-}1}$ shows a coherence peak just below ${\mathit{T}}_{\mathit{c}}$ and can be fit to the conventional Hebel-Slichter theory for spin relaxation in a superconductor with a broadened BCS density of states. The average energy gap for electronic excitations, Δ/${\mathit{k}}_{\mathit{B}}$=53(4) K or 2Δ/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$=3.6(3), is consistent with the BCS weak coupling limit.

In the existing literature, the interactions between the vortex ‘‘pancakes’’ believed to form in, e.g., ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$, have been attributed to a combination of magnetic and Josephson effects. Both these effects lead to an attraction between pancakes in neighboring ${\mathrm{CuO}}_2$ planes. We show that there exists a third mechanism of interaction, namely, the indirect effect of the Coulomb interaction, which leads to a repulsion between such pancakes. The relative strength of this coupling is estimated.