We directly detect the coherent electromagnetic radiation originating from Bloch oscillations of charge carriers in an electrically biased semiconductor superlattice structure. The oscillation frequency can be tuned with the applied bias field from 0.5 THz to more than 2 THz, the detection limit of our measurement system.

Electronic structure of 13-atom clusters of 4d nonmagnetic solids Pd, Rh, and Ru has been studied using a linear combination of atomic orbitals molecular-orbital approach within the density functional formalism. ${\mathrm{Pd}}_{13}$, ${\mathrm{Rh}}_{13}$, and ${\mathrm{Ru}}_{13}$ are all found to have nonzero magnetic moments. Unexpectedly, the ground state of ${\mathrm{Rh}}_{13}$ is found to have 21 unpaired electrons and thus a magnetic moment of 21${\mathrm{\mu }}_{\mathit{B}}$. These 4d-element magnetic moments are a result of the reduced dimensionality and the enhanced electronic degeneracy due to the symmetry of the cluster. The effect of impurities on the moments is examined through calculations on ${\mathrm{FePd}}_{12}$, ${\mathrm{FeRh}}_{12}$, ${\mathrm{RhPd}}_{12}$, and ${\mathrm{RuPd}}_{12}$ clusters.

We analyze noise suppression properties of a constant-current driven p-i-n heterojunction. It is shown that the junction capacitance and temperature determine the minimum measurement time required for obtaining sub-Poissonian electron injection. As the capacitance of the junction is reduced, the noise spectrum develops a ‘‘regulated single electron emission peak’’ at the single electron charging frequency, indicating regulation of the individual thermionic emission events and therefore a transition from macroscopic squeezing to Coulomb blockade regime.

Magnetic-field and temperature dependence of the critical current density ${\mathit{J}}_{\mathit{c}}$ is investigated in epitaxial ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ thin films. For the magnetic field H applied parallel to the c axis, the flux pinning force density ${\mathit{E}}_{\mathit{p}}$ (=${\mathit{J}}_{\mathit{c}}$B) exhibits clear scaling behavior when H is normalized by the irreversibility field ${\mathit{H}}^{\mathrm{*}}$. The maximum pinning force density scales linearly with ${\mathit{H}}^{\mathrm{*}}$. This is the first observed scaling of ${\mathit{E}}_{\mathit{p}}$ in high-quality thin films of Bi oxides, which we can reasonably explain with flux-creep theory by assuming that the activation energy is proportional to the flux line spacing.

The dynamics of a classical continuum model of a ferromagnet with exchange and anisotropy energies of hard- and easy-axis type is considered. Exact solutions of soliton pairs with vanishing center of mass velocity are presented. They reveal the counterintuitive existence of a family of bound states of a repulsive, twisted domain wall (soliton) pair. These breather states have energies which lie above the continuum of the corresponding scattering states of two solitons.

We examine the spin-S quantum Heisenberg magnet with Gaussian-random, infinite-range exchange interactions. The quantum-disordered phase is accessed by generalizing to SU(M) symmetry and studying the large M limit. For large S the ground state is a spin glass, while quantum fluctuations produce a spin-fluid state for small S. The spin-fluid phase is found to be generically gapless—the average, zero temperature, local dynamic spin susceptibility obeys χ¯(ω)∼ln(1/‖ω‖)+i(π/2)sgn(ω) at low frequencies.

Fabrication of pillar-shaped microstructures of high vacuum sputtered Fe/Cr magnetic multilayers has enabled us to measure for the first time giant magnetoresistance with the current perpendicular to the film plane from 4.2 to 300 K. At 4.2 K we find a magnetoresistance of 108% for multilayers with a Fe thickness of 3 nm and a Cr thickness of 1 nm. The pronounced temperature dependence of the perpendicular magnetoresistance is studied for samples with different Cr thicknesses and compared with current-in-plane measurements.

We use the zero-temperature random-field Ising model to study hysteretic behavior at first-order phase transitions. Sweeping the external field through zero, the model exhibits hysteresis, the return-point memory effect, and avalanche fluctuations. There is a critical value of disorder at which a jump in the magnetization (corresponding to an infinite avalanche) first occurs. We study the universal behavior at this critical point using mean-field theory, and also present results of numerical simulations in three dimensions.

Nuclear resonant scattering of synchrotron radiation from ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ nuclei at 23.87 keV has been observed. The combination of a novel nested crystal monochromator and a ${}_{}{}^{119}{}_{}{}^{}\mathrm{SnO}_2^{}{}_{}{}^{}$/Pd based grazing incidence antireflection film was used to investigate the Mössbauer parameters of a tin containing powder sample in a forward scattering experiment.

As a microbe of the environment, 20–30 ppm of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ is substituted for Cu in the ${\mathrm{CuO}}_2$ planes of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$. Mössbauer effect studies show that all the oxygenated and ‘‘deoxygenated’’ samples with x=0.00, 0.14, 0.16, and 0.18 contain large proportions of micro spin clusters. Even in the deoxygenated ceramic with x=0.16, superconducting regions coexist with about 53% of microscopic spin clusters of tyical size ∼25–250 Å. Spin clusters formation is induced by extraneous oxygen. 2D magnetic order in the electron-doped system is much stronger than that in systems hole-doped, with extremely long-lived (>${10}^{\mathrm{-}8}$ s) spin correlations far above the 3D Néel temperature.

Frequency analysis is a numerical technique for studying the long-time dynamics of nearly integrable Hamiltonian systems or symplectic maps over large regions of phase space. This technique may be especially useful because of its inherent simplicity, and we demonstrate its effectiveness in studying long-time diffusion of orbits in a simplified but nontrivial accelerator model.

We report a second example of critical behavior in gravitational collapse. Collapse of axisymmetric gravitational wave packets is computed numerically for a one-parameter family of initial data. A black hole first appears along the sequence at a critical parameter value ${\mathit{p}}^{\mathrm{*}}$. As with spherical scalar-field collapse, a power law is found to relate black-hole mass (the order parameter) and critical separation: ${\mathit{M}}_{\mathrm{BH}}$∝‖p-${\mathit{p}}^{\mathrm{*}}$${\mathrm{\Vert }}^{\mathrm{\beta }}$. The critical exponent is β≃0.37, remarkably close to that observed by Choptuik. Near-critical evolutions produce echoes from the strong-field region which appear to exhibit scaling.

Gravitational-wave interferometers are expected to monitor the last three minutes of inspiral and final coalescence of neutron star and black hole binaries at distances approaching cosmological, where the event rate may be many per year. Because the binary’s accumulated orbital phase can be measured to a fractional accuracy ≪${10}^{\mathrm{-}3}$ and relativistic effects are large, the wave forms will be far more complex and carry more information than has been expected. Improved wave form modeling is needed as a foundation for extracting the waves’ information, but is not necessary for wave detection.

The E760 Collaboration has studied the reaction p¯p→${\mathrm{\chi }}_2$→γγ using a hydrogen gas jet target in the Fermilab antiproton acumulator ring. The following values are obtained for the branching ratio and partial width to two photons; B(${\mathrm{\chi }}_2$→γγ)=(1.60±0.45)×${10}^{\mathrm{-}4}$ and Γ(${\mathrm{\chi }}_2$→γγ)=321±95 eV.

Within the scenario in which one of the scalar partners of the top quark is lighter than the top quark itself, we calculate the cross sections of t̃ t̃¯ bound state η${\mathrm{̃}}_{\mathit{t}}$($0^{++}$) production in multi-TeV pp collisions. Although the present Fermilab Tevatron cannot find such a bound state because of too small cross sections, future colliders might catch the first evidence of supersymmetry by detecting it.

We present a systematic study of transverse energy (${\mathit{E}}_{\mathit{T}}$) production in collisions of 11.4A GeV/c Au and 14.6A GeV/c Si ions with targets of Al, Au, and Pb. Comparison of data for Au+Au and Si+Al indicates that, for the heavier system, there is an increase in the amount of stopping which is accompanied by a decrease in the width of the ${\mathit{dE}}_{\mathit{T}}$/dη distribution. The ratio of the maximum ${\mathit{E}}_{\mathit{T}}$ observed for the two systems is significantly greater than the ratio of the total energy available in the center of mass frame.

Bose-Einstein correlations between charged pion and ${\mathit{K}}^{+}$ pairs are calculated using the space-time and four-momentum distributions from relativistic quantum molecular dynamics for 200 GeV/nucleon ${}_{}{}^{32}{}_{}{}^{}$S+Pb and compared to pion correlation measurements from two CERN experiments. Good agreement is found. The predicted apparent radii from the ${\mathit{K}}^{+}$ correlation functions are smaller than the corresponding pion radii.

The first observation of laser induced continuum structure on autoionizing states in both third harmonic generation and ionization is reported for atomic Ca. The possibility of controlling the shape of the induced structure, exhibiting a transition from a dip to a peak, through the laser field intensities is demonstrated. Theoretical time dependent analysis including atomic parameters verifies that the experimental observations originate from laser field dependent coherent interactions.

The triple differential cross section for double photoionization in helium has been measured for the first time. Equal energy sharing and a simple geometry for the two emitted electrons were chosen in order to facilitate the comparison with theory. Good agreement between experimental and theoretical data is found for the angular correlation pattern if the mutual Coulomb repulsion between both escaping electrons is taken into account.

When exposed to intense light of ∼580 nm, the ground state of K shifts up in energy, passing through two-photon resonances with Rydberg states and finally crossing the two-photon ionization limit. Using laser pulses of 0.5 to 13 ps duration, we have shown experimentally that ionization occurs for short pulses, but for long pulses the population is diverted into the Rydberg states where some population survives the peak intensity of the pulse, in excellent agreement with a dynamic Floquet model.