We report measurements of mm-wavelength radiation from two-dimensional arrays of underdamped Josephson junctions. All of our samples emit coherently in a novel synchronized state, which is triggered by a resonance in the array structure. Measurements of the detected power as a function of the number $N$ of active junctions show a threshold, suggesting population inversion. Above threshold, the power scales with $N^2$ up to an array size bigger than the free-space radiation wavelength. The highest measured conversion efficiency from dc to ac power is about 17%. Our data are consistent with stimulated emission causing coherence and cannot be explained by existing classical coupling mechanisms.

A new laser-accelerator injector based on the high-energy electrons ejected from a laser focus following the ionization of high-charge states of gases has been investigated. This method can generate electron pulses with MeV energies and pulse widths substantially less than the laser pulse width using terawatt lasers. Experiments at intensities of $3\times {10}^{18}\mathrm{W}/\mathrm{cm}{}^2$ show that two highly directional electron beams with energies up to 340 keV are produced in the laser polarization direction. These highly directional beams are inconsistent with simulation results using Ammosov-Delone-Krainov (ADK) tunneling theory to model the ionization process. It is shown that this is likely due to the release of electrons with larger canonical momentum than predicted by ADK theory.

The breakup of ${}^{12}\mathrm{Be}$ into ${}^6\mathrm{He}{+}^6\mathrm{He}$ and ${}^4\mathrm{He}{+}^8\mathrm{He}$ has been studied using a 378 MeV ${}^{12}\mathrm{Be}$ beam inelastically excited by ${}^{12}\mathrm{C}$ and $({\mathrm{CH}}_2{)}_n$ targets. The measurements indicate that breakup occurs from rotational states in the 10 to 25 MeV excitation energy interval, with spins in the range of $4ħ$ to $8ħ$. The inferred moment of inertia is consistent with the cluster decay of an exotic molecular structure in ${}^{12}\mathrm{Be}$, which may be associated with an $\alpha -4n-\alpha$ cluster configuration.

Recent molecular level studies of energy dissipation in sliding friction have suggested a contribution from adhesive forces. In order to observe this directly, we have constructed a scanning force microscope with decoupled lateral and normal force sensors to simultaneously observe the onset of both friction and attractive forces. Measurements made on self-assembling alkanethiol films with chemically different tail groups show that friction can increase with stronger adhesive intermolecular forces and from the associated tensile deformation and collective motion of the thiol chains.

We show the design and preparation of a novel type of magnet exhibiting two compensation temperatures; i.e., the spontaneous magnetization changes its sign twice with changing temperature. The key to obtaining this unusual behavior is the simultaneous incorporation of one antiferromagnetic and two different ferromagnetic interactions through the use of four different spin sources, as predicted by a calculation based on molecular field theory. As a prototype exemplifying this idea, we have prepared a Prussian blue analog, $\left({\mathrm{Ni}}_{0.22}^{\mathrm{II}}{\mathrm{Mn}}_{0.60}^{\mathrm{II}}{\mathrm{Fe}}_{0.18}^{\mathrm{II}}{)}_{1.5}\right[{\mathrm{Cr}}^{\mathrm{III}}\left(\mathrm{CN}{)}_6\right]̇7.6{\mathrm{H}}_2\mathrm{O}$, which exhibits magnetization reversals at 35 and 53 K.

We propose a new system for implementing quantum logic gates: neutral atoms trapped in a very far-off-resonance optical lattice. Pairs of atoms are made to occupy the same well by varying the polarization of the trapping lasers, and then a near-resonant electric dipole is induced by an auxiliary laser. A controlled-NOT can be implemented by conditioning the target atomic resonance on a resolvable level shift induced by the control atom. Atoms interact only during logical operations, thereby suppressing decoherence.

Fluids adsorbed in microporous materials can exhibit quantum sieving, where heavy isotopes are preferentially adsorbed over light isotopes. We present the first detailed predictions of quantum sieving based on realistic models of microporous materials. Carbon nanotubes, and the interstices of nanotube bundles, can act as highly effective quantum sieves for hydrogen isotopes. Quantum sieving of He and Ne in the zeolite ${\mathrm{AlPO}}_4-22$ is also discussed.

We study the propagation of an elastic excitation through a magnetorheological slurry made of hydrogen-reduced iron particles suspended in glycerine. Two different low-frequency longitudinal modes are observed, the second of them appearing as soon as a magnetic field is applied to the fluid. The first mode travels through glycerine channels within the suspension, whereas the second one propagates through the fibrillated structure formed by the iron particles. This second mode is very strong in amplitude and travels at lower speeds, depending on the magnetic field intensity.

The ground-state band of the $Z=102\mathrm{}$ isotope ${}^{254}\mathrm{No}$ has been identified up to spin 14, indicating that the nucleus is deformed. The deduced quadrupole deformation, $\beta =0.27\mathrm{}$, is in agreement with theoretical predictions. These observations confirm that the shell-correction energy responsible for the stability of transfermium nuclei is partly derived from deformation. The survival of ${}^{254}\mathrm{No}$ up to spin 14 means that its fission barrier persists at least up to that spin.

Scanning tunneling microscopy studies of etching of $\mathrm{Si}\left(100\right)-(2\times 1)$ show that the rate of terrace pit formation goes through a maximum for surface coverages of $\theta \left(\mathrm{Cl}\right)=0.77\mathrm{}\pm 0.05\mathrm{monolayer}$, in contrast to predictions of conventional models. Using recently calculated energies for different possible surface configurations, we show that a key component in desorption is the formation of a single-atom vacancy adjacent to a volatile ${\mathrm{SiCl}}_2$ unit. The demonstration of vacancy-assisted reaction establishes a self-limited reaction and the sequence of events leading to desorption.

Magnetotransport experiments on high mobility two-dimensional electron gases in GaAs/AlGaAs heterostructures have revealed striking anomalies near half filling of several spin-resolved, yet highly excited, Landau levels. These anomalies include strong anisotropies and nonlinearities of the longitudinal resistivity ${\rho }_{\mathrm{xx}}$ which commence only below about 150 mK. These phenomena are not seen in the ground state or first excited Landau level but begin abruptly in the third level. Although their origin remains unclear, we speculate that they reflect the spontaneous development of a generic anisotropic many-electron state.

We report, for magnetic fields of 0, 8.8, and 14.8 T, measurements of the temperature dependent ${}^{63}\mathrm{Cu}$ NMR spin lattice relaxation rate for near optimally doped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$, near and above $T_c$. In sharp contrast with previous work we find no magnetic field dependence. We discuss experimental issues arising in measurements of this required precision and implications of the experiment regarding issues including the spin gap or pseudogap.

We demonstrate a juggling atomic fountain by launching two laser-cooled balls of ${}^{133}\mathrm{Cs}$ atoms in rapid succession. The atoms collide near the top of the fountain and, by varying the delay between launches, we scan the collision energy from 19 to $150\mu \mathrm{K}$. We measure the differential scattering cross section and isolate the contributions from different partial waves including their interferences. We observe the $s$-wave energy dependence and the $p$-wave scattering threshold. We infer a triplet $s$-wave scattering length of ${-400a}_0$ and a $p$ wave of $({-107a}_0{)}^3$ for $|F=4,m=4\mathrm{}〉+|3,3〉$.

Extremely high energy ( $\sim {10}^{22}\mathrm{eV}$) cosmic neutrino beams initiate high energy particle cascades in the background of relic neutrinos from the big bang. We perform numerical calculations to show that such cascades could contribute more than 10% to the observed cosmic ray flux above $3\times {10}^{19}\mathrm{eV}$ if neutrinos have $\sim \mathrm{eV}$ masses. The required intensity of primary neutrinos could be consistent with astrophysical models for their production if the maximum neutrino energy reaches to $\sim {10}^{22}\mathrm{eV}$ and the massive neutrino dark matter is locally clustered. Future observations of ultrahigh energy cosmic rays will lead to an indirect but practical search for neutrino dark matter.

Large differences have been observed in the diffraction intensities of normal-hydrogen and pure para-hydrogen scattered from the surface of a LiF(001) single crystal. The observed differences are shown to result from a strong coupling between the quadrupole moment of ${\mathrm{H}}_2$ and the surface electrostatic field, which, in a zeroth order approximation, is averaged out in the case of para-hydrogen. Analogous effects are expected in scattering of all homonuclear diatomic molecules from ionic solids.

We report the first room temperature observation of an emission pattern from a single fluorescent molecule signifying that the emission dipole is aligned along the optic $\left(z\right)$ axis of a microscopic imaging system. This technique takes advantage of the $x$, $y$, and $z$-polarized evanescent fields generated in total internal reflection and the usually nettlesome aberrations commonly encountered when imaging biological samples with high numerical aperture oil-immersion objectives. For both $z$-oriented and transverse-oriented individual molecules of the carbocyanine dye ${\mathrm{DiIC}}_{18}$ embedded in polymethyl-methacrylate (PMMA), calculated images accurately model the main features of observed emission patterns.

We have experimentally discovered a new nonthermal mechanism by which submonolayer and multilayer amorphous ice films, deposited at $T<\mathrm{}100\mathrm{}\mathrm{K}$, crystallize due to UV radiation; the proposed mechanism is that photoexcited charge carriers in the graphite substrate tunnel into defect states in the amorphous film, and induce exothermic relaxation from the metastable amorphous to the stable crystalline phase.

Several approaches to QCD with two massless quarks at finite temperature $T$ and baryon chemical potential $\mu$ suggest the existence of a tricritical point on the boundary of the phase with spontaneously broken chiral symmetry. In QCD with massive quarks there is then a critical point at the end of a first order transition line. We discuss possible experimental signatures of this point, which provide information about its location and properties. We propose a combination of event-by-event observables, including suppressed fluctuations in $T$ and $\mu$ and, simultaneously, enhanced fluctuations in the multiplicity of soft pions.

We have used an atomic force microscope to make precision measurements of the Casimir force between a metallized sphere of diameter 196 μm and flat plate. The force was measured for plate-sphere surface separations from 0.1 to 0.9 μm. The experimental results are consistent with present theoretical calculations including the finite conductivity, roughness, and temperature corrections. The root mean square average deviation of 1.6 pN between theory and experiment corresponds to a $1\%$ deviation at the smallest separation.

Large-scale molecular dynamics simulations were used to study the response of carbon nanotubes to a tensile load. Plastic or brittle behaviors can occur depending upon the external conditions and tube symmetry. All tubes are brittle at high strain and low temperature, while at low strain and high temperature armchair $(n,n)$ nanotubes can be completely or partially ductile. In zigzag $(n,0\mathrm{})$ tubes ductile behavior is expected for tubes with $n<\mathrm{}14\mathrm{}$, while larger tubes are completely brittle. In both cases the curvature determines the mechanical response.