Phase transitions in the group-IV transition-metal zirconium were studied by the energy-dispersive x-ray-diffraction technique with a synchrotron source to a pressure of 32 GPa. A first-order phase transition between an ω phase and a bcc phase (isostructural with group-V transition elements) was observed during compression and decompression in a pressure range of 30±2 GPa, which is in qualitative agreement with the recent first principles theoretical predictions. The observation of the bcc phase in Zr (a group-IV element) under pressure at room temperature signifies a pressure-induced electronic configuration similar to that of a group-V element.

A self-consistent theory of metal-semiconductor electronic structure is presented. We discover that this structure is formed out of two-state systems; these include metal-like states and metal-induced semiconductorlike states. This dual aspect may provide a new unifying mechanism for existing models of metal-semiconductor electronic structures. We discuss the relevance of the discovered two-state character to the Fermi-level pinning. Eventually, the local density of states is calculated.

Experimental investigations of the Hall effect at the occurrence of autonomous current fluctuations in high-purity n-GaAs epitaxial layers at low temperatures show that the transition to chaos is accompanied by intermittent collapses in the Hall voltage. This may represent the significant third mechanism for a transition into chaos according to the Ruelle-Takens-Newhouse scenario. Basically, space charges accumulated in the filament boundaries yield different impact ionization probabilities at opposite sides of the filament destabilizing the current flow and causing multimodal oscillation phenomena originating at the filament borders.

The concept of edge channels is extended from the integer to the fractional quantum Hall effect, and the contribution of an adiabatically transmitted edge channel to the conductance is calculated. The resulting generalized Landauer formula explains the recent observation by Kouwenhoven et al. of fractional quantization of the Hall conductance at a value unrelated to the bulk properties of the sample.

A finite-two-dimensional electron gas with N full Landau levels has N branches of edge states which cross the Fermi level. In this Letter we show that in the fractional-quantum-Hall-effect regime there can be many branches of edge states for a single partly filled Landau level. The ith branch can be associated with a fractional charge, ${\mathrm{ef}}_i$, and ${\Sigma }_i$$f_i$ equals the Landau-level filling factor. The set of edge-state charges at a particular filling factor directly reflects the hierarchical structure of the incompressible ground state which occurs at the filling factor.

The nuclear-magnetic-resonance spin-lattice relaxation time ($T_1$) and electron-paramagnetic-resonance (EPR) linewidth (${\Gamma }_{\perp }^s$) are calculated for ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. The large broadening of the EPR at room temperature is shown to narrow with increasing temperature as (ξ/a${)}^3$, where ξ is the two-dimensional spin-correlation length and a is the lattice constant. The NMR $T_1$ at first lengthens with increasing temperature as (ξ/a${)}^{\mathrm{-}1}$, then shortens above T∼J/2, where J is the in-plane exchange coupling. Explicit values are calculated for $T_1$ and ${\Gamma }_{\perp }^2$. .AE

The effect of thermally activated phase slippage (TAPS) in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ grain-boundary Josephson junctions has been studied. TAPS has been found to be responsible for the dc noise voltage superimposed on the dc Josephson current near the transition temperature. Because of the reduced Josephson coupling energy of the grain-boundary junctions, which is caused by a reduced superconducting order parameter at the grain-boundary interface, TAPS is present over a considerable temperature range. The implications of TAPS on the applicability of high-${\mathrm{T}}_{\mathrm{c}}$ Josephson junctions are outlined.

We calculate the energy of flux states described by Slater determinants of single-particle levels in a uniform magnetic field with p/q flux quanta per plaquette and with no double occupancy. We find the exchange energy of flux states is lower than that of the Gutzwiller wave function by 10%–20%. The minimum kinetic energy is obtained by introducing phases associated with the holes. The phases are calculated by mapping onto a classical frustrated XY model. At 10% doping, the 1/2 flux state (π phase) is stabilized for the parameter regime J/t≳0.2. At about 20% doping, the time-reversal-breaking (1/3 flux state has the lowest energy for J/t≳1.0.

We discuss a new momentum-conserving lattice-gas model in which particles are allowed to exchange momentum between distant sites. The interactions may be tuned so that a first-order transition occurs between a dense and a light phase. An equation of state may be predicted with the assumption that the lattice is in a factorized state just after the particles have propagated. This method accurately predicts the pressure of the stable and unstable states.

We show that uncertainties in the strength of interactions of hyperons among themselves and with nucleons lead to an uncertainty in the maximum allowed neutron-star mass of nearly a factor of 2, even if the properties of nuclear and neutron matter were known with infinite precision around normal nuclear density and below. The possibility of a transition to quark matter places some constraint on the strength of the hyperonic interactions.

We report the first evidence of charmless semileptonic decays of B mesons. In the momentum interval 2.4–2.6 GeV/c where the background from b→clν is negligible, the average of the measured b→ueν and b→uμν partial branching ratios is Δ$B_{\mathrm{ub}}$(2.4,2.6) =(1.8±0.4±0.3)×${10}^{\mathrm{-}4}$. Inclusion of data from the interval 2.2–2.4 GeV/c, where the lepton yield is dominated by b→clν, gives Δ$B_{\mathrm{ub}}$(2.2,2.6) =(3.3±0.8±0.8)×${10}^{\mathrm{-}4}$. ‖$V_{\mathrm{ub}}$/$V_{\mathrm{cb}}$‖ depends on the theoretical model of b→ulν decay and is approximately 0.1.

An upper limit on the branching ratio for the decay $K^{+}$→${\pi }^{+}$νν¯ is set at 3.4×${10}^{\mathrm{-}8}$ (90% C.L.). In addition, a 90%-C.L. upper limit of 6.4×${10}^{\mathrm{-}9}$ is set on the branching ratio for decays of the form $K^{+}$→${\pi }^{+}$$X^0$, where $X^0$ is any massless, weakly interacting neutral particle. Limits &>0. .AE

We have calculated the kaon-matrix-element B parameter (${\mathrm{B}}_{\mathrm{K}}$) using lattice QCD in the quenched approximation with staggered fermions. We find the correct chiral behavior. Errors from all sources except quenching are under control. At a lattice scale of 2 GeV our result is ${\mathrm{B}}_{\mathrm{K}}$=0.70±0.01(statistical)±0.03(systematic). This translates to a renormalization-group-invariant value B${\mathrm{^}}_{\mathrm{K}}$=0.9–1.0. .AE

Following theoretical predictions of competing prolate and oblate potential-energy minima, we present experimental evidence for the coexistence of prolate and oblate nuclear shapes which are associated in a given nucleus with the same single-quasiproton orbital. Prolate ΔI=2 and oblate ΔI=1 bands observed in the ${}_{}{}^{119}{}_{}{}^{}\mathrm{I}$ nucleus are proposed as built on an ${\mathrm{h}}_{11/2}$ proton with low K and high K, respectively.

The critical behavior of the order-disorder transition in the (2×2) lattice of oxygen on the (110) molybdenum surface is studied by low-energy electron diffractometry with a coherence length of about 600 Å. The critical exponents η, γ, ν, and β of a system which belongs to the universality class of the XY model with cubic anisotropy are extracted for the first time by a new method based on the general scaling properties of the correlation function. η is found to be abnormally large for T>$T_c$ and T dependent, approaching the value 0.25 for T→$T_c$. γ, ν, and β have the (T independent) values 1.20±0.10, 0.84±0.07, and 0.19±0.02, respectively.

The excess energy of several III-V and II-VI strained-layer semiconductor superlattices (AC${)}_p$(BC${)}_p$ is studied as a function of the repeat period p and orientation G=[001], [110], [111], and [201], using first-principles calculations. We discover a number of universal features, including the predicted instability for nearly all p’s and G’s with respect to bulk disproportionation, the identification of chalcopyrite as a metastable ordered structure, and the stability of all thin epitaxial [110] and [201] and most common-anion [001] superlattices relative to coherent phase separation.

We observe, for the first time, long-range order in thick, unstrained SiGe alloys, with and without boron doping. This ordering occurs along the four equivalent 〈111〉 directions. The ordered domains are randomly shaped, and corresponds to alternating double layers of Si and Ge. Bond energy arguments are used to explain the formation of this new phase.

The existence of multipole surface-plasmon modes at simple-metal surfaces is demonstrated both experimentally and theoretically. Inelastic reflection electron-scattering experiments on smooth films of K and Na show loss peaks originating from the ordinary surface plasmon as well as from a higher-energy multipole mode. Microscopic density-functional calculations of the electron-loss function reproduce the energy and momentum dispersion of both of these surface modes. For Al, the multipole mode is shown to be too weak to be observable in electron-loss spectroscopy.

The novel idea of constructing a combined spectrometer and detector from existing semiconductor technology is proposed. Some of the advantages that such a device could be expected to have over existing spectrometers are suggested and then one possible route towards achieving such a device is discussed in detail. This uses the Fourier-transform properties of the interband magnetoabsorption oscillations between a two-dimensional electron and hole band.

The kicked rotator on a torus is a system with a bounded phase space in which a chaotic diffusion occurs for a large enough perturbation strength. The quantum version of this model exhibits localization effects which produce deviations from random-matrix-theory predictions. We show that these localization effects display a scaling behavior which is a counterpart of the scaling theory of one-dimensional Anderson localization in finite samples. We suggest that this behavior can be highly relevant to some general problems of quantum chaos.