The evolution with increasing Coulomb correlations of a semiconductor to a magnetic insulator is related to an excited-state crossover in π-electron models for conjugated polymers. We associate strong fluorescence with a lowest singlet excitation ${\mathit{S}}_1$ that is dipole allowed, on the band side, while ${\mathit{S}}_1$ becomes two-photon allowed on the correlated side. ${\mathit{S}}_1$/${\mathit{S}}_2$ crossovers in Hubbard, Pariser-Parr-Pople, or other chains with electron-hole symmetry and alternating transfer integrals t(1±δ) are based on exact results at δ=0 and 1, on molecular exciton theory at large δ, and on oligomer calculations up to twelve sites.

We present an analytical solution of a SU(N)×SU(M) generalization of the multichannel single-impurity Kondo model in the limit N→∞, M→∞, with γ=M/N fixed. Non-Fermi-liquid behavior of the single electron Green function and of the local spin and flavor susceptibilities occurs in both regimes, N≤M and N>M, with leading critical exponents identical to those found in the conformal field theory solution for all N and M (with M≥2). We explain this remarkable agreement and connect it to ‘‘spin-flavor separation,’’ the essential feature of the non-Fermi-liquid, fixed point.

We analyze relaxation of a disturbance of the wave function induced by a slowly varying perturbation localized inside a potential barrier. The perturbation is assumed to decay in time exponentially. We demonstrate that for a ‘‘flat’’ barrier, whose curvature at the highest point is zero (e.g., a box-shaped barrier), the relaxation is time scale invariant, being proportional to a certain power of time. If the curvature does not vanish but is small enough, the temporal scaling will be observed as an intermediate asymptotic regime, while at very large times the relaxation becomes exponential with a time scale determined solely by the small curvature. Dissipation slows down the relaxation, leaving it time scale invariant.

Common optical absorption features are observed near 0.5 eV in four undoped single-crystal copper oxides: ${\mathrm{La}}_2$${\mathrm{CuO}}_4$, ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$, ${\mathrm{Pr}}_2$${\mathrm{CuO}}_4$, and ${\mathrm{Sr}}_2$${\mathrm{CuO}}_2$${\mathrm{Cl}}_2$. These absorption bands are shown to be weakly electric dipole allowed excitations of the ${\mathrm{CuO}}_2$ layers. The features are ascribed to an exciton near 0.4 eV, probably of crystal field origin, with strong multimagnon sidebands. Our observations provide an important basis for understanding the doped materials.

The subgap conductivity of a normal-superconductor (NS) tunnel junction is thought to be due to tunneling of two electrons. There is a strong interference between these two electrons, originating from the spatial phase coherence in the normal metal at a mesoscopic length scale and the intrinsic coherence of the superconductor. We evaluated the interference effect on the transport through an NS tunnel juction. We propose the layouts to observe drastic Aharonov-Bohm and Josephson effects.

We have studied the spin-1/2 Heisenberg antiferromagnet on the triangular lattice by high temperature series expansions. From an analysis of the antiferromagnetic structure factor and correlation length, we deduce that the ground state has small but nonzero long range antiferromagnetic order. We also determine the temperature dependence of the uniform susceptibility and the specific heat.

Using the recently proposed density matrix renormalization group technique we show that the magnons in the S=1 antiferromagnetic Heisenberg chain effectively behave as bosons that condense at a critical field ${\mathit{h}}_{\mathit{c}}$. We determine the spin-wave velocity, v=2.49(1), as well as the gap Δ=0.4107(1)J.

Vector spin glasses with random short-range anisotropic couplings are studied using the defect-wall renormalization-group method. By calculating the correlation length exponent ν and the fractal dimension of the defect wall, ${\mathit{d}}_{\mathit{s}}$, we obtain the first direct evidence that in two dimensions these systems have a zero-temperature transition in the same universality class as the Ising spin glass. We also present results which strongly suggest that these systems have a transition at nonzero temperature in three dimensions.

The origin of giant magnetoresistance exhibited by ferromagnetic/nonmagnetic multilayered structures is examined by inserting thin layers of a second ferromagnetic material at the interfaces in ferromagnetic/nonmagnetic/ferromagnetic sandwiches. It is generally observed, for many different combinations of metals, that the magnetoresistance depends exponentially on the thickness of the interface layer, with a characteristic length, ξ. ξ is extremely short and is typically just ≃1.5 to 3 Å at room temperature. At lower temperatures ξ becomes even shorter. The giant magnetoresistance effect is thus clearly shown to be determined by the character of the ferromagnetic/nonmagnetic interfaces.

The c axis reflectance of high-quality single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ has been measured over a wide range of temperatures and frequencies. The x=0.70 crystal has a pseudogap in the conductivity at 200 ${\mathrm{cm}}^{\mathrm{-}1}$ which is present at room temperature. The gap deepens gradually as the temperature is lowered to 10 K with no discontinuity at ${\mathit{T}}_{\mathit{c}}$. In contrast, crystals with x=0.95 show no gap in the normal state and their superconducting state is characterized by low-lying states. These observations are consistent with phase diagrams proposed to explain NMR and neutron data.

We study thermally driven escape over a dichotomously fluctuating barrier and present explicit formulas for the mean escape time in three limiting frequency domains. Our analysis is based on the chemically realistic case of large potential barrier, and modest fluctuation amplitude. When the fluctuation frequency is small compared to the time scale for adiabatic adjustment, which includes the low and intermediate frequency regimes, a kinetic approximation reproduces the Fokker-Planck results.

Equilibrium polymerization in a solvent exhibits critical concentration fluctuations governed by a nonclassical critical exponent. A dramatic enhancement of the concentration fluctuations above those given by mean field theory (Flory theory) is predicted that persists even far from the transition. The predicted behavior explains recent neutron scattering results on polymerization in living polymer solutions and suggests improvements in experimental conditions. It is also in accord with recent measurements of scattering from solutions of giant polymeric micelles.

If a quantum system of Hilbert space dimension mn is in a random pure state, the average entropy of a subsystem of dimension m≤n is conjectured to be ${\mathit{S}}_{\mathit{m},\mathit{n}}$= ${\mathit{S}}_{\mathit{k}=\mathit{n}+1\mathrm{}}^{\mathit{mn}}$ 1/k-m-1/2n and is shown to be ≃lnm-m/2n for 1≪m≤n. Thus there is less than one-half unit of information, on average, in the smaller subsystem of a total system in a random pure state.

The exact free energy of SU(N) Chern-Simons theory at level k is expanded in powers of (N+k${)}^{\mathrm{-}2}$. This expansion keeps rank-level duality manifest, and simplifies as k becomes large, keeping N fixed (or vice versa)—this is the weak-coupling (strong-coupling) limit. With the standard normalization, the free energy on the three-sphere in this limit is shown to be the generating function of the Euler characteristics of the moduli spaces of surfaces of genus g, providing a string interpretation for the perturbative expansion. A similar expansion is found for the three-torus, with differences that shed light on contributions from different spacetime topologies in string theory.

Based on the Ward-Takahashi identity for the Becchi-Rouet-Stora symmetry, we prove to all orders of the loop expansion the low energy theorem of hidden local symmetry for the vector mesons (a version of the Kawarabayashi-Suzuki-Riazuddin-Fayyazuddin relation) in the U(N${)}_{\mathit{L}}$×U(N${)}_{\mathit{R}}$/U(N${)}_{\mathit{V}}$ nonlinear chiral Lagrangian.

We show that in (2+1)-dimensional quantum electrodynamics an external magnetic field applied to a finite density of massless fermions is screened, due to a (2+1)-dimensional realization of the underlying two-dimensional axial anomaly of the space components of the electric current. This is shown to imply screening of the magnetic field, i.e., the Meissner effect. We discuss the physical implications of this result.

The CCFR Collaboration presents a measurement of scaling violations of the nonsinglet structure function and a comparison to the predictions of perturbative QCD. The value of ${\mathrm{\Lambda }}_{\mathrm{QCD}}$, from the nonsinglet evolution with ${\mathit{Q}}^2$>15 ${\mathrm{GeV}}^2$ and in the modified minimal-subtraction renormalization scheme, is found to be 210±28(stat)±41(syst) MeV.

Using the CLEO-II detector, the branching ratio of the Cabibbo suppressed decay ${\mathit{D}}^{+}$→${\mathrm{\pi }}^0$${\mathit{l}}^{+}$ν, relative to the branching ratio of the Cabibbo favored decay ${\mathit{D}}^{+}$→K${\mathrm{¯}}^0$${\mathit{l}}^{+}$ν, is measured to be B(${\mathit{D}}^{+}$→${\mathrm{\pi }}^0$${\mathit{l}}^{+}$ν)/B(${\mathit{D}}^{+}$→K${\mathrm{¯}}^0$${\mathit{l}}^{+}$ν) =(8.5±2.7±1.4)%. From this ratio, the product of the ratio of Cabibbo-Kobayashi-Maskawa (CKM) matrix elements, ‖${\mathit{V}}_{\mathit{c}\mathit{d}}$/${\mathit{V}}_{\mathit{c}\mathit{s}}$${\mathrm{\Vert }}^2$, and the form factors at ${\mathit{q}}^2$=0, ‖${\mathit{f}}_{+}^{\mathrm{\pi }}$(0)/${\mathit{f}}_{+}^{\mathit{K}}$(0)${\mathrm{\Vert }}^2$, is determined to be ‖${\mathit{V}}_{\mathit{c}\mathit{d}}$/${\mathit{V}}_{\mathit{c}\mathit{s}}$${\mathrm{\Vert }}^2$‖${\mathit{f}}_{+}^{\mathrm{\pi }}$(0)/${\mathit{f}}_{+}^{\mathit{K}}$(0)${\mathrm{\Vert }}^2$ =0.085±0.027±0.014.

The spin asymmetry parameter ${\mathit{A}}_{\mathrm{\tau }}$ characterizing the angular distribution of the total hadron momentum in the decay of a polarized tau can be calculated rigorously using perturbative QCD and the operator product expansion. Perturbative QCD corrections to the free quark result ${\mathit{A}}_{\mathrm{\tau }}$=1/3 can be expressed as a power series in ${\mathrm{\alpha }}_{\mathit{s}}$(${\mathit{M}}_{\mathrm{\tau }}$) and nonperturbative QCD corrections can be expanded systematically in powers of 1/${\mathit{M}}_{\mathrm{\tau }}^2$. The QCD prediction is ${\mathit{A}}_{\mathrm{\tau }}$=0.41±0.02. A measurement in agreement with this prediction would provide strong support for the precise determination of ${\mathrm{\alpha }}_3$ from tau decay data.

We argue that off-shell corrections to the parton model may account for a significant part of the reported violation of the Gottfried sum rule in deep inelastic scattering as well as for a relative depletion of Drell-Yan pairs at high ${\mathit{x}}_{\mathit{F}}$ in nuclear targets.