Optimal channel networks (OCN’s) obtained by minimizing the local and global rates of energy expenditure evolve automatically from arbitrary initial conditions to network configurations exhibiting fractal and multifractal statistics indistinguishable from those observed in nature. It is suggested that OCN’s are spatial models of self-organized criticality and that natural fractal structures like river networks may arise as a joint consequence of optimality and randomness.

The remarkable melting behavior of partial and complete ${\mathrm{N}}_2$ monolayers deposited on graphite is examined using a Monte Carlo procedure in which various concentrations of vacancies are introduced. The calculated melting temperatures and specific heats are in agreement with experiment over the surface density range 0.2≤ρ≤1, where the upper limit is the density of the observed √3 × √3 monolayer structure. Various physical quantities are calculated, and the nature of the transition is discussed.

Reaction-diffusion interactions generate spatiotemporal patterns in a variety of uniformly active chemical media. It is of interest to know how these patterns are affected by media nonuniformity. Here, we report the first observation of a rotating temperature pulse on a nonuniformly active (i.e., polycrystalline) surface of a nickel ring during atmospheric hydrogen oxidation. The pulse had an amplitude of about 100 °C and completed one cycle around the ring every 565 s. Its width, velocity, and maximum temperature changed during each cycle, but were constant at a given position.

We generalize an approach, which was recently introduced by Brandt and Giesekus to calculate the exact ground-state energy for strongly interacting particles on special perovskitelike lattices, to the periodic Anderson model in the dimension d=1 and to extended Emery models in d=1,2 on regular lattices for arbitrary spin degeneracy. For these models we calculate the exact ground-state energy for a restricted parameter regime in the strong-coupling limit. The ground-state energy shows a simple algebraic structure. We also present an eigenfunction of the Hamiltonian with the ground-state energy as its corresponding eigenvalue.

The low frequency second-harmonic transport response of multiprobe mesoscopic systems is analyzed using perturbation theory without explicit dephasing mechanisms. The intuitive conjecture that the second order transport current may be viewed as a linear conductance probing a distorted scattering potential is explicitly demonstrated in zero magnetic field B. For nonzero B, however, this simple picture breaks down, and I(2Ω) becomes a non-Fermi-surface quantity. A novel way of generating a second-harmonic transport current is predicted.

Conductance fluctuations and weak localization in ultrasmall gold bridges with all dimensions smaller than the elastic mean free path have been studied as a function of magnetic field and applied bias voltage. In addition to the well known Thouless energy we observe a new energy scale in the dependence of both quantum interference effects on the bias voltage. This new energy scale agrees well with the level separation estimated for the states in the corresponding closed system.

We report x-ray diffraction measurements of the effects of electric fields on charge-density-wave (CDW) structure in ${\mathrm{NbSe}}_3$. For fields exceeding the CDW depinning field ${\mathit{E}}_{\mathit{T}}$, the (0 1+Q 0) CDW superlattice peak shifts along ${\mathit{b}}^{\mathrm{*}}$. The shift increases rapidly with decreasing temperature, is largest near the electrical contacts, and is in opposite directions in the two halves of the crystal. We show that the observed CDW deformations are consistent with those required for phase slip and CDW-to-normal carrier conversion at the contacts. From the magnitude of these deformations, we determine the CDW elastic constant.

We demonstrate the existence of a surface resonance below the vacuum level of a simple metal surface. Theoretical modeling of new scanning tunneling spectroscopy (STS) data from Al(111) shows that the mechanism for formation of this resonance is scattering from the bulk lattice. Correlation between the STS feature and structure seen in inverse photoemission (IPE) spectra from Al(111) suggests that the IPE feature is also attributed to this surface resonance, rather than a previously proposed matrix element effect.

A thin film of EuSe was used as a tunnel barrier between the normal metal Ag and superconducting Al. Tunneling characteristics at 0.45 K showed the following: (1) the absence of exchange splitting of the EuSe conduction band in zero magnetic field, where EuSe is antiferromagnetic; (2) field-dependent spin polarization of the tunneling electrons as high as 97% at H≥1.2 T; (3) enhanced Zeeman splitting of the Al quasiparticle states because of the exchange interaction at the Al-EuSe interface; (4) a decrease in tunnel resistance by as much as 75% in a magnetic field ≥1.7 T. The results show that EuSe barrier junctions provide field-tunable resistors and sources of low-energy polarized electrons.

A small-angle neutron study of vortex lattices in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ showed a diffraction pattern with square symmetry when the applied field was along the c axis with the four main spots aligned with the crystallographic {110} directions. With the field 30° from the c axis, a hexagonal lattice exists. At intermediate angles, a fraction of the flux lines follow the c axis and the two lattices coexist. The temperature dependence of the intensity deviates markedly from conventional predictions.

We calculate the optical conductivity of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ in the inhomogeneous Hartree-Fock plus random phase approximation using the p-d model with parameters taken from first-principles calculations. The ground state evolves from a polaron state for small and moderate doping to a conventional metal for x≳1/4 in consistency with transport and optical experiments. The calculated optical conductivity shows good agreement with experiments regarding the peak positions and relative intensities for zero and small dopings. The precursor of the midinfrared band is identified with transitions from the localized states on Cu to the extended states above the Fermi level.

Inelastic neutron scattering has been employed to study in detail the low-energy crystal-field excitations of ${\mathrm{Er}}^{3+}$ in ${\mathrm{ErBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ (6<x<7). The observed energy spectra are found to be the result of a superposition of three stable states, which we interpret in terms of local regions of semiconducting and metallic character. The superconductivity is shown to result from the formation of a two-dimensional percolative network. A two-dimensional bond percolation model correctly predicts the critical oxygen contents associated with the two-plateau structure of ${\mathit{T}}_{\mathit{c}}$.

A state-tracking method is proposed for the determination of the magnetocrystalline anisotropy (MCA) energy, in which the spin-orbit coupling perturbed occupied states are defined according to their wave functions as related to the unperturbed occupied states, instead of their energies. This ensures that the force theorem is well satisfied—a very important feature in all non-self-consistent procedures. In calculations for the effect of strain on the MCA of an Fe monolayer (using the full-potential linearized augmented plane-wave method), highly stable results were obtained even with a small number of k points (${10}^2$ in the full Brillouin zone) versus the ${10}^4$ points needed previously.

The linearized Vlasov-Maxwell system and ideal magnetohydrodynamics are examples of systems with a pseudo-Hermitian structure with respect to an indefinite inner product on the space of complex representations of real fields [J. Larsson, Phys. Rev. Lett. 66, 1466 (1991)]. This structure leads to variational principles, whose variation with respect to local wave phase yields new wave-action conservation laws, which generalize previous eikonal versions.

Based on an analysis of the extensive air shower data accumulated over the last ten years at Akeno Cosmic Ray Observatory, the value of the proton-air nuclei inelastic cross section (${\mathrm{\sigma }}_{\mathrm{in}}^{\mathit{p}\mathrm{-}\mathrm{air}}$) has been determined assuming the validity of quasi-Feynman scaling of particle production in the fragmentation region. The energy dependence of ${\mathrm{\sigma }}_{\mathrm{in}}^{\mathit{p}\mathrm{-}\mathrm{air}}$ can be represented as 290(E/1 TeV${)}^{0.052}$ mb in the energy interval ${10}^{16.2}$–${10}^{17.6}$ eV, where E is the incident proton energy. The total p-p cross section (${\mathrm{\sigma }}_{\mathrm{tot}}^{\mathit{p}\mathrm{-}\mathit{p}}$), derived using the nuclear distribution function obtained from the shell model, increases with energy as 38.5+1.37 ${\ln }^2$(√s /10 GeV) mb.

We show that the pseudo Nambu-Goldstone boson contribution to the Peskin-Takeuchi electroweak parameter S can be negative in a class of technicolor theories. This negative contribution can be large enough to cancel the positive technihadron contribution, showing that electroweak precision tests alone cannot be used to rule out technicolor as the mechanism of electroweak symmetry breaking.

The photon decay of the relativistic Coulomb excitation of the single and double giant dipole resonance (GDR) in the target has been observed in the system 1A GeV ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ on ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. For peripheral events which are dominated by relativistic Coulomb excitation, a large Lorentzian structure in the photon energy spectrum is peaked at 13.3±0.1 MeV with a width of 4.1±0.1 MeV, corresponding to the single GDR in the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ target. The sum energy of coincident γ-γ pairs shows a broad feature at 25.6±0.9 MeV with a Lorentzian width of 5.8±1.1 MeV, which we assign to the double GDR observed via the two-γ decay channel.

The U-like K-vacancy production probability in 7.5-MeV/nucleon deep-inelastic ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$${+}^{238}$U collisions has been measured for no-fission, single-fission, and double-fission exit channels. The results are interpreted using quasimolecular orbital correlations for diatomic and triatomic configurations. This leads to the determination of lower limits for the time scale for late fission of U-like products (≥8 and ≥4 as) and their probability (≥77% and ≥52%), at mean initial-excitation energies of ∼40 and ∼105 MeV, respectively.

The electric quadrupole hyperfine structure splitting constants B of the Eu isotopes 145–147, 151–153 were measured with collinear laser ion beam spectroscopy in 17 different transitions. From 105 experimental data B values of all levels in Eu ci 4${\mathit{f}}^7$${(}^8$S)5${\mathit{d}}^9$${\mathit{D}}_{\mathit{J}}$, ... 6p(7/2;1/2${)}_{\mathit{j}}$ and .... 6p(7/2;3/2${)}_{\mathit{J}}$ were determined in enhanced accuracy. The ratio ${}_{}{}^{151}{}_{}{}^{}$B${/}^{153}$B=0.39184(22) (mean value) significantly deviates from values of muonic measurements but is in good agreement with other atomic one in Eu i and Eu ii. Values for the ratios of quadrupole moments are given.

We describe the first optical experiments on atomic hydrogen in a magnetic trap, using a narrow-band source at Lyman-α wavelength. By means of transmission spectroscopy we have studied the temperature and density evolution of the gas during evaporative cooling in a fixed field geometry.