A possible new source of tunable coherent radiation is analyzed. By the passing of a relativistic electron beam (relativistic factor ${\gamma }_0$) through a homogeneous transverse ac electric or magnetic field of frequency $f_0$, electromagnetic radiation of frequency $f\sim 2{\gamma }_0^2{f}_0$ is generated by its coupling the negative-energy electrostatic beam modes with the high-frequency electromagnetic modes. Existing superconductor cavities which can supply ac fields of frequency $f_0\gtrsim 10$ GHz with amplitude $E_{max}>20\mathrm{}$ MV/m may be used to lead to a new class of submillimeter or even visible generating devices.

On the basis of data taken for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ beams impinging on a cold Cu(100) target, it is argued that the sticking of a light inert quantum particle at a metal surface occurs predominantly via resonant processes involving quasibound states. A resonance line in the sticking coefficient is expected whenever the incident particle energy nears a threshold for inelastic scattering from a long-lived local mode.

Sol-gel methods combined with specific thermal treatments were used to fabricate porous glasses with extremely uniform pore diameters and high internal surface areas. Low-temperature adsorption isotherm measurements reveal the dynamic and thermodynamic stability limits of the liquid films as a function of evolving pore diameter. Contrary to classical predictions, the results are in remarkable agreement with a universal hydrodynamic theory of film behavior which includes substrate interactions.

Experimental evidence from extended x-ray-absorption fine-structure measurements indicates that a cage of Mn atoms at the vertices of an icosahedron is the structural unit in the icosahedral MnSiAl and MnAl phases. The connections among these icosahedral units and between them and the Al atoms are different in the icosahedral and $\alpha$ phases. It is suggested that the $i$ phase grows by the random nucleating together of Mn icosahedra along their twenty threefold directions, as allowed by local steric constraints.

Conductance fluctuations due to variable-range hopping in 1D metal-oxide-semiconductor field-effect transistors are investigated in the presence of a magnetic field. With an increase in magnetic field, the Zeeman effect shifts the fluctuations to lower or higher chemical potentials. These shifts reflect the relative populations and occurrence of hopping from singly and doubly occupied sites. Combined with density-of-states measurements, they can provide an estimate of the intrasite Coulomb repulsion in 1D metal-oxide-semiconductor field-effect transistors. The orbital effect on conductance fluctuations is also discussed.

The influence of pressure $P$ on the superconducting transition temperature $T_c$ has been determined for the $\left({\mathrm{U}}_{1-x}{\mathrm{Th}}_x\right){\mathrm{Be}}_{13}$ system. The magnitude of $\frac{d{T}_c}{\mathrm{dP}}$ increases by a factor of 3 for $x>\mathrm{}1.7\%$ where an increase of $T_c$ is observed at ambient pressure. The phase diagram of $T_c\mathrm{}\left(x\right)\mathrm{}$ for $P=10\mathrm{}$ kbar shows two distinct regions of superconductivity.

A model treatment of a 1D ring-shaped wire, in which noninteracting electrons experience an induced emf and weak elastic scattering, reveals the occurrence of self-induced randomization of phases in the electronic wave functions. This novel phenomenon leads to resistive behavior in spite of the inherent time reversibility of the system.

The Landau-Khalatnikov mechanism associated with the relaxation of the order-parameter amplitude gives a peak in the longitudinal ultrasonic attenuation $\alpha$ near $T_c$, which quantitatively accounts for the observed peak in U${\mathrm{Be}}_{13}$ and U${\mathrm{Pt}}_3$. At lower temperatures this mechanism gives a power-law attenuation.

We report measurements of the de Haas-van Alphen effect in the heavy-electron metal Ce${\mathrm{Cu}}_6$ in the coherent state. Cyclotron masses of up to $40m_0$ are observed and we deduce a many-body enhancement 20 times larger than in La${\mathrm{Cu}}_6$. We conclude from the absence of any light electrons that the many-body renormalization leading to the coherent state influences all electrons, and not just those with primarily $f$ character.

The ultrafast relaxation of excitons in GaAs, coherently excited by a short optical pulse and subjected to collisions with free carriers and noncoherent excitons which are independently created by a second synchronized light pulse, is studied directly in the time domain by a probing of the excitonic phase coherence. The relaxation rates reveal strong exciton-exciton scattering with a collision efficiency of 1.6 × ${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{-1\mathrm{}}$ and even 10 times more efficient exciton-free-carrier scattering.

We use a first-principles pseudopotential approach to evaluate the thermoelectric parameters of the alkali metals. The values obtained for these parameters are extremely sensitive to the approximation used to describe the screening of the electron-ion interaction in these systems at wave vectors near $q=2\mathrm{}{k}_{\mathrm{F}}$. A criterion for the occurrence of charge-density-wave instabilities in the alkali metals is derived. We argue that the measured values of the thermoelectric parameters are inconsistent with the proximity of a charge-density-wave instability.

In this Letter we describe photoemission and inverse photoemission spectra of adsorbed xenon multilayers on Ru(001). Electron energy-loss spectra of xenon adsorbed on gold taken by Demuth, Avouris, and Schmeisser are included in the discussion. The observed layer-dependent shifts of the inverse photoemission spectra closer to the Fermi level clearly invalidate image screening effects as being the dominant cause of these shifts but support a "floating" of the adsorbed Xe potential well as a whole with the surface potential in the initial state.

This Letter reports optical third-harmonic generation in reflection from ion-implanted samples of silicon. The third-harmonic signal is found to exhibit certain novel polarization and rotational dependences, which can be understood with the aid of measurements using electron and ion beam techniques. Our observations provide quantitative information on the transformation from the crystalline to the amorphous state of silicon.

The linewidth for thermalized positrons directly annihilating with bound electrons in low-density molecular hydrogen gas has been found to be 1.56 ± 0.09 keV, by use of a Ge detector with a low-energy pulsed positron beam. This linewidth measurement together with previous experimental results makes it possible to determine precisely the composite spectrum due to positrons annihilating in a neutral-hydrogen galactic medium.

We wish to demonstrate that the recently discovered large-scale structure of the Universe (voids and filaments) can be explained by cosmic strings. We also predict the presence of pointlike structures ("knobs") of $\mathrm{masses}>{10}^7{M}_{⊚}$ in the vicinity of larger galaxies. The present analysis is limited to order-of-magnitude estimates. A detailed analysis would necessarily have to go into the full nonlinear theory of the growth of fluctuations in the Universe.

We determine the $\beta$ function of the $\mathrm{O}\mathrm{}\left(n\right)\mathrm{}$ nonlinear $\sigma$ model in $2+\epsilon$ dimensions to four-loop order using the recently calculated four-loop-order $\zeta$ function and the critical exponent $\eta$ through order $\frac{1}{n^3}$. This $\beta$ function determines completely, according to Hikami, the four-loop-order $\beta$ function for a large class of nonlinear $\sigma$ models. As an application the conductivity exponent of the Anderson metal-insulator transition is calculated for the unitary case to order $\epsilon$. This exponent turns out to be smaller than ½.

Every closed three-manifold occurs as a spacelike hypersurface of a vacuum space-time. For most of these three-manifolds, however, the vacuum space-times that contain them have no maximal slice. For asymptotically flat manifolds there are no restrictions on which three-manifolds can occur obeying the local energy conditions $\rho >{\left(J_a{J}^a\right)}^{\frac{1}{2}}$ and the space-times that contain them in most cases have no maximal slice.

An analytical method to determine the dimensionality of strange attractors in two-dimensional maps is introduced. In this method, the geometric structures of an attractor are obtained from a procedure developed previously. Such structures often appear to be the Cartesian product of a curve and a Cantor set. From the geometric structures, we determine the Hausdorff dimension first for the Cantor set, and then for the attractor. The results compare well with numerical results obtained for the Hénon, Zaslavskii, and Kaplan-Yorke maps.

Dimension and correlation entropy are measured for various settings of our hybrid optically bistable device. Measured dimension is found to be significantly less than dimensions consistent with the Kaplan-Yorke conjecture. The standard method of determining correlation entropy is shown to yield more than one value.

We have made a detailed comparison of the charged-particle flow in three-jet events ($e^{+}{e}^{-}\to q\overline{q}g$) and radiative two-jet events ($e^{+}{e}^{-}\to q\overline{q}\gamma$) from $e^{+}{e}^{-}$ annihilation at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=29\mathrm{}$ GeV. Accurate comparisons can be made because these two event types have similar topologies. In the angular region between the quark and antiquark jets, we observe substantially fewer charged tracks in the two-jet events than in the radiative three-jet events.