${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{CuO}}_4$ has been found to undergo the following sequence of transformations upon cooling: tetr.(I4/mmm) →ortho.(Bmab)→tetr.(P$4_2$/ncm), over a range of composition 0.05<x<0.20. The newly discovered low-temperature tetragonal phase can be thought of as a coherent superposition of the twin-related Bmab structures. The system can be modeled as an XY-spin system with temperature-dependent quartic anisotopy, v(T). Slight differences between orthorhombic and tetragonal structures appear to have large effects upon the superconductivity.

We investigate the statistical mechanics of a simple lattice model for the denaturation of the DNA double helix. The model consists of two chains connected by Morse potentials representing the H bonds. We determine the temperature dependence of the interstrand separation and we show that a mechanism involving an energy localization analogous to self-focusing may initiate the denaturation.

A simple model is introduced to describe the observed coexistence of gelation and phase demixing in atactic polystyrene solutions. It is shown that the multiple equilibria conditions for molecular clustering caused by physical cross linking can lead to a characteristic feature of the phase diagram. The condition is examined for the appearance of new critical phenomena, such as a tricritical point or a triple point (or a three-phase equilibrium) as a result of interference between gelation and phase separation.

We study the roughening of a growing surface near to a morphological transition within a general scaling framework. For a class of systems, where the transition can be related to directed percolation, the anomalous roughness at the critical point is at most logarithmical. In two-dimensional simulations we find ${\mathrm{w}}^2$∼logL where w is the width of the surface and L is the substrate size.

We show that the procedure of maximizing $G^{\mathrm{-}1}$(α), the inverse of Newton’s constant within the wormhole calculus, subject to the constraint that the effective cosmological constant vanish, drives the QCD θ parameter to θ=0,π which are the CP conserving values. We argue that, in fact, θ=π is the preferred value and discuss the phenomenological consistency of this choice.

We provide a geometrical interpretation of the classification of supersymmetric p-dimensional extended objects. Specifically, we show that the action describing such an object exists by virtue of a nontrivial class of the (p+2)th Chevalley-Eilenberg cohomology of superspace, considered as the super translation group.

We discuss a simple model in which muonium-antimuonium conversion can occur at a level around the recent experimental bound but the process μ→eγ is strictly forbidden. Measurements of the anomalous muon magnetic moment and high-energy Bhabha scattering $e^{+}$$e^{\mathrm{-}}$→$e^{+}$$e^{\mathrm{-}}$ together provide an indirect and interesting constraint on the conversion. The model predicts anomalous events ep→ēμμX at high-energy ep colliders.

For different up and down current quark masses, charge independence follows in the Nambu–Jona-Lasinio model from chiral-symmetry breaking; however, chiral symmetry is restored at high densities. The dependence of the constituent quark masses and the neutron-proton mass difference on the density are examined. The effect of the up-down-quark mass difference on the neutron-proton mass difference is large and in the right direction to explain the Nolen-Schiffer anomaly.

The observed saturation of temperature and of linear momentum transfer per incident nucleon in intermediate-energy nuclear reactions is studied in the model of promptly emitted particles. We demonstrate that only with the inclusion of two-body collisions is very good agreement with experimental data obtained.

Exclusive differential cross sections for the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$(γ,n${)}^{15}$O process have been measured for $E_{\gamma }$=150, 200, and 250 MeV and ${\theta }_L$=49°, 59°, and 88°. This measurement is the first of its type to be reported for the energy region above the pion production threshold. An uncollimated bremsstrahlung beam was incident on an ${\mathrm{H}}_2$O target, and neutrons were measured with a recoil proton spectrometer with resolution sufficient to identify the residual ${}_{}{}^{15}{}_{}{}^{}\mathrm{O}$ ground state. The results are compared to (γ,p) data and to calculations including effects from meson exchange currents.

We show that shallow water waves in a viscous fluid are not damped if the fluid is adequately heated from below. The critical temperature gradient needed to sustain the wave as well as its frequency are determined analytically. The nonlinear evolution of the wave is governed by the Korteweg–de Vries equation.

A novel method of frequency upshifting short (≤1 ps) pulses of laser light, which makes use of relativistic plasma waves, is described. This method makes use of the fact that a laser pulse moving in a plasma can be thought of as a packet of photons, each possessing an effective mass of $m_{\gamma }$=ħ${\omega }_{\mathrm{pe}}$/$c^2$ and moving with the group velocity of the laser pulse. These photons experience a force acting on them when in the presence of a gradient in the plasma density. By using a relativistic plasma wave (i.e., a moving density gradient) traveling with the photons, energy of the photons (thus the frequency) can be continuously increased.

Vortex turbulence generated by high-amplitude second sound in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is observed. Features of the transition to turbulence are similar to those seen in dc superfluid flow experiments. The measured frequency dependence of the critical velocity agrees with the predictions of dynamic scaling theory. The measured temperature dependence is similar to that of dc counterflow.

We report extensive Monte Carlo simulations and finite-size scaling analysis of the correlation length (${\xi }_{\mathrm{L}}$) for the spin-spin correlation function and the renormalized coupling (${\mathrm{g}}_{\mathrm{L}}$) of the simple-cubic Ising model with a wide range of lattice sizes (L≤96). New methods of analysis which avoid assumptions related to the correlation length were introduced. The exponents ν and 2-α were estimated from separate analysis of ${\xi }_{\mathrm{L}}$ and ${\mathrm{g}}_{\mathrm{L}}$, respectively. A value of ${\mathrm{\omega }}^{\mathrm{*}}$≊0.001±0.07 for the Fisher exponent is obtained. This is consistent with a very small or no violations of hyperscalings.

We report the results of measurements of the static structure factor S(κ) for gaseous ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$ at 140 K and four (subcritical) low densities from which, for the first time, we derive the well of the isotropic pair potential directly by means of a simple Fourier transform inversion.

The temperature dependence of hypersonic longitudinal and transverse sound waves in ‘‘strong’’ (${\mathrm{B}}_2$${\mathrm{O}}_3$) and ‘‘fragile’’ [${\mathrm{Ca}}_{0.4}$${\mathrm{K}}_{0.6}$(${\mathrm{NO}}_3$${)}_4$] glass-forming systems have been probed using Brillouin scattering. In the latter material the transverse modes become masked by a central component in the spectrum which broadens as the temperature is raised through the liquidus temperature; in ${\mathrm{B}}_2$${\mathrm{O}}_3$, on the other hand, well-defined (undamped) transverse modes are observed above the melting temperature of crystalline ${\mathrm{B}}_2$${\mathrm{O}}_3$ (∼723 K) all the way to 1300 K.

The first evaluation of spin-dependent extended x-ray-absorption fine structure (SPEXAFS) is reported. The SPEXAFS at the Gd L edges for Gd metal and ${\mathrm{Gd}}_3$${\mathrm{Fe}}_5$${\mathrm{O}}_{12}$ are compared to the conventional EXAFS and discussed in terms of exchange scattering from magnetic neighbors. The SPEXAFS at the Gd ${\mathrm{L}}_2$ and ${\mathrm{L}}_3$ edges reflect the change of the photoelectron spin polarization. The Fourier transform of the SPEXAFS of the ${\mathrm{Gd}}_3$${\mathrm{Fe}}_5$${\mathrm{O}}_{12}$ only shows the magnetic neighborhood, whereas the nonmagnetic oxygen neighbors remain invisible. SPEXAFS spectroscopy is expected to become a powerful tool in the determination of the magnetic short-range order.

We study a generalized Hubbard Hamiltonian which includes the Cu and O orbitals in the ${\mathrm{CuO}}_2$ planes of high-$T_c$ superconductors. We use slave-boson technique to account for the intra- and interatomic correlations. In the saddle-point approximation we obtain the metal-insulator phase diagram and conclude that, for one hole per unit cell in the ${\mathrm{CuO}}_2$ plane, ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ is a charge-transfer insulator. Our results show that doping produces a rapid metallization of the system and destroys antiferromagnetic order. Transport properties and mechanism for superconductivity are briefly discussed.

Phonon densities of states of insulating ${\mathrm{BaBiO}}_3$ and superconducting ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ are determined by neutron-scattering experiments. The main peaks in ${\mathrm{BaBiO}}_3$ occur at 35, 43, 63, and 71 meV. In ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$, the phonon spectrum softens and is comprised of two bands around 30 and 60 meV. Molecular-dynamics results, which are in good agreement with the experiment, reveal that the observed phonon peaks are due to oxygen vibrations. The softening of oxygen phonon modes around 30 and 60 meV is due to electron-phonon coupling, indicating that ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ is a normal BCS superconductor.

We present a model for fault dynamics consisting of a uniform chain of blocks and springs pulled slowly across a rough surface. The only nonlinear element of our model is a slip-stick friction force between the blocks and the surface. We find that this model gives rise to events of all sizes. Our numerical evaluation of the distribution of earthquake magnitudes results in power-law spectrum similar to what is observed in nature. Like certain other dissipative dynamical systems, the observed large fluctuations in earthquake magnitude persist because the system is in a state of marginal stability.