The critical behavior of ${\mathrm{Rb}}_2$Co${\mathrm{F}}_4$ has been studied by means of neutron scattering. In its long-range order dependence below $T_{\mathrm{N}}=103\mathrm{}$ K the material was found to behave as an almost ideal two-dimensional Ising system, with critical parameters $B=1.16\mathrm{}\pm 0.03$ and $\beta =0.119\mathrm{}\pm 0.008$. A novel phenomenon was discovered below $T_{\mathrm{N}}$: The three-dimensional stacking of the antiferromagnetic planes is sensitive to the heat treatment given at $T_{\mathrm{N}}$. Above $T_{\mathrm{N}}$ the inverse correlation length and the staggered susceptibilities were found to obey power laws with exponents $\nu =0.89\mathrm{}\pm 0.1$ and $\gamma =1.34\mathrm{}\pm 0.22$.

We report a new technique for studying the effect of uniaxial stress and hydrostatic pressure on the Néel temperature of antiferromagnets. Using magnetoelectric susceptibility measurements, we studied the shift in the Néel temperature $T_{\mathrm{N}}$ of ${\mathrm{Cr}}_2$${\mathrm{O}}_3$ caused by an externally applied uniaxial stress ($\overrightarrow{\mathrm{a}}$ and $\overrightarrow{\mathrm{c}}$ axis) or hydrostatic pressure $p$. Contrary to previously reported results, ${\left(\frac{d{T}_{\mathrm{N}}}{\mathrm{dp}}\right)}_c$ was approximately 70% greater than ${\left(\frac{d{T}_{\mathrm{N}}}{\mathrm{dp}}\right)}_a$.

Contrary to prevailing lore, a simple optical-model potential fits the pion-nucleus scattering data if an energy-dependent effective radius is used. Implications of this are discussed.

A rotational band in the "spherical" nucleus ${}_{}{}^{151}{}_{}{}^{}\mathrm{Gd}$ has been populated in the reaction ${}_{}{}^{149}{}_{}{}^{}\mathrm{Sm}(\alpha , 2n){}_{}{}^{151}{}_{}{}^{}\mathrm{Gd}$. The ($d, t$) and (${}_{}{}^3{}_{}{}^{}\mathrm{He}$, $\alpha$) cross sections, the value of $\left|\frac{(g_K-g_R)}{Q_0}\right|$ for the band, and comparisons with the ${}_{}{}^{153,155}{}_{}{}^{}\mathrm{Gd}$ isotopes identify the 1209-keV band head with the ${\frac{11}{2}}^{-}$ [505] Nilsson state.

The results of measurements of $\gamma$-ray angular distributions, $\gamma -\gamma$ coincidences, and attenuated Doppler shifts for the reaction ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}(\alpha , \gamma ){}_{}{}^{44}{}_{}{}^{}\mathrm{Ti}$ are presented, providing strong evidence that twelve of the fourteen excited states below 4.1 MeV can be grouped into four rotational-like bands.

Vector-analyzing-power angular distributions for deuteron elastic scattering and $l=0\mathrm{}$ pickup reactions on the same target are compared for ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}(d, d){}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ with ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}(d, {}_{}{}^3{}_{}{}^{}\mathrm{He}){}_{}{}^{31}{}_{}{}^{}\mathrm{P}$, ${}_{}{}^{30}{}_{}{}^{}\mathrm{Si}(d, d){}_{}{}^{30}{}_{}{}^{}\mathrm{Si}$ with ${}_{}{}^{30}{}_{}{}^{}\mathrm{Si}(d, t){}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$, and ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}(d, d){}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ with ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}(d, t){}_{}{}^{13}{}_{}{}^{}\mathrm{N}$ at a deuteron lab energy of 15 MeV. The strong similarities between the two angular distributions for each target suggest that an alternative to the distorted-wave Born-approximation description of these direct transfer reactions may be appropriate.

A method is given for generalizing the Baranger et al.-Haftel construction of the off-shell two-nucleon $T$ matrix so as to incorporate the constraints due to singular core interactions (described by the boundary condition model) and the one-pion-exchange tail of the two-nucleon potential.

It is shown that the radiation-reaction concept, which explains classical radiative effects, cannot be used exclusively to explain atomic radiative effects, and must be supplemented by consideration of the vacuum field.

Whether the origin of radiative line shifts and widths in spontaneous emission should be attributed to vacuum field fluctuations or to quantum electrodynamic radiation reaction is shown to depend on the ordering of commuting atomic and field operators.

A proper eikonalization of the radial Schrödinger equation leads at high energy to partial-wave phase shifts linear in the potential. This additivity of phase shifts for overlapping configurations implies strong cancelations in Watson's multiple-scattering series.

In a quite general Feynman-diagram model, diffractively produced inelastic states at high energies lead to a decrease in the total cross section below and above the threshold for the process. Nondiffractive channels produce the opposite effect. Asymptotically this agrees with the eikonal approach and the Gribov Reggeon calculus. Thus one can understand the broad dip in the $\mathrm{pp}$ total cross section at intermediate energies and, if the triple Pomeron decouples, the resultant intersecting-storage-rings rise.

In a coincidence experiment, we have measured the absolute differential cross section for $K$-shell ionization of Cu and Ag by 140-keV electrons. The results are discussed and compared with the calculations of Das and of Cooper and Kalbensvedt. While the agreement between experiment and theory is reasonable at 15°, there appears to be a sizable discrepancy at 45°, particularly at larger detected electron energy.

Single laser pulses at 9455 ${\mathrm{cm}}^{-1\mathrm{}}$ can generate spectra out to 30 000 ${\mathrm{cm}}^{-1\mathrm{}}$ without self-focusing. Experimental data for water are explained by primary and secondary stimulated four-photon processes.

By measuring the potential around a point-source antenna in a high-density magnetoplasma at a frequency between the electron gyrofrequency and the plasma frequency, we have detected an interference between a slow electrostatic wave and the cold plasma field. A model of quasicylindrical electrostatic waves connected with the Bernstein mode is proposed to explain the results.

In a superfluid with an anisotropic quasiparticle energy, the normal-fluid density is a diagonal tensor. The second and fourth sound velocities are distinctly anisotropic, so that heat-pulse or fourth-sound studies with an applied magnetic field $\overrightarrow{\mathrm{H}}$ may permit identification of the direction of the gap axis with respect to $\overrightarrow{\mathrm{H}}$. High-frequency sound velocities may display a weaker anisotropy.

Level differences between two baths of liquid helium connected by an orifice are generated by a charged vortex-ring beam incident on a diaphragm containing the orifice. The data are interpreted in light of phase slippage caused by vortex rings which overlap the orifice. The effects of vortex-ring interactions are discussed.

It has been found that C${\mathrm{O}}_2$-laser intensities greatly exceeding the normal breakdown fields may be transmitted in many gases. When breakdown does occur at these high intensities, the initial plasma growth is so violent as to cause frequency shifts of ∼1 ${\mathrm{cm}}^{-1\mathrm{}}$ in the beam transmitted through the focus. The interpretation of the spectrum induced by this plasma nonlinearity indicates that C${\mathrm{O}}_2$-laser pulses of duration as short as 30 psec were produced.

Measurements of the frequency- and wave-vector-dependent pair-field susceptibility have been carried out below the transition temperature of a superconductor using a tunneling technique. The results suggest that the dynamical equation of the order parameter below $T_c$ is something other than a diffusive time-dependent Ginzburg-Landau equation.

We study the nonlinear response of vortices in thin-film superconductors to strong electric fields. In the region of small magnetic fields we find unstable regions of negative differential conductivity. The voltage along the film is therfore predicted to jump from a lower flux-flow value to a higher normal-state value when the transport current is increased beyond the maximum value the flux-flow state can support.

The acoustic attenuation for a one-band free-electron metal has been derived using a generalized scattering function. If the latter has an appreciable backward lobe, the attenuation exhibits an anomalous peak when the sound wavelength is comparable to the electron mean free path. Similar effects might be observable in the alkalis as a result of umklapp scattering processes. If so, further study could enhance our understanding of the electron-phonon interaction in these metals.