We have performed optical hole-burning experiments in the temperature range between 25 and 500 mK. Experimental results for the dye molecule free-base phthalocyanine embedded in the organic polymers polystyrene and polymethlmethacrylate are presented. The hole widths in these systems show a linear temperature dependence over the whole temperature range. Extrapolating to T→0 K we obtain lifetime-limited optical linewidths. Assuming a line-broadening mechanism through two-level systems we can interpret our data with a constant density of states, an assumption which has often been questioned.

We introduce a geometrical interpretation to classify possible linear and nonlinear terms in the models for driven growth with surface relaxation. A nonlinear differential equation, distinct from the Kardar-Parisi-Zhang equation, is proposed as a relevant continuum model describing atomistic kinetic growth under conditions of chemical bonding. The scaling relations among growth exponents that we derive from a dynamic renormalization-group analysis are exact for a class of growth models with surface relaxation. The calculated exponents are in excellent agreement with our discrete atomistic growth simulation.

Anisotropic surface diffusion of CO on Ni(110) has been studied using the newly developed technique of optical second-harmonic diffraction from a laser-induced CO monolayer grating. Two distinct orthogonal diffusion channels, along [11¯0] and [001], were found. The measured diffusion coefficients yield the diffusion activation energies ${\mathit{E}}_{\mathit{d}}$(11¯0)=1.1±0.2 kcal/mol and ${\mathit{E}}_{\mathit{d}}$(001)=3.1±0.4 kcal/mol and the preexponential factors ${\mathit{D}}_0$(11¯0)=(3.8±2.0)×${10}^{\mathrm{-}9}$ ${\mathrm{cm}}^2$/sec and ${\mathit{D}}_0$(001)=(4.8±4.4)×${10}^{\mathrm{-}6}$ ${\mathrm{cm}}^2$/sec. The anisotropy in the surface diffusion appears to cross over at T∼160 K.

By means of hyper-Raman scattering, a silent soft phonon was observed in β-quartz for the first time. The soft phonon was found to become underdamped at high temperatures. Both the square of the frequency ${\mathrm{\omega }}_0^2$ and the integrated intensity ${\mathit{I}}_0$ of the soft phonon were found to obey Landau-Cochran’s soft-mode theory. The α-β phase transition of β-quartz is found to be well described by a simple displacive-type phase transition with the silent soft phonon, in contrast to many studies which suggest an order-disorder phase transition.

After proton implantation into n-type silicon at 45 K, a bistable hydrogen center with a band-gap level ${\mathit{E}}_{\mathit{c}}$-${\mathit{E}}_{\mathit{t}}$=0.16 eV is observed by deep-level transient spectroscopy. The center anneals at ∼100 K under zero bias with a decay constant ν=(3.0×${10}^{12}$ ${\mathrm{s}}^{\mathrm{-}1}$)exp[(-0.29 eV)/${\mathit{k}}_{\mathit{B}}$T] and at ∼210 K under reverse bias with ν=(1.3×${10}^8$ ${\mathrm{s}}^{\mathrm{-}1}$)exp[(-0.44 eV)/${\mathit{k}}_{\mathit{B}}$T]. In both cases the center regenerates by forward-bias injection at low temperatures. The decay without (with) reverse bias reflects capture of one (two) electron(s). The metastability is ascribed to hydrogen jumps between bond-center and tetrahedral sites as a result of changes in charge states.

The ultrafast-relaxation kinetics of the singlet excitons in polydiacetylene 4 BCMU was measured at hydrostatic pressures up to 80 kbar, using the pump-probe photoinduced-absorption technique with 70-fs time resolution at 2 eV and 5 ps in the spectra range 1.2–2.2 eV. The 100-fs decay component survives at high pressures, but the slow component evolves from a 1.5-ps exponential decay at atmospehric pressure to a much slower stretched-exponential decay at high pressures with a complete recovery in the ns time range. Results suggest that the fast decay component is a 1D relaxation process, whereas th slow component is due to exciton recombination which requires a subsequent 3D distortion of the polymer chain.

The temperature dependence of the field-cooled and zero-field cooled static dielectric susceptibilities of the deuteron glass ${\mathrm{Rb}}_{0.4}$(${\mathrm{ND}}_4$${)}_{0.6}$${\mathrm{D}}_2$${\mathrm{PO}}_4$ has been determined. Whereas above the freezing temperature ${\mathit{T}}_{\mathit{f}}$ the two susceptibilities are equal, a splitting of the two branches as well as a remanent polarization is observed below ${\mathit{T}}_{\mathit{f}}$. The fact that this splitting takes place demonstrates the occurrence of a freezing transition on crossing the Almeida-Thouless-like line.

We propose a novel mechanism which explains self-generated voltage or current oscillations induced by a transverse magnetic field on the basis of a dynamic Hall effect coupled with impact ionization. Dielectric relaxation of both the applied electric field and the Hall field combined with the generation-recombination kinetics of the carrier density yields oscillatory instabilities at a threshold value of the magnetic field. General analytic conditions and numerical simulations for p-Ge at 4 K are presented, exhibiting Hopf bifurcations, period-doubling routes to chaos, and type-I intermittency.

Electron-spin polarization in excess of 70% has been observed in photoemission from a 0.1-μm-thick epitaxial layer of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As with x≊0.13 grown on a GaAs substrate. Under these conditions, the epitaxial layer is expected to be highly strained by the 0.9% lattice mismatch. The electron polarization and the quantum efficiency have been measured as a function of the excitation photon energy from 1.25 to 2.0 eV. A significant enhancement of the electron polarization occurs in the vicinity of 1.33 eV where the expected strain-induced level splitting permits optical excitation of a single-band transition.

Magnetoresistance measurements on 1000-ppm CuMn mesoscopic spin-glass samples reveal phase-coherent electron propagation on 0.4-μm length scales observed as conductance fluctuations and Aharonov-Bohm resistance oscillations. Exploiting the broken time-reversal symmetry of the spin-glass state, a unique magnetofingerprint of the specific frozen spin configuration is extracted from a pair of resistance measurements. These ‘‘spin-glass fingerprints’’ are manifestly different after several thermal histories, but are surprisingly robust upon field cycling to over 9 times the typical exchange field.

We consider a quasi-one-dimensional spin-S quantum antiferromagnet as a function of dilution x. We show that in some regimes, dilution suppresses quantum fluctuations. For small enough interchain coupling and integer S, the ground state is disordered for x=0, but antiferromagnetically ordered for x>${\mathit{x}}_{\mathit{c}}$. We estimate that for large S, ${\mathit{x}}_{\mathit{c}}$ is exponentially small. This is a novel diffusion-induced ordering transition.

The phase diagram of the one-dimensional t-J model is investigated by analyzing the results of exact diagonalization and the exact solutions at J/t=0 and 2. Phase separation takes place above a critical value of J around ${\mathit{J}}_{\mathit{c}}$/t=2.5–3.5 depending on the electron density. In the small-J region, Tomonaga-Luttinger liquid theory holds and its correlation exponents are calculated as a function of J/t and the electron density. Superconducting correlations become dominant in a region between the solvable case (J/t=2) and phase separation. A spin-gap region is also found at low density.

We investigate the relation between the vortex structure and the dissipation mechanism in strongly layered superconductors when a large magnetic field is applied parallel to the layers. At high enough temperatures the vortex lattice undergoes a transition to a smectic state characterized by a vanishing interlayer shear. The elastic properties of this state lead to novel conditions for Kosterlitz-Thouless-type behavior, resulting in an algebraic current-voltage characteristic down to vanishing current densities.

The learing time of a simple neural-network model is obtained through an analytic computation of the eigenvalue spectrum for the Hessian matrix, which describes the second-order properties of the objective function in the space of coupling coefficients. The results are generic for symmetric matrices obtained by summing outer products of random vectors. The form of the eigenvalue distribution suggests new techniques for accelerating the learning process, and provides a theoretical justification for the choice of centered versus biased state variables.

We propose a new model of aggregation in the cellular slime mold D. Discoideum. Our approach couples the excitable signaling system to amoeba chemotaxis; the resultant system of equations is tractable to analytical and numerical approaches. Using our model, we derive the existence of a streaming instability for the concentric target aggregation pattern.

We consider fluid vesicles of toroidal topology. Minimization of the curvature energy at fixed volume and area leads to three different branches of axisymmetric shapes. By using conformal transformations, we identify a large region of nonaxisymmetric shapes in the phase diagram. For vanishing spontaneous curvature, the ground state is twofold degenerate in this region and corresponds to zero pressure difference across the membrane. The relation of these results to the recent observation of toroidal shapes for partially polymerized vesicles is discussed.

We report an elastic-light-scattering experiment for a semidilute polymer solution under a uniform laminar shear flow. The concentration fluctuations are greatly enhanced by shear flow and their structure is highly anisotropic in both the weak (γ̇${\mathrm{\tau }}_{\mathit{d}}$≪1) and strong (γ̇${\mathrm{\tau }}_{\mathit{d}}$≫1) shear regimes, where γ̇ is the shear rate and ${\mathrm{\tau }}_{\mathit{d}}$ is the longest relaxation time of the system. The anisotropy is qualitatively different from binary liquid mixtures under a shear flow as a result of the strong coupling between the concentration fluctuations and the hydrodynamic flow.

A recent anisotropy experiment with a 30’ beam is close to optimal for testing models in which cosmic structure arose from inflation-generated Gaussian density perturbations. Using a Bayesian analysis to constrain the amplitude of the perturbation spectrum, we show that adiabatic hot-dark-matter models are convincingly ruled out and cold-dark-matter models have anisotropies near our derived limits. Theories with broken scale invariance such as isocurvature baryon models are strongly constrained, as are models with extra power crafted to give significant large-scale structures and flows.

Tree-level calculations of thermal processes involving the exchange of massless gauge bosons give rise to infrared divergences which are screened by thermal loop corrections. For gauge theories at high temperatures or densities, a resummation method for the complete calculation of the effects of screening to leading order in the coupling constant is developed. The method is applied to calculte the rate of energy loss of a hot plasma from the Primakoff production of axions. Previous ad hoc prescriptions for screening significantly underestimate the contribution from magnetic scattering.

It is suggested that ${\mathit{m}}_{\mathit{d}}$>${\mathit{m}}_{\mathit{u}}$ is a natural consequence of Nambu’s four-fermion interaction model (with the BCS mechanism) if a small admixture of the charged-current interaction is included.