Low-pressure Kr and Xe gas has been irradiated by a high-intensity (3×${10}^{12}$-${10}^{14}$ W ${\mathrm{cm}}^{-2\mathrm{}}$), short-pulse (1.3 ps), tunable uv laser beam. Enhancement of the ${\mathrm{Kr}}^{+}$ yield by 2 orders of magnitude by three-photon-resonant four-photon ionization is observed at intensities 1 to 2 orders of magnitude above previous results. The $4p\to 5d{\left[\frac{1}{2}\right]}_1$ and $4p\to 4d{\left[\frac{3}{2}\right]}_1$ three-photon Kr resonance profiles are measured as functions of peak laser intensity and are well fitted by a simple model incorporating line shifts and widths scaling linearly with laser intensity.

The spatial dependence of the interaction potentials in doubly charged dimers of transition-metal atoms has been calculated by use of both the tight-binding and the self-consistent-field linear combination of atomic orbitals-molecular orbitals methods. The study reveals an interesting correlation between the stability of the doubly charged dimers and their electronic structure. Situations have been identified where the charged dimers can exist in metastable states so long that their metastability may be impossible to observe.

Squeezed light in the form of a train of pulses, each approximately 100 ps in width, has been generated by parametric down-conversion in KTiOP${\mathrm{O}}_4$ crystals. The measured noise reduction in the quiet quadrature of the field is 0.6 dB below the shot-noise limit. The quadrature noise components are measured with a balanced homodyne detector using a pulse train for the local oscillator. The noise reduction extends over a broad spectrum and has important implications for precision measurement at very short time scales.

The wave-number band accessible to a homogeneous pattern-forming system collapses to a single wave number if the control parameters are ramped slowly in space from subcritical to supercritical. This selection mechanism is investigated for Taylor vortex flow. Quantitative agreement is found with recent experiments. By variation of both radii independently, any wave number within the stable band can be selected. In addition, there exist ramps which do not permit any static patterns but force them to drift. The drift velocity is calculated. Such a drift is not automatically induced by a large-scale flow.

The new molecular-dynamics methods of Andersen and others have been used to study the structural phase transition that occurs in solid adamantane. While the transition between ordered and plastic-crystal phases is spontaneous on heating, cooling leads to metastable structures. The transition to the ordered phase can, however, be induced by extension to the complete molecular-dynamics sample of the short-range orientational correlations characteristic of the high-temperature phase.

We introduce a new type of phase of crystal surface and interfaces. This disordered flat phase appears intermediate between the familiar flat and rough phases in the presence of short-range interactions of a type common in experiments. The surface remains flat on average although it contains a disordered array of steps. The preroughening transition into the disordered flat phase belongs to a new universality class. Finite-size-scaling calculations for the restricted solid-on-solid model confirm the existence of the disordered flat phase and the preroughening transition.

The core structure of half-integer wedge disclinations in nematic liquid crystals has been investigated within the Landau-de Gennes theory, by solution of the appropriate Euler-Lagrange equations. Close to the nematic-isotropic transition the energy density exhibits a domain-wall-like structure around the core which disappears at low temperatures. The inner core does not consist of isotropic fluid, and the core is heavily biaxial at all temperatures.

It is proposed to describe the frequency-dependent transport properties in disordered systems by an effective dimension $\tilde{d}$, akin to the spectral dimension of the percolation problem. This concept is used to analyze proton nuclear relaxation and ac conductivity measurements in a conducting polymer, polypyrrole-perchlorate. Below 200 K, a one-dimensional motion along the polymer chain is observed. At higher temperature, the progressive increase of $\tilde{d}$ points out an anomalous diffusion involving a large range of interchain hopping rates.

The coherent interaction of femtosecond laser pulses and thin CdSe and GaAs samples is investigated experimentally and theoretically. Oscillatory structures in the differential probe transmission around the exciton resonance and around the pump frequency are observed when the probe pulse precedes the pump. Comparison with theory attributes the oscillations to the coherent coupling between the light field and the electron-hole transitions in the semiconductor. For nonresonant excitation, the oscillatory structures around the exciton are identified as the early stages of the optical Stark effect.

The high-resolution Bitter pattern technique has been used to reveal the magnetic structure of single-crystal samples of high-$T_c$ superconductor Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ at 4.2 K. Typical patterns consist of hexagonally correlated, singly quantized vortices of flux $\frac{\mathrm{hc}}{2e}$. That is, the structures are comparable to those that would be observed in conventional type-II superconductors under similar conditions.

Using dc sputtering in an UHV system we have prepared multilayer samples of Cu:Mn-Si with Cu:Mn thicknesses, $L$, between 4 and 500 nm. The temperature of the peak in the dc magnetic susceptibility of these spin-glass samples shifts with $L$ as $\frac{(T_{g0\mathrm{}}-T_g)}{T_{g0\mathrm{}}}\approx L^{-0.75\mathrm{}\pm 0.06}$ over the whole range of sample thickness. These results are discussed in terms of finite-size scaling and the possible effects of the electron mean free path on the effective exchange interaction.

We have employed neutron spin-echo techniques to study the dynamics of the shape fluctuation of microemulsion droplets. It is found that the scattering spectrum of a droplet system with a labeled layer of surfactants exhibits a sharp peak in the relaxation frequency spectrum. We have determined that the fluctuations are driven mainly by the bending modes on the surface, and the height of the peak allows us to measure the surface elastic bending coefficient directly.

Upward-going muons with energy greater than 1.7 GeV produced in the nearby rock by neutrinos from SN1987A were searched for in a 2140-ton underground water Cherenkov detector, Kamiokande II. No upward-going muons from the direction of SN1987A were found from 23 February 1987 to 1 September 1987; the 90%-confidence-level flux limit is 1.2×${10}^{-13\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ for an energy threshold of 1.7 GeV. For neutrino energy greater than 1.7 GeV, limits on the SN1987A neutrino flux and luminosity are 2.4×${10}^{-5\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ and 1.6×${10}^{41}$ erg ${\mathrm{s}}^{-1\mathrm{}}$ for cutoff energy $E_c={10}^{15}$ eV and spectral index $\gamma =2.1, \mathrm{and} 2.3\times {10}^{-3\mathrm{}} {\mathrm{cm}}^{-2\mathrm{}} {\mathrm{s}}^{-1\mathrm{}} \mathrm{and} 4.6\times {10}^{42} \mathrm{erg} {\mathrm{s}}^{-1\mathrm{}}$ for $E_c={10}^{12}$ eV and $\gamma =2.7\mathrm{}$.

In recent cosmological models, there is an "anthropic" upper bound on the cosmological constant $\Lambda$. It is argued here that in universes that do not recollapse, the only such bound on $\Lambda$ is that it should not be so large as to prevent the formation of gravitationally bound states. It turns out that the bound is quite large. A cosmological constant that is within 1 or 2 orders of magnitude of its upper bound would help with the missing-mass and age problems, but may be ruled out by galaxy number counts. If so, we may conclude that anthropic considerations do not explain the smallness of the cosmological constant.

An explicit parametrization is obtained of the set of all space-time solutions of Einstein's equations which are globally hyperbolic and contain a compact spatial hypersurface with constant mean curvature. This parametrization is based upon the conformal treatment of the initial-value problem for Einstein's equations, which is studied by the method of sub and super solutions for quasilinear elliptic partial differential equations. The Yamabe-Aubin-Trudinger-Schoen classification of conformal classes of Riemannian metrics plays a key role.

The local complex action is found for a closed, interacting lattice string whose configurations generate the low-temperature graphs of the three-dimensional Ising model. This action depends upon Ising degrees of freedom on the surface, as well as its extrinsic curvature.

A simple heuristic proof of the Nielsen-Ninomaya theorem is given. A method is proposed whereby the multiplication of fermion species on a lattice is reduced to the minimal doubling, in any dimension, with retention of appropriate chiral symmetries. Also, it is suggested that use of spatially thinned fermion fields is likely to be a useful and appropriate approximation in QCD—in any case, it is a self-checking one.

Interesting ambiguities associated with the possibility of charge-nonconserving decays are pointed out. Such decays or $e^{-}\leftrightarrow e^{+}$ oscillation could quench electrostatic potentials larger than several million volts. A connection to the Feynman diagrams and infrared divergence is pointed out, and a simple intuitive space-time picture suggests that $m_{\gamma }\le {10}^{-15\mathrm{}}$ eV implies very long decay times for charge-nonconserving decays.

Partial-wave unitarity implies an upper limit on the energy scale of the mechanism that generates quark and lepton masses.

Three basic pionic amplitudes $f_{\pi }^{+}$, $F_{\pi }$ (for ${\pi }^0\to \gamma \gamma$), and $F_{3\pi }$ (for $\pi \gamma \to \pi \pi$) are calculated in a QCD-oriented Bethe-Salpeter model of $q\overline{q}$ hadrons which fits the spectra of all quarkonia. The results, $f_{\pi }^{+}=157\mathrm{}$ MeV (experiment, 135), $\Gamma ({\pi }^0\to \gamma \gamma )=8.03\mathrm{}$ eV (7.9), and $F_{3\pi }=12.7\mathrm{}$ Ge${\mathrm{V}}^{-3\mathrm{}}$ (13±2), obtained with no free parameters, agree with the data and with the Wess-Zumino-Witten theory.