A single crystal of gadolinium-doped ${\mathrm{SmAl}}_2$ has zero magnetization in the midst of the ordered temperature region, despite the probable ferromagnetic spin ordering. The asymmetry in Compton-scattering intensity when switching between right- and left-handed polarization of incident 150-keV synchrotron radiation provides decisive proof that ferromagnetic order is really there, and that spin and orbital magnetic contributions cancel. The experiments also show that the spin direction at this zero-magnetization state is rather stable against the external magnetic field and, nevertheless, reversible by a preceding control of temperature and an external field.

A multidisciplinary approach based on concepts from sociology, educational psychology, statistical physics, and computational science is developed for the theoretical description of teaching-learning processes that take place in the classroom. The emerging model is consistent with well-established empirical results, such as the higher achievements reached working in collaborative groups and the influence of the structure of the group on the achievements of the individuals. Furthermore, another social learning process that takes place in massive interactions among individuals via the Internet is also investigated.

We describe the results of a search for time variability of the fine structure constant $\alpha$ using absorption systems in the spectra of distant quasars. Three large optical data sets and two $21\mathrm{cm}$ and mm absorption systems provide four independent samples, spanning $\sim 23\%$ to $87\%$ of the age of the universe. Each sample yields a smaller $\alpha$ in the past and the optical sample shows a $4\sigma$ deviation: $\Delta \alpha /\alpha =-0.72\mathrm{}\pm 0.18\times {10}^{-5\mathrm{}}$ over the redshift range $0.5<z<\mathrm{}3.5\mathrm{}$. We find no systematic effects which can explain our results. The only potentially significant systematic effects push $\Delta \alpha /\alpha$ towards positive values; i.e., our results would become more significant were we to correct for them.

The differences between neutron and proton density distributions at large nuclear radii in stable nuclei were determined. Two experimental methods were applied: nuclear spectroscopy analysis of the antiproton annihilation residues one mass unit lighter than the target mass and the measurements of strong-interaction effects on antiprotonic x rays. Assuming the validity of two-parameter Fermi neutron and proton distributions at these large radii, the conclusions are that the two experiments are consistent with each other and that for neutron rich nuclei it is mostly the neutron diffuseness which increases and not the half-density radius. The obtained neutron and proton rms radii differences are in agreement with previous results.

The technique of single atom manipulation by means of the scanning tunneling microscope (STM) applies to the controlled displacement of large molecules. By a combined experimental and theoretical work, we show that in a constant height mode of manipulation the STM current intensity carries detailed information on the internal mechanics of the molecule when guided by the STM tip. Controlling and time following the intramolecular behavior of a large molecule on a surface is the first step towards the design of molecular tunnel-wired nanorobots.

Data from satellite, balloon, and ground-station measurements show that ozone loss is strongly correlated with cosmic-ray ionization-rate variations with altitude, latitude, and time. Moreover, our laboratory data indicate that the dissociation induced by cosmic rays for ${\mathrm{CF}}_2{\mathrm{Cl}}_2$ and ${\mathrm{CFCl}}_3$ on ice surfaces in the polar stratosphere at an altitude of $\sim 15\mathrm{km}$ is quite efficient, with estimated rates of $4.3\times {10}^{-5\mathrm{}}$ and $3.6\times {10}^{-4\mathrm{}}{\mathrm{s}}^{-1\mathrm{}}$, respectively. These findings suggest that dissociation of chlorofluorocarbons by capture of electrons produced by cosmic rays and localized in polar stratospheric cloud ice may play a significant role in causing the ozone hole.

Solar neutrinos from ${}^8\mathrm{B}$ decay have been detected at the Sudbury Neutrino Observatory via the charged current (CC) reaction on deuterium and the elastic scattering (ES) of electrons. The flux of ${\nu }_e$’s is measured by the CC reaction rate to be ${\phi }^{\mathrm{CC}}\left({\nu }_e\right)=1.75\mathrm{}\pm 0.07\left(\mathrm{stat}{)}_{-0.11\mathrm{}}^{+0.12\mathrm{}}\right(\mathrm{syst})\pm 0.05(\mathrm{theor})\times {10}^6{\mathrm{cm}}^{-2\mathrm{}}{\mathrm{s}}^{-1\mathrm{}}$. Comparison of ${\phi }^{\mathrm{CC}}\left({\nu }_e\right)$ to the Super-Kamiokande Collaboration’s precision value of the flux inferred from the ES reaction yields a $3.3\sigma$ difference, assuming the systematic uncertainties are normally distributed, providing evidence of an active non- ${\nu }_e$ component in the solar flux. The total flux of active ${}^8\mathrm{B}$ neutrinos is determined to be $5.44\pm 0.99\times {10}^6{\mathrm{cm}}^{-2\mathrm{}}{\mathrm{s}}^{-1\mathrm{}}$.

First lifetime results obtained with a scanning positron microscope will be presented. A pulsed positron beam with a variable energy from 0.5 to 20 keV, with a spot diameter of $2\mu \mathrm{m}$, can be electronically scanned over an area of $0.6\times 0.6{\mathrm{mm}}^2$. This beam is formed after a double-stage stochastic cooling (moderation) of positrons emitted from a radioactive isotope. Included in the system is a conventional scanning electron microprobe for surface analysis. Three-dimensional positron lifetime spectra of a GaAs sample with a small surface scratch reveal the range due to the mechanical damage.

Using a new type of ion trap, we demonstrate that the length of a packet of charged particles oscillating between two electrostatic mirrors will remain constant under special conditions. The effect can be understood in terms of phase synchronization, where, in a rather counterintuitive way, the repulsive Coulomb interaction between the ions actually holds the packet together. Application of this effect for mass spectrometry is discussed.

We report the temperature dependent nucleation rates of ice from single water drops supporting aliphatic alcohol Langmuir films. Analysis in the context of a classical theory of heterogeneous nucleation suggests that the critical nucleus is essentially a monolayer, and that the rate-limiting steps in these nucleation processes are therefore not merely influenced by, but instead dictated by, the physics of the water-alcohol interface. Consequently, reduced dimensionality may be much more important in heterogeneous nucleation than has previously been believed.

We have reconstructed the quantum state of optical pulses containing single photons using the method of phase-randomized pulsed optical homodyne tomography. The single-photon Fock state $|1〉$ was prepared using conditional measurements on photon pairs born in the process of parametric down-conversion. A probability distribution of the phase-averaged electric field amplitudes with a strongly non-Gaussian shape is obtained with the total detection efficiency of $(55\pm 1)\%$. The angle-averaged Wigner function reconstructed from this distribution shows a strong dip reaching classically impossible negative values around the origin of the phase space.

We present the concept of a sensitive and broadband resonant mass gravitational wave detector. A massive sphere is suspended inside a second hollow one. Short, high-finesse Fabry-Perot optical cavities read out the differential displacements of the two spheres as their quadrupole modes are excited. At cryogenic temperatures, one approaches the standard quantum limit for broadband operation with reasonable choices for the cavity finesses and the intracavity light power. A molybdenum detector, of overall size of 2 m, would reach spectral strain sensitivities of $2\times {10}^{-23\mathrm{}}{\mathrm{Hz}}^{-1/2\mathrm{}}$ between 1000 and 3000 Hz.

First-principles calculations of the electronic band structure and lattice dynamics for the new superconductor $\mathrm{MgB}{}_2$ are carried out and found to be in excellent agreement with our inelastic neutron scattering measurements. The numerical results reveal that the $E_{2g}$ in-plane boron phonons near the zone center are very anharmonic and strongly coupled to the planar B $\sigma$ bands near the Fermi level. This giant anharmonicity and nonlinear electron-phonon coupling is key to quantitatively explaining the observed high $T_c$ and boron isotope effect in $\mathrm{MgB}{}_2$.

We present a new instability observed in rapid granular flows down rough inclined planes. For high inclinations and flow rates, the free surface of the flow experiences a regular deformation in the transverse direction. Measurements of the surface velocities imply that this instability is associated with the formation of longitudinal vortices in the granular flow. From the experimental observations, we propose a mechanism for the longitudinal vortex formation based on the concept of granular temperature.

We have observed capillary condensation in nanometer-thin films of nematic liquid crystals using force spectroscopy. The liquid crystal was confined to a submicron gap between a flat substrate and a microsphere, attached to the cantilever of an atomic force microscope. A long-ranged and strongly attractive force due to capillary condensation of a nematic phase was observed at temperatures close to the bulk isotropic-nematic phase transition. The critical point, terminating the first-order confined isotropic-nematic transition line, was determined for the first time.

We present the first demonstration of the use of ultrafast extreme-ultraviolet pulses to directly monitor a surface chemical reaction on femtosecond time scales. By adsorbing molecular oxygen onto a platinum surface and exciting it with an ultrafast laser pulse, changes in the oxygen-platinum chemical bond on a subpicosecond time scale were observed through changes in the photoelectron spectra. This work demonstrates a powerful new technique for studying reactions of interest in catalysis and for probing changes of local order on surfaces on their fundamental time scales.

The smallest quantum code that can correct all one-qubit errors is based on five qubits. We experimentally implemented the encoding, decoding, and error-correction quantum networks using nuclear magnetic resonance on a five spin subsystem of labeled crotonic acid. The ability to correct each error was verified by tomography of the process. The use of error correction for benchmarking quantum networks is discussed, and we infer that the fidelity achieved in our experiment is sufficient for preserving entanglement.

We propose that large, reversible shape changes in solids, of between $10\%–400\%$, can be induced optically by photoisomerizing monodomain nematic elastomers. Empirical and molecular analysis of shape change and its relation to thermal effects is given along with a simple model of the dynamics of response. Our experiments demonstrate these effects for the first time and theory is compared qualitatively with our results.

We report inelastic neutron scattering measurements of the phonon density of states in $\mathrm{Mg}{}^{11}{\mathrm{B}}_2$, which has a superconducting transition at 39.2 K. The acoustic phonons extend in energy to 36 meV, and there are highly dispersive optic branches peaking at 54, 78, 89, and 97 meV. A simple Born–von Kàrmàn model reproduces the mode energies, and provides an estimate of the electron-phonon coupling of $\lambda \sim 0.9$. Furthermore, the estimated boron and magnesium contributions to the isotope effect are in qualitative agreement with experiment. The data confirm that a conventional phonon mechanism, with moderately strong electron-phonon coupling, can explain the observed superconductivity.

The vector nature of magnetic fields and the geometrical interpretation of gravity introduced by general relativity guarantee a special coupling between magnetism and spacetime curvature. This magnetogeometrical interaction effectively transfers the tension properties of the field into the spacetime fabric, triggering a variety of effects with profound implications. Given the ubiquity of magnetic fields in the universe, these effects could prove critical. We discuss the nature of the magnetic-field–curvature coupling and illustrate some of its potential implications for cosmology.