We report neutron-scattering measurements on ${\mathrm{SrCr}}_{8\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{4+\mathit{x}}$${\mathrm{O}}_{19}$, a layered compound containing Kagomé planes of S=3/2 ${\mathrm{Cr}}^{3+}$ ions. From magnetic-susceptibility data, the Curie-Weiss constant is large and negative; ${\mathrm{\theta }}_{\mathrm{CW}}$≃-500 K. Even so, our measurements for a sample with x=0.87 reveal only short-range antiferromagnetic order with a correlation length ξ=7±2 Å≊2a, where a is the inter-Cr spacing. Below 8 K, there is a spin-freezing transition to a state where the magnetic fluctuations have large amplitude and a spectrum (averaged over reciprocal space) characteristic of two-dimensional antiferromagnets with ξ≫a.

We present numerical results for the thermodynamic limit of the one-dimensional symmetric Kondo lattice over the full temperature range, obtained by using a recently developed Monte Carlo technique. In particular, we compare spin correlations with RKKY predictions and two-impurity results, and note the opening of a gap as the temperature is lowered. We discuss the relevance of our results to various theoretical treatments of Kondo and Anderson lattice models.

THe ground state of the typical spin-1 linear-chain Heisenberg antiferromagnet Ni(${\mathrm{C}}_2$${\mathrm{H}}_8$${\mathrm{N}}_2$${)}_2$${\mathrm{NO}}_2$(${\mathrm{ClO}}_4$) containing a small amount of ${\mathrm{Cu}}^{2+}$ is studied by the electron-spin-resonance (ESR) technique. The ESR results are qualitatively explained by the model that the valence bonds are broken at the ${\mathrm{Cu}}^{2+}$ sites resulting in spin-1/2 states at the ${\mathrm{Ni}}^{2+}$ sites neighboring the ${\mathrm{Cu}}^{2+}$. Thus the present study gives experimental evidence for the existence of the valence-bond-solid ground state in S=1 linear-chain Heisenberg antiferromagnets.

X-ray-resonance magnetic scattering has been used to study antiferromagnetic ordering in the small-moment (μ¯≃0.02${\mathrm{\mu }}_{\mathit{B}}$) heavy-fermion superconductor ${\mathrm{URu}}_2$${\mathrm{Si}}_2$. The intensity of the magnetic (003) reflection develops abruptly at ${\mathit{T}}_{\mathit{N}}$≃17 K and grows linearly to T=3 K, where it saturates. Long-range antiferromagnetic order (${\mathrm{\zeta }}_{003}^{\mathit{c}}$≃450 Å) persists into the superconducting state at ${\mathit{T}}_{\mathit{c}}$=1.3 K demonstrating the microscopic coexistence of these two ground states. At saturation, a remarkable peak intensity of 8 counts/sec was measured.

Neutron scattering from a high-quality single crystal of the heavy-fermion superconductor ${\mathrm{URu}}_2$${\mathrm{Si}}_2$ has shown the onset of the small, antiferromagnetic ordered moment (0.037${\mathrm{\mu }}_{\mathit{B}}$) occurs abruptly at ${\mathit{T}}_{\mathit{N}}$, unlike the gradual onset in previously studied samples. The results are in good agreement with x-ray-resonance magnetic scattering. The magnitude of the moment and the magnetic excitation spectrum are the same as previously observed, including the anomalous damping of excitations with a component of momentum transfer along the ${\mathit{c}}^{\mathrm{*}}$ axis. This shows that these are intrinsic properties of ${\mathrm{URu}}_2$${\mathrm{Si}}_2$ and confirms the microscopic coexistence of antiferromagnetism and superconductivity.

We propose a novel quantum detection mechanism for photon absorption in a two-dimensional superconductor that exhibits a vortex-unbinding transition. Well below the transition, absorption of a single photon of energy hf can result in the creation of a vortex-antivortex pair, which can be broken apart in an applied current, thus transferring a single flux quantum ${\mathrm{\Phi }}_0$ across the film. This results in a quantum-limited voltage responsivity ${\mathrm{\Phi }}_0$/hf=1/2ef, and may account for some reports of enhanced nonbolometric detection of infrared radiation in thin granular superconducting films.

It is shown that in anisotropic superconductors uniform current flow can be unstable for current densities ${\mathit{j}}_{\mathit{f}1}$<j<${\mathit{j}}_{\mathit{f}2}$, where ${\mathit{j}}_{\mathit{f}1}$ and ${\mathit{J}}_{\mathit{f}2}$ essentially depend on the orientation of j. The instability leads to a stratification of the magnetic flux into macroscopic domains (macrovortices). Densities of closed shielding currents circulating inside the macrovortces can be well above the mean density of the transport current. The relation of such a macrovortex structure to the magnetic granularity of high-${\mathit{T}}_{\mathit{c}}$ oxides is discussed. The anisotropy is shown to result in the instability of both the Bean critical-state model and the uniform vortex glass with current-voltage characteristic of the form V∝exp(-const/${\mathit{j}}^{\mathrm{\mu }}$).

An unusual form of superconducting transition has been observed in some of the superconducting compounds ${\mathit{L}}_{2\mathrm{-}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ (L=Pr,Nd,Sm,Eu; M=Ce,Th) under low magnetic fields. When the temperature is reduced below the critical one, the sample’s resistance first increases, then drops to a nonzero value, increases again, and finally decreases at low temperature, creating a characteristic double-peak transition. The behavior is explained in terms of intra- and intergranular superconducting transitions.

We study the direct incoherent energy transfer for a single excited donor molecular to acceptors, all of which are attached to chainlike polymers, randomly distributed in viscous solvent. We present both analytical and simulation results that document the essential departure from Forster-type decay laws due to correlations in acceptor placement.

The bulk conductivity ${\mathrm{\sigma }}^{\mathrm{*}}$(p) of the bond lattice in ${\mathit{openZ}}^{\mathit{d}}$ with a fraction p of conducting bonds is analyzed. Assuming a hierarchical node-link-blob (NLB) model of the conducting backbone, it is shown that ${\mathrm{\sigma }}^{\mathrm{*}}$(p) (for this model) is convex in p near the percolation threshold ${\mathit{p}}_{\mathit{c}}$, and that its critical exponent t obeys the inequalities 1≤t≤2 for d=2,3, while 2≤t≤3 for d≥4. The upper bound t=2 in d=3, which is realizable in the NLB class, virtually coincides with two very recent numerical estimates obtained from simulation and series expansion.

The Husimi distribution is computed for a particle in a double-well potential and an oscillatory driving force. The extended phase space of the classical system contains two disjoint stable tubes of regular orbits, embedded in a chaotic sea. For the quantum system we find coherent oscillatory tunneling between these stability tubes, at a rate many orders of magnitude greater than the rate of ordinary undriven tunneling.

Measurement of the free precession of nuclear spins in optically pumped ground-state ${}_{}{}^{201}{}_{}{}^{}\mathrm{Hg}$ atoms has allowed a test of Weinberg’s nonlinear generalization of quantum mechanics. Shifts among the three resolved precession frequencies in the spin-3/2 system were studied as a function of the projection of spin polarization along the precession axis. Our results set an upper limit on ε, Weinberg’s nonlinearity parameter, of ‖ε‖/2πħ<3.8μ Hz, which corresponds to 2.0×${10}^{\mathrm{-}27}$ of the binding energy per nucleon. This is the most stringent such limit to date.

We study the transition to chaos for random dynamical systems. Near the transition, on the chaotic side, the long-time particle distribution (which is fractal) that evolves from an initial smooth distribution exhibits an extreme form of temporally intermittent bursting whose scaling we investigate. As a physical example, the problem of the distribution of particles floating on the surface of a fluid whose flow velocity has a complicated time dependence is considered.

The idea of supersymmetry at the weak scale should be tested without regard to the Planck-scale origin of any specific model. A class of low-energy effective supersymmetric theories is derived from four assumptions: minimal field content, R-parity conservation, absence of quadratic divergences, and naturalness of near-flavor conservation. Current experiments are testing and constraining this wide class of supersymmetric models, and not just a specific N=1 supergravity model.

We consider the effects of an anomalous quadrupole moment of the W on the ${\mathit{Z}}^0$ branching ratios and asymmetries at the CERN ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ collider LEP. Stringent experimental constraints from the ρ parameter are evaded if ${\lambda }_{\gamma }$=${\lambda }_{\mathit{Z}}$==λ. We therefore choose a scheme based on a custodial global SU(2${)}_{\mathrm{weak}}$ symmetry, implemented with the W-dominance mechanism such that ρ=1 is enforced. With the expected accuracy at LEP, we find that, for the scale of new physics Λ=1 TeV, a limit ‖λ‖≤0.34 can be obtained for this coupling. This is as good a limit as can be set at LEP 200.

Using the Columbia University–Stony Brook (CUSB-II) detector we have studied the inclusive photon spectrum from 2.9×${10}^4$ Υ(5S) decays. We observe a strong signal due to ${\mathit{B}}^{\mathrm{*}}$→Bγ decays. From this we obtain (i) the average ${\mathit{B}}^{\mathrm{*}}$-B mass difference, 46.7±0.4 MeV, (ii) the photon yield per Υ(5S) decay, 〈γ/Υ(5S)〉=1.09±0.06, and (iii) the average velocity of the ${\mathit{B}}^{\mathrm{*}}$’s, 〈β〉=0.156±0.010, for a mix of nonstrange (B) and strange (${\mathit{B}}_{\mathit{s}}$) ${\mathit{B}}^{\mathrm{*}}$ mesons from Υ(5S) decays. From the shape of the photon line, we find that both B and ${\mathit{B}}_{\mathit{s}}$ mesons are produced with nearly equal values for the hyperfine splitting of the B and ${\mathit{B}}_{\mathit{s}}$ meson systems.

We describe a test of factorization comparing the decays of the B${\mathrm{¯}}^0$ meson to ${\mathit{D}}^{\mathrm{*}+}$${\mathrm{\pi }}^{\mathrm{-}}$ with ${\mathit{D}}^{\mathrm{*}+}$${\mathit{l}}^{\mathrm{-}}$ν${\mathrm{¯}}_{\mathit{l}}$. Assuming that factorization works and using the Isgur and Wise theory of a universal form factor, we estimate the decay constant of the ${\mathit{D}}_{\mathit{S}}$ meson to be 276±45±44 MeV.

We demonstrate that a cancellation mechanism between quark and gluon contributions to the axial-vector singlet form factor is operative in a generalized Skyrme model, The calculation of the non-electromagnetic part of the neutron-proton mass difference plays a crucial role in this analysis.

Light-front field theory may provide a promising avenue of research for nuclear and particle physics, but a Tamm-Dancoff truncation of field theory is required for practical computations. Such a truncation limits the number of virtual mesons allowed in hadronic field theories, or the number of quarks and gluons allowed in bound states described by quantum chromodynamics. Past Tamm-Dancoff renormalization problems are analyzed and a solution is proposed.

Effects of physics beyond the standard model on electroweak observables ares studied using the Peskin-Takeuchi isospin-conserving, S, and -breaking, T, parametrization of ‘‘new’’ quantum loop corrections. Experimental constraints on S and T are presented. Atomic parity-violating experiments are shown to be particularly sensitive to S with existing data giving S=-2.7±2.0±1.1. That constraint has important implications for generic technicolor models which predict S≃0.1${\mathit{N}}_{\mathit{T}}$${\mathit{N}}_{\mathit{D}}$ (${\mathit{N}}_{\mathit{T}}$ is the number of technicolors, ${\mathit{N}}_{\mathit{D}}$ is the number of technidoublets).