We have studied the picosecond time dependence of luminescence from a two-dimensional electron system following absorption of an ultrashort light pulse. From our measurements we determine the temporal evolution of the carrier temperature, finding that the cooling of hot carriers is suppressed by a factor 60 below that predicted on a three-dimensional nondegenerate-electron model. Additionally, we determine the electron-hole radiative life-time and invoke a hole trap to explain shortened luminescence lifetimes at low carrier densities.

This Letter reports measurements of the magnetic field dependence of the resistive critical temperature $T_c$ of a regular square network of superconducting aluminum. We find new effects of flux quantization corresonding to both integral (1, 2, 3,...) and fractional (¼, $\frac{1}{3}$, $\frac{2}{5}$, ½, $\frac{3}{5}$, $\frac{2}{3}$, $\frac{3}{4}$) numbers of flux quanta per unit cell of the network. The fractal fine structure of the upper critical-field line is identified as the edge of the Landau-level spectrum for a tight-binding problem on a square lattice.

Negative-muon spin precession has been investigated in ferromagnetic nickel in zero applied field. A comparison of the hyperfine field for ${\mu }^{-}$ Ni with that for ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$ in nickel yields a giant hyperfine anomaly of -2.4(3)% to -2.8(5)%, over the temperature range 23-303 K. The results indicate that the electron spin density near the Ni nucleus decreases with increasing radial distance more steeply than the $s$-state electron charge density, and they are in good agreement with an unrestricted Dirac-Fock calculation by Freeman et al.

A tandem Fabry-Perot interferometer has been used to locate Raman bands at 12 ${\mathrm{cm}}^{-1\mathrm{}}$ ($B$-DNA) and 15 and possibly 12 ${\mathrm{cm}}^{-1\mathrm{}}$ ($A$-DNA). The Raman shift of these bands is dispersionless and the scattering process appears to require quasimomentum conservation. We assign these bands to the lowest-lying Raman-active optic phonons.

We introduce a noncanonical ("new-time") transformation which exchanges the roles of a coupling constant and the energy in Hamiltonian systems while preserving integrability. In this way we can construct new integrable systems and, for example, explain the observed duality between the Hénon-Heiles and Holt models. It is shown that the transformation can sometimes connect weak- and full-Painlevé Hamiltonians. We also discuss quantum integrability and find the origin of the deformation $-\frac{5}{72}{\hslash }^2{x}^{-2\mathrm{}}$.

When Einstein-Brillouin-Keller quantization is possible, it applies to all conserved dynamical variables (not only to the Hamiltonian) and in particular to the time average of any dynamical variable. Thus, for an integrable system of $n$ degrees of freedom, the eigenvalues of $n$ independent constants of motion form a locally regular $n$-dimensional lattice, in the limit $h\to 0$. Failure of doing that may be an indication of quantum chaos.

Quantum chaos of a single wave function is defined in terms of a path correlation function, and used to show how chaos sets in, in the particle-in-a-stadium problem. The criterion is shown to be equivalent to the requirement of a random distribution of the wave-function values.

It is discussed whether the recently discovered resonance at 8.3 GeV can be interpreted as a weakly coupled fundamental pseudoscalar. Such a particle is readily incorporated in an SU(2) ⊗ U(1) gauge theory framework. The importance of mixing with ${\eta }_B$, ${\eta }_B^{\prime }$ for the phenomenology of such a particle is emphasized, and critical tests of their identification are proposed.

We present the first results for the nonperturbative $\beta$ function along the Wilson axis for the SU(3) pure gauge theory using the Monte Carlo renormalization-group method. The results show that there is no asymptotic scaling for the coupling in the fundamental representation $K_F<6.1\mathrm{}$. The renormalized action generated by the $\sqrt{3}$ block transformation is also estimated for use in future calculations.

The reaction $\bar{p}+\mathrm{Ta}\to V^0+X$ ($V^0=\Lambda , \bar{\Lambda }, or K_s^0$) at 4 GeV/c has been studied by comparing with the results in $\bar{p}p$ interaction. Strong enhancement of $\Lambda$ production was observed, while $\bar{\Lambda }$ production is suppressed. The $\frac{\bar{\Lambda }}{\Lambda }$ production ratio is obtained as (2.0 ± 1.0) × ${10}^{-2\mathrm{}}$. $\Lambda$ reveals evaporationlike behavior with a nuclear temperature of 97 MeV. Evaporationlike behavior with a temperature of 135 MeV, but via a short-range process, was observed also in $K_s^0$ production.

We report the results of a free-electron laser experiment which has produced 1-2-μsec duration pulses of ∼ 30-GHz radiation at power levels up to 4 MW. This is the first free-electron laser in the high-current operating regime to employ no external electron-focusing field. Consequently, there is no cyclotron emission, and the output radiation is unambiguously identified as free-electron laser emission.

The absorption spectrum of a system of $N$ atoms interacting with a single mode of the quantized radiation field is exactly calculated. Such a spectrum shows vacuum-field Rabi splittings, and thus microwave absorption by Rydberg atoms in a cavity should be a useful way to observe these splittings.

The apparently large long-time-tail contribution to the stress-tensor autocorrelation function observed in computer simulations is explained on the basis of a generalized modecoupling theory. This theory includes effects that occur on a molecular length scale where structural relaxation is important. These structural effects are also shown to be responsible for the appearance of propagating shear waves in dense liquids at large wave numbers.

These experiments studied the Raman instability occurring at densities below the quarter-critical density of the incident 0.53-μm laser light. Solid gold targets were irradiated by up to 4 kJ of energy in a 1-ns pulse with focal spots of 150 to 1880 μm. The angular distribution of the Raman scattering shows efficient Raman sidescatter. The energy in electron plasma waves accounts for the hot-electron production over almost three orders of magnitude. We report the results and discuss the implications for laser fusion.

We present the first direct observation of the structure of a driven global Alfvén eigenmode in a tokamak plasma using C${\mathrm{O}}_2$ laser interferometry.

A new argument is given for a lower critical dimension $d_l=2\mathrm{}$ for the Ising model in a random magnetic field. It forms the basis for a proof that the three-dimensional model exhibits long-range order at zero temperature and small disorder. This settles the controversy between the values $d_l=2\mathrm{}$ and $d_l=3\mathrm{}$.

A mean-field model of a Heisenberg spin-glass with weak random anisotropy is solved. It is shown that an Ising-like phase transition in a finite magnetic field $H$ occurs at $T_c\mathrm{}\left(H\right)\mathrm{}\sim T_c\mathrm{}\left(0\right)-A{H}^{\frac{2}{3}}$ in the limit of small $H$. The crossover to an Ising behavior occurs when $\frac{D}{J}>{\left(\frac{\mu H}{J}\right)}^{\frac{2}{3}}$, where $D$ and $J$ are the anisotropy and exchange coupling constants, respectively.

By use of a Monte Carlo technique, it is shown that high-field electron transport in the Si${\mathrm{O}}_2$ conduction band is controlled by LO-phonon scattering for fields below 3 × ${10}^6$ V/cm and by nonpolar scattering with acoustic phonons at higher fields. This accounts for recent experimental results indicating that an energy-loss mechanism, previously neglected, is effective at high fields.

Tm${\mathrm{Se}}_{0.32}$ ${\mathrm{Te}}_{0.68}$ undergoes a continuous pressure-induced semiconductor-to-metal transition (at 1.4 GPa) in which the Tm ions change their valence. This has been studied by electrical-resistivity, Hall-effect, volume, elastic-constant, and magnetic-susceptibility measurements under pressure. Already in the semiconducting phase all the lattice-related properties indicate strong valence mixing, whereas the magnetic susceptibility remains typical of pure divalent Tm. These new results are discussed in terms of the pressure-induced $4f-5d$ hybridization and local correlation effects.

Positive muons were injected into both trans- and cis-${\left(\mathrm{CH}\right)}_x$ as well as ${\left(\mathrm{CD}\right)}_x$. A marked difference was found in the field dependence of the muon spin-relaxation functions between cis- and trans-polyacetylene. A quantitative explanation was obtained by considering the unpaired electron produced by a muon: It is localized at the nearby carbons in cis-polyacetylene forming a radical state; on the other hand, in trans-polyacetylene, it becomes highly mobile and takes a one-dimensional solitonlike motion as clearly indicated by the inverse-root field dependence found for the relaxation rates.