It is shown formally that the Peierls expression is inadequate for studying the instabilities of a physically realizable one-dimensional chain of atoms, due to the long-range nature of the Coulomb interaction. Detailed total-energy studies of trans-polyacetylene, for both suggested crystal structures (P$2_1$/n,P$2_1$/a) and for the one-dimensional chain, establish that no appreciable total-energy minimum occurs for nonzero dimerization amplitude within the local-density approximation. The measured dimerization is discussed within the context of recent models of electron correlation.

A theory is developed for the recently discovered magnetoresistance oscillations in a two-dimensional electron gas subject to a weak periodic potential. The effect is explained as a resonance between the periodic cyclotron orbit motion and the oscillating E×B drift of the orbit center induced by a potential grating.

Cyclotron-resonance experiments done in a novel superlattice, HgZnTe-CdTe, provide the first determination of the conduction-band dispersion in a type-III superlattice. The measured electron-mass anisotropy is strongly dependent on the CdTe layer thickness, in agreement with band-structure and Landau-level calculations. These calculations show also that the studied superlattices are semimetallic, consistent with the large valence-band offset (300 meV). Another cyclotron-resonance transition is observed, evidencing strongly the formation of a two-dimensional electron gas.

The subjects of this paper are the proper inclusion of long-range electrostatic terms in the theory of electronic deformation potentials, the way to include these terms by using supercells in ab initio density-functional methods, and calculations for selected semiconductors. We describe the connection with the heterojunction problem, and compare our values with previous model theories and with experiment.

We have measured the frequency dependence of the thermoelectric power, $S$, in the charge-density-wave state of ${\mathrm{K}}_{0.3}$Mo${\mathrm{O}}_3$. A strong frequency dependence with both real and imaginary components was observed, and we interpret our finding in terms of a two-fluid model. We also discuss the criteria of observability of this novel phenomenon in other materials where the electrical conductivity is frequency dependent.

Angle-resolved photoemission measurements of the Fermi surfaces of several surface localized states of clean and hydrogen-covered W(110) are reported. Three hole orbits and one electron orbit have been characterized. The hole-orbit states are rapidly attenuated by hydrogen, while the electron-orbit states are shifted to higher binding energy resulting, initially, in an expansion of the Fermi surface, and ultimately, in its conversion to two hole orbits. These data represent the most detailed study of the behavior of the two-dimensional Fermi surface to date.

We have demonstrated experimentally that magnetic-flux quantization in a superconducting circuit is obeyed in an absolute sense, and have ruled out the possibility of a fractional quantum of magnetic flux. We use an optically switched superconducting microbridge to destroy and reestablish the phase coherence of the order parameter around a superconducting circuit. Changes in the vector potential while the global phase coherence is destroyed are still observed when the coherence is reestablished, even though the magnetic field is inacessible to the superconductor.

The critical current density $J_c$(B,T) is measured to 15 T for c-axis-perpendicular expitaxial thin films of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ with $J_c$(0,77 K) of order ${10}^6$ A/${\mathrm{cm}}^2$. Even in the least-favorable perpendicular orientation, critical currents can exceed 5×${10}^5$ A/${\mathrm{cm}}^2$ at 20 K in 15-T fields. Thermally activated flux motion (flux creep) is prominently observed, and can in large part explain the magnitude, temperature, and field dependence of the high-field critical currents.

We report measurements of the conductivity and dielectric constant as a function of frequency and temperature on samples of ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ from dc to 100 GHz. An analysis of the frequency dependence of the complex conductivity indicates that the high-frequency dielectric constant is large (≃50) and weakly temperature dependent. The dc charge carriers are massive (1100${\mathrm{m}}_{\mathrm{e}}$), weakly damped, and partially pinned.

High-energy inelastic neutron scattering is used to resolve spin waves in ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. The corresponding long-wavelength velocity is 0.85±0.03 eV Å, well above previous bounds set by thermal neutron scattering. Within experimental error, conventional spin-wave theory gives an excellent description of the magnetic dynamics for momentum and energy transfers below 0.15 A${̊}^{\mathrm{-}1}$ and 0.1 eV, respectively. Doping with Ba changes the magnetic dynamics; the data indicate a combination of spin-wave softening and damping effects.

In many continuous spin systems, competing interactions give nontrivial degeneracies of the classical ground states. Degeneracy-breaking free-energy terms arise from thermal (or quantum) fluctuations, which select for collinear states, and from dilution, which selects for anticollinear (yet long-range ordered) states. They are explicitly computed for an XY square-lattice antiferromagnet dominated by second-neighbor antiferromagnetic exchange. The predicted phase diagram agrees qualitatively with simulations.

Using confocal laser optical signal processing and neutrally buoyant particle laser tracking techniques, we have investigated the formation and breakdown of the diffusion layer which develops at the dendritic interface of a metal alloy model material (28 wt.% ${\mathrm{NH}}_4$Cl in ${\mathrm{H}}_2$O) undergoing directional solidification. The character of the breakdown for the system is shown to be a solutal Rayleigh instability with a Benard-cell–type structure which is internal to the fluid, i.e., not terminated by physical or free boundaries.

The Julia set associated with the Newtonian map for the solution of $z^3$-1=0 in the complex plane is investigated by purely analytic means. The hierarchical organization of this strange repeller is explained from basic principles. The family of generalized fractal dimensions and the equivalent spectrum of scaling indices is evaluated using an extension of the Hentschel-Procaccia renormalization scheme. Our quantitative results are in excellent agreement with direct numerical computations.

This paper gives a new classification of the eigenphase shifts caused by a noncentral potential in the Schrödinger equation and generalizes the well-known low-energy behavior of phase shifts for central potentials. It also presents a new Levinson theorem for each eigenphase shift.

Motivated by recent models of Bak, Tang, and Wiesenfeld we study dissipative transport in open systems. A simple continuum equation is constructed to describe fluctuations around a steady state in a flowing ‘‘sandpile.’’ The principle of scale invariance and self-similarity is understood in terms of a conservation law in dynamics. A dynamic renormalization-group calculation allows us to determine various critical exponents exactly in all dimensions.

We perform the canonical quantization for a system of non-Abelian chiral bosons. We show that unlike in the Wess-Zumino-Witten model, the left- and right-handed current densities do not simultaneously span Kac-Moody algebras in the quantum theory. At the critical values of the coupling constants, only one Kac-Moody algebra results.

Measurements of the exclusive branching fractions B($D^0$→${\pi }^{\mathrm{-}}$$e^{+}$${\nu }_e$) and B($D^0$→$K^{\mathrm{-}}$$e^{+}$${\nu }_e$), using data collected at the ψ(3770) with the Mark III detector at the SLAC $e^{+}$$e^{\mathrm{-}}$ storage ring SPEAR, are used to determine the ratio of the Kobayashi-Maskawa matrix elements ‖$V_{\mathrm{cd}}$/$V_{\mathrm{cs}}$${\Vert }^2$ =0.057${\pm }_{0.015}^{+0.038\mathrm{}}$±0.005.

An analysis of pp¯ collision events at √s =1.8 TeV with jets and large missing transverse energy finds no event with missing transverse energy >40 GeV. This result yields a 90%-C.L. limit on the cross section for one-jet-event production of <0.1 nb for events with the jet in the pseudorapidity range ‖η‖<1.0 and with jet $E_T$>52 GeV. Limits on the masses of squarks and gluinos in a minimal supersymmetry model are also set. At the 90% C.L., $m_{q̃}$>74 GeV and $m_{g̃}$>73 GeV.

We report on the first observation of subthreshold antiproton production in nucleus-nucleus collisions. This measurement was made for the system ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ at a bombarding energy of 2.1 GeV/nucleon (kinetic energy per NN pair in the c.m. frame ∼850 MeV). A differential cross section $d^2$σ/dP d Ω of 80±40 nb/sr (GeV/c) was measured for p¯ production at 1.9 GeV/c and 0°. This result is 3 orders of magnitude larger than that predicted by a calculation incorporating internal motion of the nucleons in the colliding nuclei.

When observed in singles, complex fragments associated with the 18A-MeV ${}_{}{}^{139}{}_{}{}^{}\mathrm{Ni}$ reaction present a complex invariant-cross-section pattern without an obvious source, while the center-of-mass velocities of the coincident binary events span the entire range from complete fusion to near projectile velocities. By gating on a given value of the source velocity, it is possible to characterize precisely the product formed in the corresponding incomplete-fusion process and its decay.