In CuBr, biexcitons are excited resonantly by the absorption of two photons which are provided by two different laser sources. By a change in the geometrical configuration of the experiment, the wave vector $\overrightarrow{\mathrm{K}}$ of these biexcitons can be tuned. Thus, for the first time, a momentum spectroscopy of biexciton states is achieved. The biexciton dispersion $E\left(\overrightarrow{\mathrm{K}}\right)$ is established and a splitting of biexciton states, degenerate at $\overrightarrow{\mathrm{K}}=0\mathrm{}$, is observed. This dispersion is fully explained by the band parameters known from the exciton case.

Stimulated spin-flip scattering reported in $p$-InSb by Ebert et al. is reinterpreted as being due to excitons. The analysis explains the observation of Raman shifts which change magnitude as functions of pump wavelength at constant magnetic field. The Raman shifts satisfy the unusual equation $\Delta \omega =\pm {\mu }_{\mathrm{B}}{g}^{*}(H-H_0)$, where $H_0\cong 29$ kG, and $g^{*}$ is about 6. The number of transitions (six) and their spacing as triplet, doublet, and singlet are explained by the model.

It is shown that the cosmological problem of too many relic monopoles generated in the big bang according to standard grand unification theories can be avoided by several straightforward machanisms, one of which may make monopoles useful as seeds for galaxy formation.

Proton emission in relativistic nuclear collisions is examined for events of low and high multiplicity, corresponding to large and small impact parameters. Peripheral reactions exhibit distributions of protons in agreement with spectator-participant decay modes. Central collisions of equal-size nuclei are dominated by the formation and decay of a fireball system. Central collisions of light projectiles with heavy targets exhibit an enhancement in sideward emission which is predicted by recent hydrodynamical calculations.

This Letter reports a first measurement of the fast oscillations in ${\mathrm{Ar}}^{+}$ on Ar charge-transfer collisions. The high resolution needed to observe these oscillations is obtained with use of crossed ion and atom beams. The observed frequency is very different from that calculated from available ${\mathrm{Ar}}_2^{+}$ potentials. The difference is attributed to spin change between the fine-structure levels of ${\mathrm{Ar}}^{+}$.

Resonant degenerate four-wave mixing with two close optical frequencies ($\omega , \omega +\delta$) is used to perform high-frequency optically heterodyned saturation spectroscopy. Doppler-free spectra of ${\mathrm{I}}_2$ ($\lambda =514.5\mathrm{}$ nm) are obtained in this way for $20 \mathrm{kHz}<\delta <20\mathrm{} \mathrm{MHz}$. Lock-in detection for $\delta <1\mathrm{}$ MHz allows relaxation studies and line assignments. At higher frequencies, rf-power detection yields Doppler-free doublets split by $\frac{3\delta }{2}$.

A variant of the double-optical-resonance method is proposed in which the pumping step is a two-photon transition, either stimulated Raman scattering or two-photon absorption. It is shown that the probe linewidth can be minimized by proper choice of pump frequencies and assignment of pump beam propagation directions relative to the probe beam. The minimum linewidth is smaller than the value obtained by the usual one-photon pumping technique, and the method does not require any relations among level spacings.

The diffusion coefficients of ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{H}$ on the (110) plane of W have been measured by the field-emission fluctuation method. Below 140 K ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ diffusion corresponds to pure tunneling. The barrier is shown to be 0.21 eV high and ∼ 0.5 Å wide, and probably corresponds to passage over the saddle formed by two nearest-neighbor W atoms. Evidence for a phase transition at 89 K (at $\theta =0.6\mathrm{}$) is also presented.

Either the free-energy density describing the spin-density-wave state of chromium must be allowed to depend on the electron density and local state of strain of the crystal (as well as the order parameter) or highly nonlocal contributions to the free energy must be included. The theory describes the linear polarization of the spin-density-wave state of chromium, and other observable properties.

This article gives a microscopic description of magnetism in the Fe group, based on tetratomic spin-polarized Jahn-Teller resonance states. Ferromagnetism is due to Hund's-rule intra-atomic exchange coupling between molecular magnetic states having a common atom. Magnetic anisotropy is determined by the molecular symmetry axis of the resonance state.

Relaxation of photoinduced optical absorption following pulsed laser excitation was measured between 0.5 μs and 10 ms in doped and undoped $a$-Si: H as a function of temperature. The recombination was found to be bimolecular diffusion limited. The diffusion coefficient of the excess carriers is time dependent ($\sim t^{-0.3\mathrm{}}$) in agreement with the drift mobility of photocarriers and the predictions of the continuous-time random-walk theory of dispersive transport in disordered materials.

The $L_{\mathrm{III}}$ x-ray absorption edge of Tm in TmSe yields a temperature-independent valence of 2.58±0.07. The thermal damping of the extended x-ray absorption fine structure of TmSe is nearly identical to that of ErSe. This indicates very weak coupling of breathing shell charge fluctuations of the Tm ions to the phonons and very little static disorder of the lattice.

The $L_{\mathrm{III}}$ x-ray absorption edge of Tm in TmSe yields a temperature-independent valence of 2.58±0.07. The thermal damping of the extended x-ray absorption fine structure of TmSe is nearly identical to that of ErSe. This indicates very weak coupling of breathing shell charge fluctuations of the Tm ions to the phonons and very little static disorder of the lattice.

X-ray absorption spectrum of the Sm $L_{\mathrm{III}}$ edge has been measured in mixed-valence ${\mathrm{Sm}}_{0.75}$${\mathrm{Y}}_{0.25}$S. From analysis of the edge structure and extended x-ray absorption fine structure, the valence is determined and it is shown that the S neighbors of each Sm atom adopt an average distance rather than a dynamically distorted environment with two distances corresponding to the two valence states. From this it is concluded that the characteristic Sm $4f$-band width is not greatly modified by polaron effects.

Strong azimuthal effects have been observed in the angular-resolved photoemission from the $1e$ orbital of N${\mathrm{H}}_3$ chemisorbed on Ir(111). Some of the anisotropy is clearly due to substrate scattering. Other spectral features appear to be dominated by initial-state effects from the N-H bonds. Analysis of the diffraction features based on near-neighbor substrate scattering shows the bonding site to be the $\mathrm{ABC}$ site, i.e., a threefold hollow with a vacancy underneath.

Magnetism in iron at high temperature is investigated by calculating the total electronic band-structure energy for four types of spin arrangements. A slow smooth spatial variation of spin direction costs relatively little energy and the atomic moment $m$ is reduced only ∼ 10%. More rapid variations have considerably higher energy, which may explain the high degree of short-range order and small $\delta m$ observed at $T\gtrsim T_{\mathrm{C}}$. Other aspects are also discussed.

Exact bounds for the complex, bulk, effective dielectric constant ${\epsilon }_e$ of a two-component macroscopic composite that depend on the available information about the composite are presented and discussed. Some of these bounds are readily ascribable to special, exactly solvable, microscopic geometries. As a consequence, it is shown that there can exist composites where the real part of ${\epsilon }_e$ diverges as $\omega \to 0$ while the dc conductivity ${\sigma }_e\ne 0$.

Interaction effects in disordered Fermi systems are considered in the metallic regime. In two dimensions, logarithmic corrections are obtained for conductivity, density of states, specific heat, and Hall constant. These results are compared with a recent theory of localization as well as some experiments.

The prediction of the local band theory of itinerant-electron ferromagnetism for the temperature dependence of the single-hole Green's function is obtained. Qualitative agreement with observation is found, but the interpretation is novel. The underlying spectral function is more complex than the simple two-peaked structure heretofore assumed. Although the width of this complex shrinks with temperature, the shrinkage does not correspond to a diminution of the exchange splitting as that quantity is most conveniently defined.

Neutron scattering by the magnons of a spin-density-wave antiferromagnet is investigated with a one-dimensional local-moment model with long-range exchange. The magnon dispersion curves are unusual and the neutron inelastic scattering cross section exhibits peaks at a variety of neutron momentum transfers including the commensurable and incommensurable points. The general features of the calculation correlate remarkably well with recent neutron inelastic scattering data on chromium and strongly suggest a new interpretation of the data.