We explain the deep electrical conductivity minimum near x=0.05 in the perovskite-type ceramic ${\mathrm{LaCr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{O}}_3$ as a crossover between two different regimes of hopping conduction. At low Mn concentrations the diffusion of small polarons among Cr ions is limited by multiple trapping at energetically lower Mn sites. At higher concentrations a percolating path of Mn sites forms and direct transport between Mn ions dominates the conduction process.

Stacey et al. have reported evidence for a breakdown of Newton’s law based on measurements in a deep mine. We have tested the reproducibility of this result by analyzing gravity data from boreholes in Nevada. One interpretation of our resuls suggests a breakdown of the Newtonian theory which is much larger than the effect previously reported. But the lack of consistency between the results suggests that it is not fundamental physics that has failed, but rather the experiments are subject to large systematic uncertainties which are caused by mass anomalies at intermediate distances from the holes.

Even where a symmetry is spontaneously broken, there are circumstances in which it can be used to classify particles of definite mass as belonging to one or a few of its irreducible representations. Examples are given of applications to quantum chromodynamics and technicolor models.

An explanation is offered why quarks in the constituent quark model should be treated as particles with axial coupling ${\mathit{g}}_{\mathit{A}}$=1 and no anomalous magnetic moment.

Using the CLEO detector we have observed the Cabibbo-suppressed decays ${\mathit{D}}^0$→${\mathit{K}}^0$K${\mathrm{¯}}^0$, ${\mathit{K}}^{+}$${\mathit{K}}^{\mathrm{-}}$, and ${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^{\mathrm{-}}$. We determine B(${\mathit{D}}^0$→${\mathit{K}}^0$K${\mathrm{¯}}^0$)= (0.${13}_{\mathrm{-}0.05\mathrm{-}0.02}^{+0.07+0.02\mathrm{}}$)%. Our measurement B(${\mathit{D}}^0$→${\mathit{K}}^{+}$${\mathit{K}}^{\mathrm{-}}$)/B(${\mathit{D}}^0$→${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^{\mathrm{-}}$)=2.35±0.37±0.28 represents asubstantial improvement over previous results. We find no evidence for the decay ${\mathit{D}}^0$→${\mathrm{\pi }}^0$${\mathrm{\pi }}^0$, and place a 90%-confidence-level upper limit of B(${\mathit{D}}^0$→${\mathrm{\pi }}^0$${\mathrm{\pi }}^0$)<0.46%.

A search for decays of the form ${\mathit{K}}^{+}$→${\mathrm{\pi }}^{+}$γγ was performed for pion kinetic energies between 117 and 127 MeV. A 90%-C.L. upper limit for the branching ratio of B(${\mathit{K}}^{+}$→${\mathrm{\pi }}^{+}$γγ)<1.0×${10}^{\mathrm{-}6}$ is set for direct decays assuming a ${\mathrm{\pi }}^{+}$ energy distribution given by phase space. Limits are set for decays of the type ${\mathit{K}}^{+}$→${\mathrm{\pi }}^{+}$${\mathit{X}}^0$, ${\mathit{X}}^0$→γγ, where ${\mathit{X}}^0$ is any short-lived neutral particle with mass smaller than 100 MeV/${\mathit{c}}^2$.

Parity nonconservation was studied for seventeen states in ${}_{}{}^{239}{}_{}{}^{}\mathrm{U}$ by measuring the helicity dependence of the total cross section for epithermal neutrons scattered from ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$. The root-mean-squared parity-violating matrix element for the mixing of p-wave and s-wave states was determined to be M=0.${58}_{\mathrm{-}0.25}^{+0.50\mathrm{}}$ meV. This corresponds to a parity-violating spreading width of ${\mathrm{\Gamma }}^{\mathrm{PV}}$=1.0×${10}^{\mathrm{-}7}$ eV. Under plausible assumptions this gives a value of 4×${10}^{\mathrm{-}7}$ for ‖${\mathrm{\alpha }}_{\mathit{P}}$‖, the ratio of strengths of the P-odd and P-even effective nucleon-nucleon interactions.

No evidence of neutron emission was observed following deuterium loading into palladium and titanium in both electrochemical and pressurization experiments. Upper limits obtained with a detector having a very clean neutron signature are at least 100 times lower than values reported in recent publications giving evidence of cold fusion. The deduced fusion rate limits were lower than 2×${10}^{\mathrm{-}26}$ per second per pair of deuterons.

A feature associated with continuous-wave two-photon optical gain has been observed in the absorption spectrum of an ensemble of barium atoms driven by a strong near-resonant optical field. In the dressed-atom picture, the observed gain is attributable to inverted two-photon transitions with nearly resonant intermediate states. A cw optical two-photon laser utilizing this gain appears feasible.

We have employed an ultrasharp tip, prepared by field-ion techniques, as a coherent point source of low-energy electrons. The source, which constitutes the origin of a spherical electron wave, is positioned close to small carbon fibers, at which some of the electrons are elastically scattered. At macroscopic distances a hologram is observed, which arises from the interference of the scattered electrons with the reference wave. The high magnification of the interference pattern is brought about purely by geometric projection. No electron lenses are emloyed and, thus, the high-resolution microscopic information gained by this novel technique is not affected by aberrations.

The linearized coupled-mode equations in two dimensions are solved numerically within the paraxial approximation to find the absolute-mode threshold of parametric instabilities driven by an incoherent laser beam. The results show that, for plasma lengths only a few times threshold, a surprisingly small temporal bandwidth Δω that is substantially less than the growth rate ${\gamma }_0$ is effective in suppressing absolute growth. Only if the plasma length greatly exceeds the threshold is the necessary temporal bandwidth the larger value obtained from homogeneous plasma theory.

The order-parameter texture has been studied in rotating ${}_{}{}^3{}_{}{}^{}\mathit{B}$ in the vortex-free state with NMR techniques. A sequence of textural phase transitions is observed with increasing rotation speed. It is generated by competing interactions with magnetic field, surfaces, and rotation. Two surface interactions are extracted: the susceptibility anisotropy at the wall and the gyromagnetism due to a magnetic-field-induced surface supercurrent. The results are consistent with an order-parameter structure approaching the planar state with diffuse quasiparticle scattering at the wall.

Formation of radiation damage and its recovery in ${\mathrm{UO}}_2$ single crystals were investigated in the temperature range from 5 to 293 K. The samples were implanted with Xe ions at 5 K and subsequently in situ analyzed using the ion-channeling technique. Successive measurements were performed during the warming-up of the sample to 293 K. Two recovery stages in the U sublattice were revealed: the first one in the vicinity of 77 K and the second one at 110±5 K.

The pressure dependence and isotope effect of cobalt diffusion have been measured in amorphous ${\mathrm{Co}}_{76.7}$${\mathrm{Fe}}_2$${\mathrm{Nb}}_{14.3}$${\mathrm{B}}_7$ and in fcc Co. Within experimental accuracy no pressure dependence and an isotope effect E=(${\mathit{D}}_{\mathrm{\alpha }}$/${\mathit{D}}_{\mathrm{\beta }}$-1)/[(${\mathit{m}}_{\mathrm{\beta }}$/${\mathit{m}}_{\mathrm{\alpha }}$${)}^{1/2}$-1]=0.1 were found in ${\mathrm{Co}}_{76.7}$${\mathrm{Fe}}_2$${\mathrm{Nb}}_{14.3}$${\mathrm{B}}_7$. These results show that diffusion in the metallic glass is not mediated through quasivacancies in thermal equilibrium similar to crystalline Co. Diffusion rather occurs via a direct mechanism involving about ten atomic masses.

A new symmetry-based phenomenological model for the modulated structures in the dielectric ${\mathit{A}}_2$${\mathit{BX}}_4$ family is constructed. The model contains a novel competing-interaction mechanism which gives a rich phase diagram. Furthermore, the model gives a universal prediction for both the space groups and the modulation wave vectors for the sequences of phase transitions observed experimentally in the entire ${\mathit{A}}_2$${\mathit{BX}}_4$ family.

Si molecular-beam epitaxy (MBE) on smooth Si(100) surfaces is shown to occur at room temperature. We demonstrate for the first time that Si deposition becomes amorphous after growth of a limiting epitaxial thickness (${\mathit{h}}_{\mathrm{epi}}$). ${\mathit{h}}_{\mathrm{epi}}$ is ≊10–30 Å at room temperature and increases rapidly at higher temperatures with a rate-dependent activation energy in the range 0.4–1.5 eV. The effect is tentatively linked to surface roughening during growth at low temperatures, and is probably general in MBE, also occurring for Si/Si(111), Ge/Si(100, and GaAs/GaAs(100).

In Brillouin-light-scattering studies of a series of Ag/Pd superlattice films we have observed scattering from longitudinal guided modes (LGMs) whose velocities are determined primarily by the ${\mathit{c}}_{11}$ elastic constant. From the measured velocity of the first-order LGM, we have obtained the first accurate determination of ${\mathit{c}}_{11}$ for metallic thin films. For the Ag/Pd superlattice films, large increases in both the longitudinal (${\mathit{c}}_{11}$) and shear (${\mathit{c}}_{55}$) elastic constants were observed as the superlattice modulation wavelength Λ was decreased below Λ=60 Å.

The presence of a nonrandom fractional number of atomic planes in each layer of a superlattice produces unusual diffraction patterns in which the peaks cannot be indexed in the usual fashion as due to a single series. The x-ray line broadenings are distinctly different from earlier measurements and calculations in which the interfacial roughness is due to random variations of the scattering function. Therefore, interfacial roughness encountered at a single interface may just be a consequence of controlled, but not random, roughness and that under proper growth conditions superlattices with atomically sharp interfaces may be produced. These results are in good agreement with experimental measurements.

Results of ab initio computations for a single chain of trans-polyacetylene are reported. Starting from self-consistent-field calculations within a Gaussian-type-orbital basis set, correlations are included by using the local ansatz. The dependence of the total energy on dimerization is then analyzed in order to explain the origin of dimerization. The position of the equilibrium dimerization is dominantly determined by atomic correlation effects. Contributions to the exchange energy from nearest-neighbor Coulomb interactions play a non-negligible role concerning the depth of the energy minimum although their strength is strongly reduced in comparison to self-consistent-field results. Electron-lattice coupling alone leads only to a marginal dimerization. These findings are in contrast to assumptions often made in connection with soliton theories.

Combining high-resolution absorption and electron-emission spectra for ${\mathrm{CaF}}_2$, we have investigated the nature of the electronic transitions near the Ca L edge and the F K edge. The presence or absence of spectator and participator lines in the Auger decay of the core-hole excited states is direct spectroscopic evidence which enables us to infer whether or not the final state in the absorption transition is a bound state. Additionally we obtain information about the location of the excited-state electron relative to the ion cores.