Photodesorption of ${\mathrm{SiF}}_3$ groups, which are the principal adsorbates on a silicon surface during etching by ${\mathrm{XeF}}_2$, is found to be responsible for the etch-rate enhancement observed under illumination by low-power, cw band-gap radiation. It is proposed that desorption is stimulated by photogenerated-charge-carrier–mediated chemical reaction, and not the simple charge trapping and recombination mechanism usually invoked for desorption from semiconductor surfaces.

We give experimental evidence that long-range ferromagnetic order exists at finite temperatures, over a macroscopic size and in the absence of magnetic field for 1 monolayer of Co on Cu(100). The magnetization as a function of the applied magnetic field follows an hysteresis loop. At remanence (zero-applied magnetic field) the film is in a single-domain state. The magnetization reversal occurs by breaking down of the film into magnetic domains.

Two-photon magnetoabsorption measurements in a GaAs-AlGaAs multiple quantum well are presented for the first time. With the use of high-power picosecond pulses from a ${\mathrm{CO}}_2$ laser and a tunable dye laser, Landau transitions are clearly resolved in magnetic fields up to 6.6 T. Two-photon selection rules for Landau transitions (Δn=±1) give the unique possibility to determine the cyclotron frequencies of the valence band and conduction band separately.

Low-noise, high-resolution, far-infrared absorption spectra of free excitons in silicon are presented as a function of temperature. A complex thermalizing structure is revealed, resolving for the first time the electron-hole exchange splitting of an indirect-free-exciton ground state. A remarkable simplification of the spectrum is achieved with uniaxial stress. The relative intensities of the transitions suggest that the existing model for the free-exciton excited-state structure may be flawed, and an alternate scheme is proposed.

The Bardeen-Cooper-Schrieffer gap equation for Δ(k) is solved analytically for a simple metal having a spiral or linear spin-density wave. Δ falls to zero at the spin-density-wave energy gap for the spiral case, but falls to a finite (but small) value if the spin-density wave is linearly polarized. The electronic heat capacity in the superconducting state acquires a low-temperature tail, far in excess of a BCS exponential falloff, and similar to the (hitherto enigmatic) behavior observed in pure Pb.

We study the ${\mathrm{BiO}}_{\mathrm{m}}$ high-${\mathrm{T}}_{\mathrm{c}}$ oxides and compare them to the ${\mathrm{CuO}}_{\mathrm{n}}$ systems (n,m=2 or 3). Both are described by a general Hamiltonian and studied by perturbation theory in the hopping energy. It is shown that a common polarization mechanism can give rise to pairing of the O holes added to the antiferromagnetic or disproportionated semiconducting states. The necessary conditions are proximity of the M (Cu or Bi) and O levels, a gap in the electronic spectrum of the system, and important nearest-neighbor M-O repulsion.

Band-inversion phenomena, found in recent computer simulation and real experiment of steady-state gel electrophoresis, is theoretically analyzed by a simple model that describes the conformational change of DNA reptating through a gel. The model predicts distribution function of the end-to-end vector and the steady-state mobility, which agrees quite well with the result of computer simulation without use of any adjustable parameters.

We show the existence of a generic integrate-and-fire phenomenon which separates the transition to complete phase locking from the transition to chaos. We argue that our picture provides a natural explanation of "missing" hysteretic bands in the Belousov-Zhabotinskii reaction. In addition, we explain the origin of experimentally observed "deviations from universality" and bumps in the Arnold tongues in driven relaxation oscillators.

The thermally activated escape from a metastable well is studied in a region where both recrossing and depletion effects are important. It is shown that the relevant variable for the escape dynamics is the energy in the unstable normal mode at the barrier. In terms of this quantity the theory becomes easily perceptible and allows for a calculation of the escape rate in the turnover region between the low damping and strong damping Kramers limits.

It is shown that the geometric phase accompanying an arbitrary cyclic change of the state vector can be naturally understood as a canonical phase term in the coherent-state path integral. The adiabatic phase is shown to be derived as a part of the canonical phase.

A quantum system whose classical limit is at the critical threshold to global chaos is studied, i.e., in the small Planck's constant limit the last invariant torus is about to disappear. The quantum states are compared with classical phase-space structure near this torus. We exhibit the extension of the scaling properties of the latter to the quantum regime and find a new scaling of the quantum eigenvalues.

The Dirac equation for hydrogenic atoms has a well known instability when $z\alpha >1\mathrm{}$. A similar instability occurs for the "relativistic Schrödinger equation" with $\frac{p^2}{2m}$ replaced by ${(p^2{c}^2+m^2{c}^4)}^{\frac{1}{2}}-m{c}^2$ at $z\alpha =\frac{2}{\pi }$. These instabilities concern only the product $z\alpha$, but when the many-electron-many-nucleus problem is examined (in the relativistic Schrödinger theory) we find that a bound on $\alpha$ alone (independent of $z$) is then required for stability. If $\alpha <\frac{1}{94}$ we find that stability occurs all the way up to the critical value $z\alpha =\frac{2}{\pi }$, whereas if $\alpha >\frac{128}{15\pi }$ then the system is unstable for all values of $z$. Some implications of these findings are also discussed.

Large-angle $\mathrm{pp}$ elastic and quasielastic ($p,2p$) scattering have been simultaneously observed in hydrogen and each of several nuclear targets (Li, C, Al, Cu, Pb) at incident proton momenta of 6, 10, and 12 GeV/c. The nuclear transparency is the ratio of such a cross section in a nucleus to the free $\mathrm{pp}$ cross section. The transparency of aluminum increases with incident momentum by more than a factor of 2 from 6 to 9.5 GeV/c and falls significantly between 9.5 and 12 GeV/c. This occurs in a region where the free-proton nucleon-absorption cross section exhibits little energy dependence. QCD predicts an increase in transparency with energy.

In an energy-dependent phase-shift analysis of all $\mathrm{np}$ scattering data below $T_{\mathrm{lab}}=30\mathrm{}$ MeV, we find a large breaking of charge independence in the $\pi \mathrm{NN}$ coupling constants $g_0^2$ and $g_c^2$. For 495 degrees of freedom we obtain ${\chi }_{min}^2=533.8\mathrm{}$ and $\Delta g^2\equiv g_c^2-g_0^2=1.28\mathrm{}\pm 0.15$. A combined analysis, including also all $\mathrm{pp}$ scattering data below $T_{\mathrm{lab}}=30\mathrm{}$ MeV, results in ${\chi }_{min}^2=847.2\mathrm{}$ for 840 degrees of freedom. We then obtain $\Delta g^2=1.62\mathrm{}\pm 0.27$. The effect of such large values for $\Delta g^2$ is most strikingly seen in the ${}_{}{}^3{}_{}{}^{}P$ waves. The $\mathrm{np}{}_{}{}^3{}_{}{}^{}P$ phase shifts at $T_{\mathrm{lab}}=25\mathrm{}$ MeV are much larger than those of our phase-shift analyses.

Longitudinal response functions for ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ have been obtained from inclusive electron scattering cross sections, for $200\le q\le 550$ MeV/c. The response functions are compared with several theoretical calculations, all of which use exact three-body ground-state wave functions. The response functions are found to be more similar than the calculations predict. When a direct calculation of the Coulomb sum rule is compared with our data, the agreement is good and evidence for two-proton correlations in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is clear.

The distribution of $M1\mathrm{}$ strength in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ has been measured between 5.8 and 7.4 MeV with highly polarized tagged photons. $\Sigma \frac{{\Gamma }_0^2\mathrm{}\left(M1\mathrm{}\right)\mathrm{}}{\Gamma }={14.6}_{-1.3\mathrm{}}^{+1.5\mathrm{}}$ eV corresponding to $\Sigma B\left(M1\mathrm{}\uparrow \right)=\left({10.7}_{-0.9\mathrm{}}^{+1.1\mathrm{}}\right){\mu }_N^2$ was found, and can fully account for the much discussed "missing" $M1\mathrm{}$ in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. When the present result is combined with known $1^{+}$ transitions above neutron threshold, an $M1\mathrm{}$ giant resonance emerges at 7.3 MeV, 1 MeV wide, with $\Sigma B\left(M1\mathrm{}\uparrow \right)\approx 15.6{\mu }_N^2$. Smaller $1^{+}$ resonances are also seen at both 5.85 and 6.24 MeV. The total $M1\mathrm{}$ strength below 6.4 MeV amounts to $\Sigma B\left(M1\mathrm{}\right)\mathrm{}=\left({1.9}_{-0.4\mathrm{}}^{+0.7\mathrm{}}\right){\mu }_N^2$.

It is shown that there is an essential limit on the focusing of electron beams due to synchrotron radiation in the final focusing lens of a linear collider. The minimum possible beam size is determined almost only by an invariant emittance of a beam which is given at the entrance of a linear accelerator.

A simple physical model which describes the dynamics of turbulence and the spectrum of density fluctuations in compressible, self-gravitating matter and self-binding, phase-space density fluctuations is presented. The two systems are analogous to each other in that each tends to self-organize into hierarchical structures via the mechanism of Jeans collapse. The model, essential physical ingredient of which is a cascade constrained by the physical requirement of quasivirialization, is shown to exhibit interesting geometric properties such as intrinsic intermittency and anisotropy.

Neutral-beam current-drive experiments in the DIII-D tokamak with a single null poloidal divertor are described. A plasma current of 0.34 MA has been sustained by neutral beams alone, and the energy confinement is of $H$-mode quality. Poloidal $\beta$ values reach 3.5 without disruption or coherent magnetic activity suggesting that these plasmas may be entering the second stability regime.

Profiles of the poloidal magnetic field in Ohmically heated, sawtoothing discharges are determined in the Texas Experimental Tokamak (TEXT) from a precise measurement of the partial circular polarization of an impurity-ion line, TiXVII 3834 Å. The sawtooth-period-averaged safety factor (inverse of the rotational transform) on the plasma axis is found to be close to 1.0, independent of its value on the plasma edge. The measurements indicate a decoupling of the current and temperature profiles and a flat current density profile in the center of the discharge.