A stimulated photon-echo study has verified the instantaneous shift (≤1 MHz) and subsequent restoration (∼2 ms) of the transition frequency of an impurity ion in a solid when its neighboring ions are exposed to pulsed optical excitation. Experiments were conducted by monitoring the echo intensity for the ${}_{}{}^7{}_{}{}^{}$${\mathit{F}}_{0\mathrm{-}}^5$${\mathit{D}}_0$ transition of Eu ions in one site of ${\mathrm{Y}}_2$${\mathrm{SiO}}_5$, while exciting the other site. A photon-echo theory taking account of the stochastic frequency recovery is developed to explain the observations.

This Letter reports experimental evidence for the existence of localized excitons at above-barrier energies in type-I superlattices. By using magnetoabsorption measurement on a series of ${\mathrm{Zn}}_{0.86}$${\mathrm{Cd}}_{0.14}$Se/${\mathrm{Zn}}_{0.75}$${\mathrm{Mn}}_{0.25}$Se superlattices, where the subbands localized in nonmagnetic and magnetic layers undergo drastically different Zeeman splittings, we show conclusively that above-barrier excitons are localized in the barrier rather than in the well regions.

We consider photon emission in electron tunneling to a small metal particle. The radiative (dipole) plasmon mode of the particle can be excited resonantly by a tunneling electron either when it is in the barrier or else as a decay channel of the hot electrons injected into the particle via elastic tunneling. Analytical estimates show that for a particle with a few hundred angstrom radius the former process dominates over the latter by a factor of ∼${10}^3$, and that the integrated light intensity is ∼${10}^{\mathrm{-}3}$ photon per tunneling electron under optimum conditions, in agreement with recent observations.

The tunneling characteristics of a two-junction system created with extremely small metal particles is studied in the voltage region within the Coulomb blockade. We observe a conductance orders of magnitude lower than outside the blockade, which matches predictions for a superposition of elastic and inelastic macroscopic quantum tunneling of the charge variable Q.

Renormalization group arguments are used to give a complete parametrization of the disorder dependence of the mean-field superconducting ${\mathit{T}}_{\mathit{c}}$ within a BCS model. The theory describes destruction of superconductivity in bulk materials well before the metal-insulator transition is reached, and rapid degradation of ${\mathit{T}}_{\mathit{c}}$ in thin films with increasing sheet resistance. Comparison of explicit calculations with experiments yields good agreement.

We present exact diagonalization results of an electron-phonon model Hamiltonian for the O(4)-Cu(1)-O(4) cluster in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. For large enough electron–infrared-phonon coupling the motion of holes and ions becomes strongly correlated, i.e., polaronic, leading to a double-well structure for the infrared mode, as encountered in x-ray-absorption fine structure. In contrast, the associated Raman mode shows a single-well behavior even for large electron–Raman-phonon coupling. The appearance of polarons accompanies nonadiabatic behavior yielding optical spectroscopy predictions differing from those obtained from harmonic or anharmonic lattice dynamics models.

High-resolution electron-energy-loss spectroscopy has been used to determine the magnitude and temperture dependence of the energy gap for oxygen annealed ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ single crystals. Below the ${\mathit{T}}_{\mathit{c}}$, low-energy excitations are detected in the energy-loss spectra. These spectra are proportional to the frequency-dependent resistivity, ρ(ω), and thus the energy gap can be determined directly from the onset of ρ(ω). We find that at low temperature 2Δ=6${\mathit{kT}}_{\mathit{c}}$ and that Δ develops sharply for T<${\mathit{T}}_{\mathit{c}}$. The similarity of these results to the predictions of models that consider spin interactions suggests that higher ${\mathit{T}}_{\mathit{c}}$’s may be obtainable in this material.

We study the N-fold degenerate Hubbard model in the limits U→∞, N→∞, t→0 with ${\mathit{Nt}}^2$=const (U, on-site Coulomb correlation; t, nearest-neighbor hopping). In leading nontrivial order in 1/N one does not obtain simply a renormalized tight-binding band, which the slave-boson mean-field theory would predict, but a correlation-induced splitting of the lower Hubbard band. There exists an effective interaction between the electrons in different spin channels at nearest-neighbor sites in the case U→∞, in which the usual antiferromagnetic J≃${\mathit{t}}^2$/U interaction vanishes.

In a general class of one- and two-dimensional Hubbard models, we prove upper bounds for the two-point correlation functions at finite temperatures for electrons, electron pairs, and spin. The upper bounds decay exponentially in one dimension, and with power laws in two dimensions. The bounds rule out the possibility of the corresponding condensation of superconducting electron pairs, and of the corresponding magnetic ordering. Our method is general enough to cover other models such as the t-J model.

Sputtered Fe(20 Å)/Nb superlattices were prepared with Nb thicknesses ranging from ∼2 to 40 Å, and oscillatory interlayer magnetic coupling was observed by means of magnetoresistance (MR), magnetometry, and Kerr rotation measurements. As many as five oscillations between ferromagnetic and antiferromagnetic coupling were observed at room temperature with a Nb periodicity of ∼9 Å. The largest negative MR measured, however, was only 0.3%. The low MR values of high resistivity (60–120 μΩ cm) suggest that the structural disorder associated with the lattice mismatch (13% in this system) plays an important role in governing the transport phenomenon.

We use supersymmetric quantum mechanics to show that the spectrum of three anyons in a harmonic potential exhibits an almost complete mirror symmetry about semions. Barring a subset of exactly solved states, all states come in pairs such that the energy of one state with the statistics parameter θ is equal to the energy of its partner state at π-θ. Bosons, semions, and fermions have θ=0,π/2, and π, respectively. From this, we show that the third virial coefficient of an ideal anyon gas is exactly mirror symmetric about semions.

First, we demonstrate Bell’s theorem, without using inequalities, for an experiment with two particles. Then we show that, if we assume realism and we assume that the ‘‘elements of reality’’ corresponding to Lorentz-invariant observables are themselves Lorentz invariant, we can derive a contradiction with quantum mechanics.

The response of an overdamped bistable system to a weak periodic force in the presence of external noise is investigated using linear-response theory and by analog electronic experiment. It is shown, both theoretically and experimentally, that a phase lag $\phi$ exists between the force and the response, and that $\left|\phi \right|$ passes through a maximum when the noise intensity is tuned through the range where stochastic resonance occurs. These results clarify the interrelation between stochastic resonance and conventional resonance phenomena, and confirm that stochastic resonance can usefully be treated by linear-response theory.

The power-law behavior of the average time between intermittent bursts in the NMR-laser dynamics near a heteroclinic tangency crisis is investigated. Using symbolic-dynamical techniques, new crisis-induced sequences are identified in the strange attractors reconstructed from both experimental and simulated data. Our approach provides a precise criterion for the onset of the attractor widening due to the collision of the stable and the unstable manifold belonging to different unstable periodic orbits. The results show the predictability power of our laser model.

We investigate the chaotic temperature dependence of the renormalized couplings in a short-range Ising spin glass. Some of our analytic and numerical results, obtained by the Migdal-Kadanoff renormalization scheme but argued to have general validity, differ from those of the scaling and droplet theory. First, chaos is present also at and above the critical temperature. Second, between ${\mathit{d}}_{\mathit{l}}$=2.46 (the lower critical dimension) and ${\mathit{d}}_{+}$=3.4, the chaos in the critical region is characterized by a new critical exponent ${\mathrm{\zeta }}_{\mathit{c}}$ that has hitherto escaped attention.

We show that in the world-sheet SL(2,R)/U(1) coset model there exists a hidden nonlinear W${\mathrm{^}}_{\mathrm{\infty }}$ current algebra, which incorporates all higher-spin (s≥2) currents including the energy-momentum tensor. Our proof is based on a free-field realization of W${\mathrm{^}}_{\mathrm{\infty }}$ in terms of two scalars, as well as an elegant generating function for the W${\mathrm{^}}_{\mathrm{\infty }}$ currents in the classical model. After quantization, the deformed W${\mathrm{^}}_{\mathrm{\infty }}$ current algebra still results in an infinite set of commuting charges, which might be related to a space-time W symmetry for the 2D black hole arising in the corresponding string theory.

A new measurement of the ${\mathrm{\pi }}^{+}$→${\mathit{e}}^{+}$ν branching ratio gives ${\mathit{R}}_{\mathrm{\pi }\mathit{e}\nu }$=Γ(π→eν+π→eνγ)/Γ (π→μν+π→μνγ) =[1.2265±0.0034(stat)±0.0044(sys)]×${10}^{\mathrm{-}4}$. This result is in agreement with standard model calculations and confirms the hypothesis of electron-muon universality at the 0.2% level.

We have measured the asymmetry parameter (${\mathrm{\alpha }}_{\gamma }$) in the hyperon radiative decay ${\mathrm{\Sigma }}^{+}$→pγ with a sample of 34 754±212 events obtained in a polarized charged hyperon beam experiment at Fermilab. We find ${\mathrm{\alpha }}_{\gamma }$=-0.720±0.086±0.045, where the quoted errors are statistical and systematic, respectively.

A statistical method to analyze the gamma decay of excited, rapidly rotating nuclei is presented. It is based on the fact that the cooling path of these systems goes through regions of both suppressed and enhanced fluctuations typical of large and low level density, respectively. The method is applied to the study of the high-spin quasicontinuum of ${}_{}{}^{167,168}{}_{}{}^{}\mathrm{Yb}$.

We present the first polarization measurements for pion absorption on a nucleus heavier than the deuteron. The polarization of protons resulting from ${\mathrm{\pi }}^{+}$ absorption in the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ was measured at bombarding energies of 120 and 250 MeV. Protons from absorption in a quasideuteron were selected by applying kinematical constraints. A significant discrepancy was observed between the experimental results and theoretical predictions. At 120 MeV the measured polarizations for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ are consistent with those of the deuteron. At 250 MeV the angular distribution of the polarization is significantly different than for the deuteron, showing sensitivity to the nuclear density, and thus may be sensitive to short range correlations between nucleons.