The magnetization direction switching is investigated in epitaxial Fe/Cu(100) films by spin-polarized scanning electron microscopy. We follow the transition from perpendicular to in-plane magnetization with both increasing film thickness and varying temperature. No variation of magnetic moment with magnetization direction change is found. A perpendicular stripe domain configuration is identified which evolves from the low-temperature single-domain state during the reorientation phase transition.

The multiphoton ionization of liquid water that occurs in femtosecond laser photolysis is investigated with an experimental setup in which 2 eV photons are sent into the sample. Hydrated electrons are identified as the generated absorbing species and their absorbance A is measured as a function of the laser irradiance E. The power law A∝${\mathit{E}}^{1.8}$ which is found differs markedly from that expected from energetic considerations. This is explained in terms of a resonant four-photon absorption involving the à ${}_{}{}^1{}_{}{}^{}$${\mathit{B}}_1$ electronic state of water which, in the liquid phase, is partly autoionizing.

Calculations of exciton radiative lifetimes in quantum wires free of defects are presented. Only excitons near zero wave number can spontaneously undergo radiative decay while conserving energy and momentum. The lifetime of the lowest-energy exciton in 100-Å-diam GaAs quantum wires, ${\mathrm{\tau }}_{\mathrm{spon}}$=150 ps, is an order of magnitude longer than in quantum wells. This is due to the finite spatial coherence in the lateral direction. Thermalization effects lead to an effective lifetime of τ(T)=140 √T ps ${\mathrm{K}}^{\mathrm{-}1/2}$.

High-order scattering is found to be essential for the convergence of the multiple-scattering (MS) theory of x-ray-absorption fine structure, both in the near-edge and the extended regimes. These contributions are calculated using an ab initio curved-wave scattering-matrix formalism. Convergence to full MS accuracy is demonstrated for fcc Cu, as well as for molecular ${\mathrm{O}}_2$ and ${\mathrm{N}}_2$, where our approach provides a high-order MS interpretation of the ${\mathrm{\sigma }}^{\mathrm{*}}$ shape resonances.

A small-angle neutron scattering study of the kinetics of growth of semiconductor crystallites in a glass matrix is presented. The scattered intensity I(k) exhibits a peak at a nonzero scattering vector and decreases to zero as k goes to zero. The I(k) patterns measured at different annealing temperatures present a scaling behavior consistent with the ripening approximation. We show that I(k) is in good agreement with the predictions of a simple phenomenological model, based on local mass conservation, which describes a dilute gas of crystallites surrounded by depletion zones.

A two-component network is studied by Monte Carlo simulation to model the lipid/spectrin membrane of red blood cells. The model predicts that the shear modulus decreases rapidly with the maximum length of the model spectrin and should be in the ${10}^{\mathrm{-}7}$ J/${\mathrm{m}}^2$ range for human red blood cells. A simplified model for the isolated spectrin network shows a negative Lamé coefficient λ. Transverse fluctuations of the dual membrane are found to be fluidlike over the range of wavelengths investigated.

Statistical ensembles of flexible two-dimensional fluid membranes arise naturally in the description of many physical systems. Typically, one encounters such systems in a regime of low tension but high stiffness against bending, which is just the opposite of the regime described by the Polyakov string. We study a class of couplings between membrane shape and in-plane order which break 3-space parity invariance. Remarkably there is only one such allowed coupling (up to boundary terms); this term will be present for any lipid bilayer composed of tilted chiral molecules. We calculate the renormalization-group behavior of this relevant coupling in a simplified model and show how thermal fluctuations effectively reduce it in the infrared.

We analyze a random perturbation applied to the baker’s map, a prototype for chaotic Hamiltonian evolution. We compare two strategies for following the perturbed evolution: (i) tracking the perturbed pattern in fine-grained detail; (ii) coarse graining by averaging over the perturbation. We find that the Landauer erasure cost associated wtih the information needed to implement the first strategy is overwhelmingly larger than the standard free-energy reduction associated with the second strategy. This finding provides a quantitative justification for coarse graining and thus an explanation of the second law of thermodynamics.

Long-wavelength vacuum oscillations between neutrinos can explain all the existing results from the chlorine, water-Cherenkov, and new gallium solar neutrino detectors. They predict distinctive energy dependences and seasonal time dependences that can be measured in solar neutrino experiments currently being constructed.

It is known that the existing experimental limit for CPT violation is rather poor—the 10% level at best. For this reason, we study experimental tests for CPT violation in B decays. We show that BB¯ mixing as well as CP-violating asymmetries can reveal CPT violation. We also point out that, with possible CPT violation, further experimental study is necessary in order to obtain the value for Δm/Γ from the rate of same-sign dilepton events.

The question of mass splitting within a doublet of heavy fermions, in particular a fourth generation of quarks and leptons, is reexamined with emphasis on the contribution of possible heavy-fermion ‘‘vector-meson’’ bound states to the ρ parameter. The heavy-fermion bound states are formed by the exchange of the Higgs boson. We have found that, in the minimal model, the constraint on mass splitting which now also depends on the Higgs boson mass gives rise to some interesting phenomena, including the possibility of having a top-quark mass greater than 180 GeV.

We measure the differential cross section with respect to Feynman-x (${\mathit{x}}_{\mathit{F}}$) and transverse momentum (${\mathit{P}}_{\mathit{T}}$) for charm meson production using targets of Be, Al, Cu, and W. In the range 0.1<${\mathit{x}}_{\mathit{F}}$<0.7, dσ/${\mathit{dx}}_{\mathit{F}}$ is well fit by the form (1-${\mathit{x}}_{\mathit{F}}$${)}^{\mathit{n}}$ with n=3.9±0.3. The difference between n values for ${\mathit{D}}^{\mathrm{-}}$ and ${\mathit{D}}^{+}$ is 1.1±0.7. However, we find an asymmetry of 0.18±0.06 favoring the production of ${\mathit{D}}^{\mathrm{-}}$ compared to ${\mathit{D}}^{+}$. In the lower ${\mathit{P}}_{\mathit{T}}$ range, <2 GeV, dσ/${\mathit{dP}}_{\mathit{T}}^2$ is well fit by the form exp(-b×${\mathit{P}}_{\mathit{T}}^2$) with b=1.03±0.06 ${\mathrm{GeV}}^{\mathrm{-}2}$, while in the higher ${\mathit{P}}_{\mathit{T}}$ range, 0.8 to 3.6 GeV, it is well fit by the form exp(-b’×${\mathit{P}}_{\mathit{T}}$) with b’=2.76±0.08 ${\mathrm{GeV}}^{\mathrm{-}1}$. The shape of the differential cross section has no significant dependence on atomic mass of the target material.

We present results of new measurements of the β spectrum of ${}_{}{}^{35}{}_{}{}^{}\mathrm{S}$, using the Caltech double-focusing, iron-free β spectrometer. Our data show no evidence for a heavy neutrino with a mass between 12 and 22 keV admixed to the usual light neutrino. In particular, we rule out, at the 6σ level, a 17 keV neutrino admixed at 0.85%, and give an upper limit (90% C.L.) of 0.2% for such a neutrino admixture. To demonstrate that our experiment is sensitive to spectral features such as those from heavy neutrinos we have induced an artificial kink by means of an absorber foil covering part of the source.

The problem of the existence of heavy-quarkonia bound states in nuclei is discussed. The method of multipole expansion and low-energy QCD theorems are used to calculate the cc¯-nucleon scattering amplitude. It is shown that an effective interaction is too weak to produce bound states with light nuclei for S-wave cc¯ systems. S-wave quarkonia can be bound in nuclei with A>10 with binding energy of the order of some MeV.

We present novel Monte Carlo methods for treating the interacting shell model that allow exact calculations much larger than those heretofore possible. The two-body interaction is linearized by an auxiliary field; Monte Carlo evaluation of the resulting functional integral gives ground-state or thermal expectation values of few-body operators. The ‘‘sign problem’’ generic to quantum Monte Carlo calculations is absent in a number of cases. We discuss the favorable scaling of these methods with nucleon number and basis size and their suitability to parallel computation.

We have measured the three photon ionization and electron energy spectra of Na from 5-ps laser pulses over the range 16 300 to 17 800 ${\mathrm{cm}}^{\mathrm{-}1}$. Together these measurements suggest that multiphoton ionization can be understood in terms of a Floquet description. Sharp discontinuities in the ionization probability and Rabi splittings in the electron energy spectra are direct consequences of adiabatic and diabatic evolution, respectively, from the initial zero-field ground state.

We report the quantum-nondemolition measurement of the photon number of an optical soliton. Taking advantage of the conservation properties of quantum solitons, we were able to read out the photon number of a ‘‘signal’’ soliton via the phase of a ‘‘probe’’ soliton after a soliton collision in a single-mode low-loss optical fiber. Subsequent noise measurements showed the photon number fluctuations of the signal soliton at the shot-noise limit to be correlated to the phase fluctuations of the probe soliton, signifying a backaction-evading measurement of the photon number.

It is shown that the unstable steady state of a multimode laser system may be stabilized by the occasional proportional feedback technique for dynamical control of chaotic systems. The range of pump excitations over which stabilization can be maintained is extended by more than an order of magnitude through application of a procedure for tracking the unstable steady state as the pump excitation is slowly varied.

It is shown that an inverse cascade of the turbulence of inertio-gravity waves produces a long-scale wave condensate. A new nonlinear equation is derived for long waves on rotating shallow water. It is proven that steady localized solutions are absent and that the condensate (a uniform inertial oscillation) is stable with respect to small perturbations. Wave self-interaction could not thus stop an inverse cascade of mesoscale geophysical turbulence. The implication of the existence of a condensate for the problem of tidal dissipation and retardation of the Earth’s rotation is discussed.

We derive a set of nonlinear partial differential equations describing the space-time evolution of Langmuir waves in a beam-plasma system, taking into account the propagation effects due to the difference between the beam and wave velocities. We demonstrate the existence of superradiant spikes behind the electron beam front that can be represented by a self-similar solution of the system of equations. These spikes are associated with self-bunching in the electron beam. The conditions in which the predicted spiking behavior could be experimentally observed are discussed.