A simple approximation for one- and two-dimensional Heisenberg ferromagnets is proposed. This is a modification of the spin-wave theory for a three-dimensional ferromagnet and is expected to give correct low-temperature properties. The free energy and susceptibility of the linear chain are expanded in powers of square root of temperature. At S=(1/2) the results agree excellently with those of Bethe-Ansatz integral equations. The susceptibility of the two-dimensional square lattice diverges as exp(4π${\mathrm{JS}}^2$/T).

Density fluctuations of axionic dark matter decaying to photons are shown to have possibly measurable luminosities.

The connected-moments expansion (CMX) for the ground-state energy is derived from the Horn-Weinstein theorem. This new technique possesses several striking advantages over traditional approaches to calculation of the ground-state energy. Like perturbational theories, the CMX series is conceptually simple but is easier to derive and program and is usually convergent even for very ‘‘crude’’ trial kets. Numerical examples reveal potential applicability of CMX for calculation of the correlation energy when the trial ket has the form of multideterminant wave function.

A new approach to Monte Carlo simulations is presented, giving a highly efficient method of simulation for large systems near criticality. The algorithm violates dynamic universality at second-order phase transitions, producing unusually small values of the dynamical critical exponent.

We study the one-loop amplitudes for the emission of four gravitons and/or neutral gauge bosons and their superpartners, and prove their modular invariance and finiteness.

The static potential is studied in Helfrich and Polyakov’s model of smooth strings. For large separations R, the potential is linear in R with corrections of order 1/R. While the physical string tension is renormalized by the extrinsic curvature coupling, the coefficient of the 1/R term has Lüscher’s universal value to any finite order in the loop expansion. For very small separations, the leading term in the potential is proportional to 1/R, with a coefficient exactly twice that of the Lüscher term, and with corrections that are logarithmically small.

The structure function $F_2^{\gamma }$ for a quasireal photon has been measured in the reaction ee→eeX for $Q^2$ in the range 0.2<$Q^2$<7 ${\mathrm{GeV}}^2$, by use of 9200 multihadron events obtained with the TPC/Two-Gamma detector at the SLAC storage ring PEP. The data have been corrected for detector effects by a regularized unfolding procedure and are presented as $F_2^{\gamma }$(x,$Q^2$). The structure function shows scaling in the region 0.3<$Q^2$<1.6 ${\mathrm{GeV}}^2$, x<0.3, and rises for higher $Q^2$ and x>0.1. Below $Q^2$=0.3 ${\mathrm{GeV}}^2$, scaling breaks down in accordance with the finite cross-section bound for real photons.

On the basis of recent ${}_{}{}^{193}{}_{}{}^{}\mathrm{Ir}(2n, \gamma ){}_{}{}^{195}{}_{}{}^{}\mathrm{Ir}$ reaction measurements, we have identified new low-spin positive-parity states and their electromagnetic deexcitations in ${}_{}{}^{195}{}_{}{}^{}\mathrm{Ir}$. All of the low-lying states can be classified as those expected in a nucleus with a $\mathrm{U}\left(\frac{6}{4}\right)$ supersymmetry within the framework of the interacting bosonfermion approximation model. In particular, the spacings of the lowest levels (with $\sigma ={\sigma }_{max}=\frac{13}{2}$) are repeated for another sequence, suggesting $\sigma =\frac{11}{2}$ and, in consequence, the applicability of a supersymmetry description to ${}_{}{}^{195}{}_{}{}^{}\mathrm{Ir}$.

Experimental results are presented for the electron-impact ionization of negatively charged water clusters to form singly and doubly charged positive clusters. A cluster initially containing 325 water molecules loses an average of 42 and 47 molecules, respectively, in the formation of the singly and doubly charged species with 100-V electrons. Measurement of the appearance potentials of these positively charged species yielded results consistent with ionization taking place at multiple sites within the cluster and not at a single site.

The ${\mathrm{H}}^{+}$ secondary-ion yield from the surface of solid targets of Au, C, and nitrocellulose has been used to measure mean charges of energetic ions of ${}_{}{}^{84}{}_{}{}^{}\mathrm{Kr}$ and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ (at 1.16 MeV/u) at the beam-exit surface. Values for Kr ions were found to be 1.6 to 3 electronic charges ($e$) less than those found 70 ns later, after Auger decay. No such difference was observed for Ar ions of the same velocity.

Experimental evidence is presented for collisionally aided excitation of $4D-3P_{\frac{3}{2}}$ electronic-state, optical coherence in Na. A simple theoretical model of the reaction is proposed which gives agreement with experiment.

For the beam-beam phenomenon in $e^{+}{e}^{-}$ colliding rings, a simple model is presented, which illustrates several common features of the observed phenomenon (a blowup of one beam above a certain current, the unavoidable beam-beam limit, etc.). The tune-shift limit is given as a function only of betatron tune and radiation damping rate per collision point. This implies the universality of the phenomenon.

The electrical resistance of a porous material during mercury injection decreases in steps on a devil's staircase. These thermodynamically first-order steps result from the filling of pores that short a percolation cluster even though the average conductance is not described by a critical exponent. The consistency of the percolation description is supported by modeling of the devil's staircase and the gravitational influence on injection. The results raise significant questions about applications of percolation theory to the thermodynamics and kinetics of fluid displacement processes in porous media.

Mattis has derived six relations among the partial-wave amplitudes for the reaction $\pi N\to \rho N$. From these relations we deduce that the six independent helicity amplitudes for the $t$-channel isospin-0 reaction may be simply expressed in terms of one unknown function. This leads to definite predictions for the $\rho$-meson density matrix and the absence of nucleon polarization effects, which can be tested directly, without recourse to partial-wave analysis.

With use of a large green laser system (5 kJ, 500 psec, ten beams), scattered light from cavity and cannonball targets (1500 μm diam) is investigated. Strong suppression of stimulated Brillouin scattering is observed for the cavity target, while weak to modest suppression of stimulated Brillouin scattering is measured for the cannonball targets. The observed damping is attributed to sudden temperature rise due to collisions of two plasmas in the cavity.

Laboratory experiments have examined particular elements of proposed mechanisms for ion conic formation seen in the Earth's auroral-zone magnetosphere. A laser-induced fluorescence diagnostic measured the ion distribution function at many angles in velocity space, allowing tomographic techniques to reconstruct the multidimensional ion distribution function. Ion conics, as well as drifting Maxwellians, were observed.

Evidence is presented that many-body interactions in fluids have important consequences at the liquid-vapor critical point. In particular, three-body interactions in lattice-gas models are shown to lead to revised scaling variables and to a singularity in the coexistence-curve diameter with an amplitude proportional to the molecular polarizability. This is confirmed in experiments reported here. A companion van der Waals theory explains several other observed correlations between nonuniversal critical amplitudes.

In order to determine how the sample density of states is reflected in topographic images obtained in the scanning tunneling microscope, the apparent size of an adsorbed atom is studied as a function of bias voltage for several different atoms. This quantity is found to reflect state-density features that lie well above the Fermi level, but it shows a striking absence of any trace of $d$-state peaks below the Fermi level.

Combining first-principles self-consistent local-density total-energy calculations with the cluster variation method, we calculate the phase diagram of a semiconductor alloy. It is demonstrated that inclusion of both elastic and chemical interactions in the total-energy functional leads to new features, including the appearance in the same phase diagram of ordering and phase separation, and strain stabilization of both stable and metastable ordered phases.

Quantum mechanical calculations of the surface stress tensor at the aluminum (111) and (110) surfaces are presented. In each case the stress is tensile, favoring contraction in the plane of the surface. This tensile stress is caused by smoothing of the electronic wave functions at the crystal surface.