A new mechanism for enhancing optical nonlinearities in quantum well structures, subject to an electric field normal to the layers, is investigated. It is found that large nonlinearities are induced by virtual transitions of Wannier excitons with "giant dipole moments." In contrast to other mechanisms, this enhancement mechanism is independent of correlations between excitons, for any order of nonlinearities. This fact results in a resonant enhancement at half the excitonic energy, and an ultrafast response, even faster than other virtual-process-induced mechanisms.

We apply the Schwinger boson mean-field theory to the square-lattice Heisenberg antiferromagnet at low temperatures. For spin ½ we confirm the renormalized classical behavior of the correlation length ${\kappa }^{-1\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$ without appealing to spin-wave theory. We also present a detailed calculation of the dynamical structure factor. A quasielastic peak is featured near $\mathbf{q}=(\pi ,\pi )$, while for $|\mathbf{q}-(\pi ,\pi \left)\right|\gg \kappa$, spin-wave ridges appear. The uniform susceptibility interpolates between spin-wave results at $T=0\mathrm{}$ and the high-temperature series of Rushbrooke and Wood. Our theory is distinct from theories in which fermionic spinon excitations determine the low-temperature spin dynamics.

We have used magnetotransport measurements to determine the phase diagram of ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$Cl${\mathrm{O}}_4$ from 0.5 to 10 K in magnetic fields to 30 T. This organic metal becomes a spin-density-wave semimetal as temperature is reduced in a magnetic field greater than a threshold field $H_c\mathrm{}(T=0\mathrm{})\mathrm{}\sim 3$ T. The critical temperature $T_c\mathrm{}\left(H\right)\mathrm{}$ increases with field in agreement with present theories, and reaches a maximum of 5.6 K at 18 T. Above this field, however, $T_c\mathrm{}\left(H\right)\mathrm{}$ decreases, extrapolating to zero temperature at 17 T. This reentrant phase diagram is not present in any theoretical model.

We present arguments, based on local gauge invariance, that the multicritical point of Ising spin-glasses should be located on a particular line of the phase diagram known as Nishimori's line [$tanh\left(\beta J\right)=2p-1\mathrm{}$ for the $\pm J$ distribution]. One scaling axis is along the line, and the other is along the temperature direction. This scenario is generic for any random Ising model with a Nishimori line, in any number of dimensions, if the transitions are second order. The renormalization-group fixed point located inside Nishimori's manifold is expected to control multicriticality for a wider class of models.

The phase diagram for the nuclear magnetic ordering of Cu in a magnetic field is theoretically found to exhibit three distinct phases in accordance with recent measurements. It is a consequence of the frustrated ground-state properties of an antiferromagnetic fcc structure with nearest-neighbor Heisenberg and dipolar interactions, a model also of relevance for the resonating-valence-bond problem. The adiabatic process is discussed, and nonadiabaticity caused by off-diagonal spin-spin interactions in the ordered, canted phase is found to be in accordance with the experimental value.

Hyperfine-enhanced nuclear antiferromagnetic resonance of ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho} (I=\frac{7}{2})$ in ${\mathrm{Cs}}_2$NaHo${\mathrm{Cl}}_6$ was observed below $T_{\mathrm{N}}\simeq 4.5$ mK following demagnetization cooling. Measurements were carried out at several frequencies by cw NMR with a low-field sweep. From the temperature dependence of the resonance field, the resonance frequency versus applied-field diagram at 0 K was obtained. This is the first observation of enhanced nuclear antiferromagnetic resonance.

An electron-spin polarization $P$ of as much as 80% has been observed in the tunnel current in Au/EuS/Al tunnel junctions. $P$ can be explained by the different heights of the tunnel barriers for the two spin directions. The Zeeman splitting of the Al quasiparticle density of states is greatly enhanced by the exchange interaction at the EuS/Al interface. Spin polarization was even seen in zero applied field. The value of $P$ calculated from the tunneling theory using known barrier heights in EuS is consistent with the measured values.

We have used the direct energy-transfer method to study the internal morphology of a nonaqueous-dispersion-polymerization latex. These 1-μm particles contain 97 mol% poly(methyl methacrylate) (PMMA) and 3 mol% polyisobutylene (PIB). Phenanthrene (donor) and anthracene (acceptor) were doped selectively into the PIB phase. These dyes sample length scales up to 34 Å and indicate an "effective local dimension" $\overline{d}=2\mathrm{}$, a result which conveys important information about the PIB phase in this material.

It is shown that the coarsest discretization of positions and velocities in molecular dynamics leads to qualitatively correct transport coefficients and quantitatively predictable long-time tails in the velocity autocorrelation function, but requires orders of magnitude less computer time than standard molecular-dynamics methods.

We report the result of a search in the Kamiokande II detector for the interactions of electron-type antineutrinos in the energy level 19 to 35 MeV. We measure directly a 90%-C.L. upper limit of 226 ${\mathrm{cm}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ on the flux of electron antineutrinos with energies in that interval. An upper limit on the total flux of relic antineutrinos from past supernovae is obtained as a function of the effective equilibrium temperature of the relic antineutrino sea.

In an attempt to demonstrate that any parity-nonconserving term can be expected to contribute to the axial-vector part of the current-correlation function, the coupling of charged vector mesons is considered. It is confirmed that parity nonconservation does in fact affect the relevant form factor at zero momentum transfer. Although the result is generally divergent, a finite correction is obtained when a certain sum rule on inverse masses is satisfied. In that limit a single expression is seen to unify the spin-0, spin-½, and spin-1 contributions to the so-called topological mass.

It is argued that the high-temperature gluon-plasma damping constants previously calculated by use of the background-field method and the Vilkovisky-DeWitt effective action do not have a physical interpretation since they are not derived from correlation functions relevant to linear-response theory. A correct application of the gauge- and parametrization-independent formalism does not affect the plasmon frequency, but yields the following modified results for the longitudinal and transverse damping constants: ${\gamma }_L={\gamma }_T=-\left(\frac{9}{32\pi }\right)g^2\mathrm{NT}$.

The current controversy over the damping of color oscillations in a quark-gluon plasma is resolved through a gauge-invariant linear-response analysis of relevant physical processes. The plasmon decay rate is shown to be $\gamma =-\frac{11N_c{g}^{2}T}{24\pi }$ to lowest perturbative order, but this result does not survive at higher orders. A self-consistent estimate suggests that plasmon decay is in fact a nonperturbative phenomenon governed by static screening effects.

Electroproduction of the $\Delta$ isobar in C and Fe has been studied for incident electrons with energies between 0.6 and 1.6 GeV. A longitudinal-transverse decomposition of the inclusive cross section has been made for $q_{\mu }^2=0.1\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. The residual longitudinal cross sections measured do not provide the additional strength needed above the quasifree region to satisfy the Coulomb sum rule. The transverse cross sections are in good agreement with data from photoabsorption.

Well-known variational principles for second-order perturbation matrix elements are extended to higher-order multiphoton processes for atomic hydrogen. Numerical applications to two- and three-photon ionization of the hydrogen ground state provide results comparable to those of analytic calculations, even near intermedaite-state resonances, yet without the complications involved in summation over intermediate states. Extensions to multiphoton processes for many-electron atoms are discussed briefly.

A traveling-wave state observed in full numerical simulations of cylindrical convection is reported. These waves arise with large amplitude via a global bifurcation of the steady convective state in systems with highly conducting sidewalls. For poorly conducting sidewalls, transition occurs instead to a disconnected branch of steady states. The Prandtl-number dependence of these transitions is interpreted in light of a wavelength selection mechanism previously proposed for axisymmetric convection.

To investigate the interaction of hydrodynamics and domain growth in phase-separating fluids, the spinodal decomposition of a binary mixture subjected to a uniform and steady shear flow has been studied by light-scattering techniques. We find that the effects of hydrodynamics become important when the distortion produced by shear is faster than the growth of the domains. When this happens, the growth becomes anisotropic and new and unexpected features are detected. Finally, a steady state predicted by theory is observed and compared to the theory.

A theory of transitions between tilted hexatic-$I$ and -$F$ phases in liquid-crystal films is developed. A renormalization-group analysis leads to four tilted hexatic phases: the hexatic-$I$ and -$F$ phases, an intermediate hexatic-$L$ phase, and an unlocked phase. All these phases except the unlocked phase also exist if the films are crystalline rather than hexatic. These results are consistent with recent experiments on thermotropic and lyotropic liquid crystals.

We have measured the longitudinal resonance frequency ${\nu }_l$ in superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}-B$ in a magnetic field strong enough to distort the superfluid energy gap. At low temperatures, the resonance is narrower than in zero field, and the measurements are precise enough to determine both the quadratic and quartic dependences of ${\nu }_l$ on the field. The quadratic term has been fitted by the quasiclassical theory to give the $l=3\mathrm{}$ pairing interaction. At extremely low temperatures ($T\lesssim 0.2T_c$), an unexplained structure in the resonance line is observed.

Novel universal behavior of the equilibrium crystal shape is reported: The Gaussian curvature, a product of two principal curvatures, assumes a universal jump across the facet contour at any temperature below the roughening temperature. This behavior is shown to be a consequence of a universal relation between the coefficients ${\gamma }_s$ and $B$ in the small-p expansion (p is the surface gradient) of the interface free energy, $f\left(\mathbf{p}\right)=f\left(0\right)+{\gamma }_s\left|\mathbf{p}\right|+B{\left|\mathbf{p}\right|}^3+O\left({\left|\mathbf{p}\right|}^4\right)$. Both exact results on a solvable model and Monte Carlo calculations support this behavior—universal Gaussian-curvature jump at the facet edge.