The electron areal concentration $n_s$ of a one-sided 130-Å-thick GaAs-Ga(Al)As modulation-doped quantum well is continuously varied from 0 to 6.6×${10}^{11}$ ${\mathrm{cm}}^{-2\mathrm{}}$ by use of a Schottky gate. This allows one to study the precise evolution of the luminescence and excitation spectra with increasing $n_s$. From a comparison between the measured peak positions and Hartree calculations of the energy levels, a measurement of the band-gap renormalization is obtained.

The effective "miniband" mass approximation is shown to break down in strong magnetic fields when the magnetic length is smaller than the superlattice period. Under these conditions a real-space description must be used and features in the inhomogeneously broadened cyclotron resonance can be indexed by the position of the cyclotron orbit with respect to the barriers and wells. The line shape agrees with a calculation of the quantum states in a magnetic field in a one-dimensional periodic potential and these experiments give us a rare view of Landau states in the limit where the periodic potential has a period that is larger than the cyclotron diameter.

We present the results of a model-system study of the competition between superconductivity and disorder in narrow superconducting wires. As one moves from the two-dimensional regime toward the one-dimensional limit, large and systematic reductions in the superconducting transition temperature are obtained. The observed behavior extrapolates to the total destruction of superconductivity in the disordered one-dimensional limit. Our findings are in clear disagreement with a recent theoretical treatment. In addition, the superconducting fluctuations appear to be modified by disorder for the narrowest samples.

Extended x-ray-absorption fine-structure measurements have been made on the Ge and Pb edges of ferroelectric ${\mathrm{Pb}}_{1-x}{\mathrm{Ge}}_x\mathrm{Te}$ disordered alloys. Our results show that Ge atoms are displaced approximately 0.8 Å from the cell center both below and above the transition, implying that the ferroelectric transition involves long-range ordering of the dipoles. We have also measured the displacement of the Pb sublattice relative to the Te sublattice, thus showing evidence for a simultaneously order-disorder and displacive transition.

Using laser frequency-modulation spectroscopy, we have observed statistical fine structure (SFS) in the inhomogeneously broadened optical absorption of pentacene in $p$-terphenyl at liquid-helium temperatures. SFS is time-independent structure on the inhomogeneous line profile caused by statistical variations in the spectral density of absorbers in each frequency interval. The size of the SFS varies as the square root of the number of centers, and the autocorrelation of the SFS yields an estimate of the homogeneous linewidth without the requirement of spectral hole burning or coherent transients.

Reversible cellular automata are used to investigate the thermodynamic behavior of large systems. Additive conserved quantities are regarded as the energy of these models. By the consideration of a large system as the sum of a subsystem and a heat bath, it is numerically shown that a canonical distribution is realized under certain conditions concerning the conserved quantities.

We describe a mechanism for generic $\frac{1}{f}$ noise in nonintegrable Hamiltonian systems. The phenomenon is observed in the velocity fluctuations of a particle in a 2D periodic potential and is associated with anomalously enhanced deterministic diffusion. It is explained in terms of a renewal process and trapping in a hierarchy of nested cantori.

We generalize to a local relativistic quantum field theory a proposal of Ignatiev and Kuzmin for a single oscillator which has small violation of the Pauli principle and thus provide a theoretical framework which, for the first time, allows quantitative tests of the Pauli principle. Our theory provides a continuous interpolation between fully hindered parafermi statistics of order 2 ($\beta =0\mathrm{}$), which is equivalent to Fermi statistics, and ordinary parafermi statistics of order 2 ($\beta =1\mathrm{}$). We suggest two types of experiments which can place bounds on $\beta$.

We consider the Becchi-Rouet-Stora-Tyutin quantization of the $N=2\mathrm{}$ supersymmetric Siegel Lagrangean which is obtained by truncation of the $N=2\mathrm{}$ spinning string. We show that this Lagrangean describes four chiral bosons and that it can be consistently coupled to $N=(1, 0)$ supergravity. Using this system we propose a Lagrangean for the heterotic string in the bosonic formulation.

A minimal extension of the standard model is proposed to incorporate the light-neutrino masses and the resolution of the strong $\mathrm{CP}$ problem. A unique light pseudo-Goldstone boson, whose coupling to the electron is 1 or 2 orders of magnitude greater than that of the conventional hadronic axion, arises from the model. The scale of Peccei-Quinn symmetry breaking is shown to be greater than ${10}^{10}$ GeV through its connection to the light-neutrino masses, and is further constrained if the (hypothetical) fourth family of quarks and leptons exists.

We report a new and precise measurement of the lifetime of the $\tau$ lepton. The data were taken with the High Resolution Spectrometer at the SLAC $e^{+}{e}^{-}$ colliding-beam facility PEP operating at 29 GeV center-of-mass energy. The flight distances of 1311 $\tau$ decays to three charged particles were measured with a four-layer tubular-cell vertex chamber in conjunction with the main drift chamber. The resulting lifetime of the $\tau$ lepton is ${\tau }_{\tau }=(2.99\mathrm{}\pm 0.15\pm 0.10)\times {10}^{-13\mathrm{}}$ sec.

In a search for highly ionizing particles produced in $p\overline{p}$ collisions at $\sqrt{s}=1800\mathrm{}$ GeV, polycarbonate and CR-39 plastic track detectors were exposed to an integrated luminosity of ∼5×${10}^{32}$ ${\mathrm{cm}}^{-2\mathrm{}}$, and UG-5 glass detectors were exposed inside the vacuum system to look for short-range particles. No highly ionizing particles other than hadronically produced spallation recoils were detected.

Uranium-uranium collisions at the energy of the Brookhaven Relativistic Heavy-Ion Collider (100 + 100 GeV per nucleon) are predicted to produce an average of nearly 100 jets with $p_T>3\mathrm{}$ GeV. These jets will on average carry of 70 GeV of transverse energy $E_T$ per unit rapidity. Central collisions produce more transverse energy than this; the $E_T$ distribution extends up to about $\frac{\left(5\mathrm{}{A}^{\frac{4}{3}} {\mathrm{GeV}}^4\right)}{p_{Tmin}^3}$ per unit rapidity, which is 4 times the average. It is estimated that the minijets are likely to undergo further collisions and become thermalized.

First results from measurements of all the shower particles produced in 191 nearly central interactions of a 200-GeV/nucleon ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ beam with nuclear emulsion are presented. The lower limit of the average energy density for the events with multiplicity $n_s\ge 200$ is $〈\epsilon 〉\simeq 1$ GeV/${\mathrm{fm}}^3$.

Out of 30 000 interactions of 3.2-TeV ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ nuclei in Pb, we found that no fast fragments had a charge $Z>\mathrm{}8\mathrm{}$, none with $Z\gtrsim 5.5$ had a charge that differed from an integer by as much as $\frac{1}{3}$, and none with $Z\gtrsim 5.5$ had an angle to the beam > 0.8 mrad. The cross section for production of a fast fragment with $Z>\mathrm{}8\mathrm{}$ or a stable particle with charge $\frac{19}{3}e$, $\frac{20}{3}e$, $\frac{22}{3}e$, or $\frac{23}{3}e$ is less than 240 μb at 90% confidence level. Using a measurement technique with a position resolution of 30 μrad, we found the transverse-momentum distributions for C, N, and O to be Gaussians with widths ≃ 100 MeV/c, similar to those measured in projectile fragmentation of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ at a factor 100 lower energy.

In the limit of unbroken SU(3) symmetry, we exhibit a selection rule which forbids the decay of certain hypernuclear states involving a coherent admixture of $\Lambda$ and $\Sigma$ hyperons. This may provide an explanation of the narrow widths of some hypernuclear excitations observed in the $\Sigma$ continuum.

The ${}_{}{}^2{}_{}{}^{}\mathrm{H}(\gamma , p)n$ cross section was measured for the first time simultaneously at ${\theta }_p^{\mathrm{c}.\mathrm{m}.\mathrm{}}=0\mathrm{}°, 90°, \mathrm{and} 180°$. The photon energies were 170 and 210 MeV. A quasimonochromatic photon beam was used and the photon spectrum measured on line by a pair spectrometer. The results confirm the consistency among all experimental data from monochromatic photon beams and indicate that the angular distributions are less isotropic than predicted by theory. This problem might be solved by the introduction of relativistic corrections.

The spontaneous radiation of a classical oscillator and that of a two-level atom both in the vicinity of a phase-conjugate mirror are calculated and compared. In either case the phase-conjugate mirror may be replaced by a phase-conjugate "image" of the source. We study the dependence of the decay constants and the characteristic frequencies on the phase-conjugate reflectivity and the difference between the characteristic frequency of the radiator and the pump frequency of the phase-conjugate mirror.

We report the observation of the transition state ${\mathrm{HD}}_2$${\mathrm{}}^{++}$ in the chemical reaction H+${\mathrm{D}}_2$→(${\mathrm{HD}}_2^{++}$→)HD+D. The ${\mathrm{HD}}_2$ was detected by resonantly enhanced multiphoton ionization. It was identified by velocity-selected time-of-flight mass spectrometry, and by comparison of the time dependence of its concentration with that of reagent H and product D, also studied by resonantly enhanced multiphoton ionization.

We have used a two-mode optical parametric oscillator operating above threshold to generate high-intensity twin beams which exhibit quantum correlations. The noise power measured on the intensity difference between two such beams is reduced by 30% below the shot-noise limit. Noise reduction is observed over a broad range of frequencies.