The spin relaxation of photocreated carriers in GaAs/GaAlAs quantum wells in high magnetic fields is investigated by luminescence experiments under selective optical excitation. The complete quantization of the two-dimensional energy structure in high magnetic fields implies a simultaneous exchange of energy and momentum for spin relaxation and makes spin-conserving relaxation processes, both in thermalization and recombination, much faster than spin-flip processes. This results in a new spectroscopic tool to identify magnetoexcitonic states with the same electronic spin orientation.

Mössbauer-effect investigations under applied magnetic field and magnetization studies of neptunium intermetallics in their paramagnetic state are reported. It is shown that the bulk and local susceptibilities at 4.2 K (${\mathrm{\chi }}_{\mathrm{B}}$ and ${\mathrm{\chi }}_{\mathrm{L}}$, respectively) agree even for highly hybridized 5f-electron systems. The results indicate dominant orbital contributions to the magnetic moment also for paramagnetic neptunium heavy-fermion compounds. For these Np systems proportionalities between ${\mathrm{\chi }}_{\mathrm{L}}$, ${\mathrm{\chi }}_{\mathrm{B}}$, density of states at the Fermi energy, and the isomer shift are pointed out.

The results of a study of the superconducting state of very small diameter PbIn wires are described. The smallest samples, which had diameters below 200 Å, exhibited significant dissipation at all temperatures below $T_c$. In addition, their voltage-current characteristics exhibited oscillatory structure, which became more pronounced as the temperature was decreased. These results are consistent with a model in which the phase of the order parameter is treated as a quantum degree of freedom. The observed behavior is then due to quantum tunneling of the order parameter, and the existence of discrete energy levels.

Using a simple model of single vortex depinning, we show that the temperature dependence of both resistivity and ac susceptibility in the superconducting state in moderate magnetic fields can be understood in terms of the activation of vortices from homogeneous and densely distributed pinning centers. Furthermore, the field dependence of the activation energy is explained by including the interaction of vortices above H∼1 T, below which the distribution of pinning strengths becomes important.

An external magnetic field does not destroy superconductivity in type-II superconductors. As the field increases, the Abrikosov flux lattice crosses over into a new quantum limit, characterized by a transition temperature which is an increasing function of the field, the absence of the Meissner effect, and a supercurrent flow along the field direction. The transition temperature remains finite in an arbitrarily strong external field. We discuss the relevance of our results for the studies of a flux lattice in high-temperature superconductors and other superconducting systems.

A molecular-dynamics computer experiment is set up to study the stationary properties of an exothermic gas phase chemical system. A discrepancy is observed between the results of the simulation and predictions from macroscopic chemical kinetics. The origin of this discrepancy is found to arise from the deformation of the Maxwell distribution by reactive collisions. Quantitative agreement with the corrected rate law is established.

In this paper the crossover from ‘‘small’’ to ‘‘large’’ chaotic systems is studied. The behavior of the largest Lyapunov exponent in a system of coupled chaotic maps shows that this crossover is remarkably sharp, and allows us to define a coherence length beyond which the system is effectively large. Between the coherent chaos of the small system and the incoherent chaos (turbulence) of the large one there is a stable window starting at the linear instability point for the uniform chaotic state in which the lattice becomes effectively one dimensional. The scaling of the coherence length close to the onset of turbulence is investigated and compared to recent predictions.

We compute the velocity autocorrelation function of a tagged particle in a two-dimensional lattice-gas cellular automaton using a method that is about a million times more efficient than existing techniques. A ${\mathrm{t}}^{\mathrm{-}1}$ algebraic tail in the tagged-particle velocity autocorrelation function is clearly observed. The amplitude of this tail is predicted to within a few percent by a lattice-gas version of mode-coupling theory. The magnitude of logarithmic corrections to the ${\mathrm{t}}^{\mathrm{-}1}$ tail is much smaller than expected for continuous fluids.

In currently used lattice formulations of quantum electrodynamics, even though gauge fields are introduced as noncompact degrees of freedom, the constraints of gauge invariance imply that fermion fields are sensitive to features best described by compact or periodic variables such as magnetic monopoles. Results from a quenched simulation are presented, showing the existence of a percolation threshold for monopole current networks near the chiral-symmetry-breaking transition already known to occur from previous studies. Implications for the mechanism driving the chiral transition are discussed.

We have measured the mass of the Z boson to be 91.14±0.12 GeV/$c^2$, and its width to be ${2.42}_{\mathrm{-}0.35}^{+0.45\mathrm{}}$ GeV. If we constrain the visible width to its standard-model value, we find the partial width to invisible decay modes to be 0.46±0.10 GeV, corresponding to 2.8±0.6 neutrino species, with a 95%-confidence-level upper limit of 3.9.

An upper limit on the branching ratio of the decay $K^{+}$→${\pi }^{+}$H, H→${\mu }^{+}$${\mu }^{\mathrm{-}}$ is set at 1.5×${10}^{\mathrm{-}7}$ at the 90% confidence level, for a Higgs-boson mass in the interval 220<$m_H$<320 MeV/$c^2$. In addition, 90%-confidence-level upper limits on the branching ratios of the decays $K^{+}$→${\pi }^{+}$${\mu }^{+}$${\mu }^{\mathrm{-}}$ and $K^{+}$→${\mu }^{+}$ν${\mu }^{+}$${\mu }^{\mathrm{-}}$ are set at 2.3×${10}^{\mathrm{-}7}$ and 4.1×${10}^{\mathrm{-}7}$, respectively.

A search for the decays $K_L^0$→μe and $K_L^0$→ee has produced no examples of either process. When normalized to the decay $K_L^0$→${\pi }^{+}$${\pi }^{\mathrm{-}}$, the 90%-C.L. upper limits on the branching ratios are B($K_L^0$→μe)<2.2×${10}^{\mathrm{-}10}$ and B($K_L^0$→ee)<3.2×${10}^{\mathrm{-}10}$. .AE

Concurrent with our search for the decays $K_L^0$→μe and $K_L^0$→ee, we have observed 87 $K_L^0$→μμ events. Normalizing this sample to the simultaneous observation of the decay $K_L^0$→${\pi }^{+}$${\pi }^{\mathrm{-}}$, we obtain the branching ratio B($K_L^0$→μμ) =[5.8±0.6(stat)±0.4 (syst)] ×${10}^{\mathrm{-}9}$.

A new parametrization of the Kobayashi-Maskawa (KM) matrix is proposed. It is based on the eigenvalues and the eigenvectors of the KM matrix. In this parametrization the experimental data for the KM matrix (including CP violation) can be reproduced with only two angles. This suggests that CP violation is intimately connected with the Cabibbo-type rotations. We also predict that ‖$V_{\mathrm{ub}}$/$V_{\mathrm{cb}}$‖=0.128±0.001. .AE

We present a measurement of subthreshold antiproton, kaon, and energetic pion production at 0° for the reactions ${}_{}{}^{28}{}_{}{}^{}$Si${+}^{28}$Si at 2.0 GeV/nucleon ($E_{\mathrm{available}/\mathrm{A}=0.82\mathrm{}}$ GeV) and 1.65 GeV/nucleon ($E_{\mathrm{available}/\mathrm{A}=0.70\mathrm{}}$ GeV), and for ${}_{}{}^{20}{}_{}{}^{}$Ne+NaF at 2.0 GeV/nucleon. For a given reaction, the particle yields, plotted as a function of the required excitation energy, exhibit a scaling behavior independent of particle species. Implications for collectivity and a common production mechanism are discussed.

Using the Heidelberg-Darmstadt Crystal Ball spectrometer the γ decay of the superdeformed shape isomer in ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ has been investigated employing the ${}_{}{}^{235}{}_{}{}^{}$U(d,p) reaction at 11 MeV. The isomer was isolated by requiring that the prompt and delayed γ sum energy add up to the initial excitation energy of ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ as determined from the energy of the recoiling proton. The shape isomer was found to have an excitation energy of E${(0\mathrm{}}_{\mathrm{II}{}^{+})=2.75\mathrm{}\pm 0.01}$ MeV and to decay by four different γ transitions in competition with its well known fission mode. The corresponding branching ratio of γ decay to fission was determined to be ${\Gamma }_{F\gamma }$/${\Gamma }_{\mathrm{Ff}}$=8±3, thus resolving the long-standing problem of the missing γ decay branch.

Multiple superdeformed rotational bands have been identified in a nucleus for the first time. Cascades of 14, 13, and 11 transitions have been assigned to three bands in ${}_{}{}^{153}{}_{}{}^{}\mathrm{Dy}$. Despite the small intensities, it has been possible to follow the decay of these bands from an angular frequency of 0.7 down to 0.4 MeV/ħ. In all three cases, the dynamic moment of inertia ${\mathrm{scrI}}^{\mathrm{}\left(2\right)\mathrm{}}$ is nearly constant. Assignments to high-N intruder orbitals are suggested through comparison of these values of ${\mathrm{scrI}}^{\mathrm{}\left(2\right)\mathrm{}}$ with theoretical calculations based on the cranked shell model.

Deformed single-particle levels are derived from a boson-fermion Hamiltonian in which the odd fermion occupies several j orbits. The geometric-oriented approach applied to ${}_{}{}^{169}{}_{}{}^{}\mathrm{Tm}$ clarified the role of algebraic interactions and provides an intuitive interpretation and guidance to numerical calculations in deformed nuclei.

We report photoelectron energy spectra from multiphoton ionization of xenon by 150-fs pulses wavelength tuned between 575 and 675 nm. The spectra show a number of structures that are interpreted as Stark-induced resonances. Measuring the energy of these structures for different wavelengths yields the intensity and wavelength dependencies of the corresponding Stark shifts. For intensities up to 3×${10}^{13}$ W ${\mathrm{cm}}^{\mathrm{-}2}$, states between 7p and 6f were found to undergo giant shifts (up to 1.5 eV). These results are compared with the predictions of a second-order perturbation calculation.

Studies of multiphoton ionization of noble gases have been carried out using 1-μm, 1-ps laser pulses with intensities up to the mid- ${10}^{16}$ W/${\mathrm{cm}}^2$. To our knowledge, this work represents the first study of the production of highly ionized noble-gas ions done exclusively in the tunneling regime. It is found in this regime that the ionization at a given intensity depends on both the ionization potential and the charge state of the species. The onset of ionization occurs when the sum of the Coulomb and laser electric potentials causes the electron to be unbound.