We have measured the energy spectrum of positronium (Ps) thermally desorbed from Al(111) surfaces. For sample temperatures as low as 84 K and Ps perpendicular energies between 5 and 50 meV the spectrum is a beam Maxwellian, i.e., a simple exponential. We argue that the surface must be essentially a blackbody for Ps emission, and that we thus have the first example of a system that fails to exhibit the expected perfect reflection of a very slow particle from a cold surface. We suggest that the effect is associated with the breakdown of the perturbation expansion for the Ps reflection probability.

A new strain-relief mechanism in strained III-V semiconductor structures is identified. The signature defect of the proposed mechanism is a microtwin along the {111} plane spanning an embedded strained layer. This defect can form when two partial dislocations with antiparallel Burgers vectors of the 1/6〈112〉 type are generated inside the strained layer and glide to the opposite interfaces, leaving a stacking fault between them. This is a low-energy strain-relaxation channel and poses fundamental limitations for strained-layer device structures.

A self-consistent nonperturbative screening calculation of the surface-phonon dispersion curves has been carried out for the Na(001) surface. The Bloch states were caluclated by self-consistently solving the Kohn-Sham equations in the local-density approximation. From the Bloch states, the static electronic polarizability and density response function were obtained and used to calculate the surface-phonon dispersion curves. The changes in interatomic force constants at the surface are discussed, as are the effects of three-body interactions.

The complex of a divalent silicon interstitial and a pair of adjacent oxygen intersitials is proposed as the core of the 450 °C oxygen thermal donor in silicon. The proposal is supported by theoretical calculations which suggest that this complex is stable relative to a self-interstitial and two separated oxygen interstitials, and that it has a doubly occupied level close to the computed conduction-band edge. The calculated spin distribution is in qualitative agreement with the one found experimentally for the NL8 center.

We present the first atomic-resolution images of interfacial ordering occurring in ultrathin (${\mathrm{Si}}_{\mathit{m}}$${\mathrm{Ge}}_{\mathit{n}}$${)}_{\mathit{p}}$ superlattices. The observed asymmetric interfacial abruptness is associated with several distinct ordered configurations, which differ from phases considered previously. This is explained by a novel Ge-atom pump mechanism which is chemically driven, and takes place principally at the rebonded edge configuration during type-${\mathit{S}}_{\mathit{B}}$ step propagation, leading to lateral (2×2) compositional ordering.

We show that the reaction ${\mathrm{Al}}_{1/2}$${\mathrm{O}}_3$+3/${4\mathrm{H}}_2$⇆3/2AlOOH+1/2Al proceeds to the right at 300 K and at pressures above 136 GPa, producing the metal aluminum in the process. The system of neighboring particles of ${\mathrm{Al}}_2$${\mathrm{O}}_3$ coated with aluminum hydroxide oxide and covered with aluminum is a dc conductor and exhibits a resonance in the infrared which appears as a plasma edge. In view of this, claims of production of metallic hydrogen based on reflectivity studies on composite ${\mathrm{Al}}_2$${\mathrm{O}}_3$-${\mathrm{H}}_2$ samples are unwarranted.

We report direct detection of light emission from single-crystal silicon cleaved in vacuum. The emission occurs in at least two wavelength regions, one above 1 eV energy (A) and the other between 0.25 and 0.36 eV (B). Intensities for about 0.1-${\mathrm{cm}}^2$ cleaved areas are in the region of ${10}^{12}$ photons in region A and ${10}^{11}$ photons in region B. The B radiation is of significantly longer duration than the A radiation. Some of the latter can be explained by radiative bulk band recombination, and the B radiation by radiative surface band recombination. The duration of the B radiation shows that the surface-state gap is indirect.

Stretched exponential relaxation of stress-induced quadrupolar order is found in ${\mathrm{K}}_{0.989}$${\mathrm{Li}}_{0.011}$${\mathrm{TaO}}_3$ at temperatures T<30 K. In contrast to expectations for glasses, however, the stretching exponent increases upon cooling and reaches unity as T→0. In parallel with the observed loss of polydispersivity the relaxation time crosses over from Arrhenius to dissipative-quantum-tunneling behavior, τ(T)=τ(0)exp(-${\mathit{aT}}^4$) at T<20 K. The absence of quadrupolar freezing seems to be due to the onset of tunneling and to coupling with the frozen-in dipolar degrees of freedom.

We investigate the cause of compositional ordering in fcc ${\mathrm{Ni}}_{0.50}$${\mathrm{Pt}}_{0.50}$ on the basis of a first-principles theory of electronic structure and finite-temperature concentration fluctuations. We treat the size effect, electronegativity, and band filling on an equal footing and find that the first of these dominates, leading to L$1_0$ ordering in agreement with experiment. An electronic origin is given for the metallurgical ‘‘rule of thumb’’ that alloys comprised of big atoms and little atoms tend to order into simple stuctures.

Delayed coherent nuclear forward scattering of synchrotron radiation by polycrystalline Fe foils (both isotopically enriched and naturally abundant) has been measured. Quantum beats wre observed in the time spectra. With the chosen experimental conditions these can only arise from coherent scattering. The 5-meV bandwidth provided by a special silicon crystal monochromator obviates the need for a nuclear resonant filter. This direct approach offers the possibility of performing a wide range of Mössbauer spectroscopy experiments using synchrotron radiation.

We report the observation of a local vibrational mode (LVM) in hydrostatically stressed, Si-doped GaAs. The corresponding infrared absorption peak is distinct from the ${\mathrm{Si}}_{\mathrm{Ga}}$ shallow-donor LVM peak and is assigned to the Si DX center. The relative intensities of the Si DX LVM and the Si shallow-donor LVM peaks and photoquenching behavior of the sample are consistent with the appearance of a defect which binds two electrons as it undergoes a large lattice relaxation at approximately 23 kbar.

We present quantum Monte Carlo results for a two-dimensional electron-phonon model. At half filling, a Peierls–charge-density-wave (CDW) instability dominates the behavior of the system and suppresses superconductivity. The CDW structure factor scales with system size and is consistent with a finite-size Ising-like scaling analysis which we use to estimate ${\mathit{T}}_{\mathit{c}}$ for the Peierls-CDW transition. Away from half filling, the Monte Carlo calculations show superconducting pairing in an s-wave channel, with a peak in the pair field susceptibility near a region in which there are strong Peierls-CDW correlations.

The effect of Coulomb and electromagnetic interactions on the dynamics of spin-density waves is discussed. We show that effects analogous to the Anderson-Higgs mechanism for superconductors occur, and discuss the experimental evidences for such a mechanism.

On spear-anvil-type point contacts of As both positive and negative ${\mathit{d}}^2$V/${\mathit{dI}}^2$ spectra are observed, corresponding to, respectively, an increase and a decrease in the contact resistance at characteristic phonon energies. Under the influence of an applied magnetic field, contacts with positive electron-phonon-interaction spectra show a positive magnetoresistance indicating ballistic metallic transport, while contacts with negative spectra give a negative magnetoresistance indicating the presence of a localization area inside the contact. The spectral intensity depends on the applied magnetic field (≤ 20 T) due to the magnetic-field-induced one-dimensional current spreading through the contact and the magnetic breakdown of weak localization.

Hexagonal ${\mathrm{Er}}_3$${\mathrm{Si}}_5$ films epitaxially grown on Si show strong anisotropies in magnetization and magnetotransport below the ordering temperature. The magnetoresistance has a cusplike positive anomaly or is negative and featureless for a magnetic field applied, respectively, along or perpendicular to the [0001] axis. A noncollinear structure, composed of an antiferromagnetic and a ferromagnetic component accounts for the magnetization data. The latter used in conjunction with the Yamada-Takada theory of magnetotransport accounts for the magnetoresistance data.

We present the first observation of the magnetophonon effect in the frequency-dependent conductivity of electrons in a polar semiconductor. The cyclotron-resonance position and linewidth oscillate as a function of magnetic field and show resonances for ${\mathrm{\omega }}_{\mathrm{LO}}$≊N${\mathrm{\omega }}_{\mathit{c}}$. The resonances grow rapidly with increasing temperature, giving a direct measure of the optic-phonon scattering rate. Good agreement is found with a one-electron theory of the absorption spectrum, showing that the oscillations have the same origin as magnetophonon-resonance oscillations in the resistivity.

The S=1 quantum antiferromagnetic chain with the Hamiltonian H= ${\mathcal{J}}_{\mathit{i}}$ ${\mathrm{S}}_{\mathit{i}}^{\mathit{x}}$${\mathit{S}}_{\mathit{i}+1\mathrm{}}^{\mathit{x}}$+${\mathit{S}}_{\mathit{i}}^{\mathit{y}}$${\mathit{S}}_{\mathit{i}+1\mathrm{}}^{\mathit{y}}$+λ${\mathit{S}}_{\mathit{i}}^{\mathit{z}}$ ${\mathit{S}}_{\mathit{i}+1\mathrm{}}^{\mathit{z}}$+D(${\mathit{S}}_{\mathit{i}}^{\mathit{z}}$${)}^2$ is studied. By developing a (path-integral-type) stochastic geometric representation and using the ideas of percolation, we find that the Haldane phase with a unique disordered ground state and a gap can be regarded as ‘‘liquid’’. The large-D phase and the Néel ordered phase are identified as ‘‘gas’’ and ‘‘solid’’, respectively. We introduce a new order parameter that distinguishes the Haldane phase from other disordered phases such as the large-D phase or the dimer phase.

The fractional quantum Hall states with non-Abelian statistics are studied. Those states are shown to be characterized by non-Abelian topological orders and are identified with some of the Jain states. The gapless edge states are found to be described by non-Abelian Kac-Moody algebras. It is argued that the topological orders and the associated properties are robust against any kinds of small perturbations.

An arrangement for obtaining Aharonov-Bohm oscillations of basic periodicity qh/e (q>1) is discussed. The relaxation towards h/e periodicity is characterized by a decay time exponential in the system size at zero temperature, and linear in this size at finite temperature.

The dynamics of a dissipative, quantized macroscopic two-state system have been experimentally investigated through the measurement of tunneling transitions between the adjacent, equal-energy fluxoid states of a superconducting loop containing Josephson junctions. The tunneling rate Γ has been observed to approach zero as a power law in the temperature, giving a damping parameter α=1.44, in the incoherent relaxation regime. The predicted exponential dependence of Γ on barrier height has been quantitatively confirmed.