Atomic resolution scanning tunneling microscopy of Au deposited on Ni (110) shows that even though Au is completely insoluble in bulk Ni, it replaces Ni in the first surface layer forming a surface Au-Ni alloy and the squeezed out Ni atoms agglomerate in Ni islands on the surface. This picture is supported by total energy calculations within the effective-medium theory, which also provide a detailed understanding of this surprising phenomenon.

Optical second-harmonic generation has been used to study the effect of coadsorbed small volatile molecules on the orientational order of a liquid crystal monolayer at the interface between a mica surface and a liquid crystal film. As the chemical potential of the volatile molecules was increased, a first-order orientational transition was observed in the monolayer. This surface transition is shown to drive the reorientation of the bulk liquid crystal previously observed in these systems.

Luminescence spectra, both emission and excitation, and the excitation dependence of the resonance Raman spectra, have been measured for the quasi-one-dimensional charge-density-wave material [Pt(en${)}_2$][Pt(en${)}_2$${\mathrm{Cl}}_2$](${\mathrm{ClO}}_4$${)}_4$, en=1,2-diaminoethane. While the luminescence experiments show the existence of tail states at low temperature in the band gap region, the Raman measurements conclusively demonstrate that this tail does not arise from ordinary static structural disorder. These results can be explained by considering the zero-point motion of the lattice.

We show by neutron diffraction that metallic ${\mathrm{V}}_{2\mathrm{-}7}$${\mathrm{O}}_3$ develops a spin density wave below ${\mathit{T}}_{\mathit{N}}$≊9 K with incommensurate wave vector q≊1.7${\mathit{c}}^{\mathrm{*}}$ and an ordered moment of 0.15${\mathrm{\mu }}_{\mathit{B}}$. The weak ordering phenomenon is accompanied by strong, nonresonant spin fluctuations with a velocity c=67(4) meV Å. The spin correlations of the metal are very different from those of the insulator and place ${\mathrm{V}}_{2\mathrm{-}\mathit{y}}$${\mathrm{O}}_3$ in a distinct class of Motte-Hubbard systems where the wave vector for magnetic order in the metal is far from a high symmetry commensurate reciprocal lattice point.

A new echo phenomenon is suggested, which we call the current echo. A disordered one-dimenisonal tight-binding conductor subject to two very short voltage pulses is considered. While the current response following the first pulse decays due to scattering off the disorder, a delayed current pulse is predicted following the second voltage pulse, its delay being equal to the temporal separation of the two voltage pulses. This prediction is illustrated for an ensemble of finite, disordered chains.

We study the Fermi edge singularity in the absorption and emission spectra of quantum Hall states at filling fraction ν=1/k in double-layer systems. We found that power-law anomalies remain in these systems because of the neutral gapless mode. The exponents are calculated and we found that the ‘‘orthogonality catastrophe’’ exponent is positive. The ‘‘excitonic’’ exponent for optical recombination if the electron is located in a different layer has the opposite sign. So there exists two power-law anomalies for each localized core hole. We suggest that the Fermi edge singularity might be measurable in order to detect the neutral gapless modes.

We investigate the properties of the one-electron Green’s function in an interacting two-dimensional electron system in a strong magnetic field, which describes an electron tunneling into such a system. From finite-size diagonalization, we find that its spectral weight is suppressed near zero energy, reaches a maximum at an energy of about 0.2${\mathit{e}}^2$/ε${\mathit{l}}_{\mathit{c}}$, and decays exponentially at higher energies. We propose a theoretical model to account for the low-energy behavior. For the case of Coulomb interactions between the electrons, at even-denominator filling factors such as ν=1/2, we predict that the spectral weight varies as ${\mathit{e}}_0^{\mathrm{-}\mathrm{\omega }}$/‖ω‖, for ω→0.

The temperature dependence of the magnetic penetration depth of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ single crystals and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films has been measured with a parallel plate resonator technique at microwave frequencies. Both materials show a ${\mathit{T}}^2$ temperature dependence between 10 and 25 K, and systematic deviations towards a flatter temperature dependence below 10 K. We have also calculated the real part of the complex conductivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films from our penetration depth and surface resistance data and compare them with those of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals.

Using a micro-Hall probe, measurements of field of first flux penetration ${\mathit{H}}_{\mathit{p}}$ and the critical current density ${\mathit{J}}_{\mathit{c}}$ have been made near ${\mathit{T}}_{\mathit{c}}$ of superconducting ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$. In the temperature region just below ${\mathit{T}}_{\mathit{c}}$=86.0 K; ${\mathit{T}}_{\mathit{s}}$<T<${\mathit{T}}_{\mathit{c}}$, it was found that ${\mathit{H}}_{\mathit{p}}$ and ${\mathit{J}}_{\mathit{c}}$ are strongly suppressed, leaving a reversible diamagnetic susceptibility. No similar effect is observed n ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ which suggests that the highly two-dimensional character of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ is responsible for its particular behavior. The results are compared with recent theoretical predictions.

A generating function is introduced to determine the probabilty P(q) of the overlap q in disordered systems via a product of random transfer matrices. In one-dimensional models, the overlap is obtained by the Lyapunov exponent λ of the product. Replica symmetry breaking at zero temperature corresponds to a discontinuity of the derivative of λ with respect to an appropriate coupling variable in the replica space. The method is illustrated in a frustrated magnetic model where q≠0.

Spin-resolved photoelectron spectra of a monolayer of xenon atoms physisorbed on magnetized Fe(110) and Co(0001) surfaces have been taken using unpolarized VUV light. The spin-resolved spectra reveal the Xe 5p signal being split by up to 0.24 eV. This spin splitting can be explained by magnetic adsorbate-substrate interactions which have previously been observed only for strongly chemisorbed species. The results indicate physisorbed rare-gas atoms acting as a kind of ‘‘atomic probe’’ for local surface magnetism.

The K-edge shift and the absorption profile of aluminum along the shock Hugoniot have been calculated using density functional methods taking account of finite temperature effects, lattice melting, and liquid structure. Our first principles calculation is in good accord with available experiments.

Electrons measured around the direction of specular reflection for 15–100 keV ${\mathrm{H}}^{+}$, ${\mathrm{H}}_2^{+}$, and ${\mathrm{He}}^{+}$ scattering from single crystals show a strong dependence of their energy distribution with the surface topography. The maximum of the distribution appears at the convoy electron energy for rough surfaces and shifted to higher energies for smooth surfaces. This energy shift is visible for all the projectiles at 15 keV and increases with ion energy. The results are discussed in terms of transfer to the continuum of the ion Coulomb potential screened by the ion and electron surface induced charge.

We consider an exactly solvable random matrix model related to the random transfer matrix model for disordered conductors. In the conventional random matrix models the spacing distribution of nearest neighbor eigenvalues, when expressed in units of average spacing, has a universal behavior known generally as the Wigner distribution. In contrast, our model has a single parameter, as a function of which the spacing distribution crosses over from a Wigner to a distribution which is increasingly more Poisson-like, a feature common to a wide variety of physical systems including disorder and chaos.

In this paper we reconsider recently derived bounds on MeV tau neutrinos, taking into account previously unaccounted for effects. We find that, assuming that the neutrino lifetime is longer than ∼100 sec, the constraint ${\mathit{N}}_{\mathrm{eff}}$<3.6 rules out ${\nu }_{\mathrm{\tau }}$ masses in the range 0.5<${\mathit{m}}_{\nu \mathrm{\tau }}$<35 MeV for Majorana and 0.3<${\mathit{m}}_{\nu \mathrm{\tau }}$<35 MeV for Dirac neutrinos. Given that the present laboratory upper bound is 31 MeV, our results imply an upper bound of 0.5 MeV for Majorana neutrinos and of 0.3 MeV for Dirac neutrinos.

The QED effective action at finite temperature and density is calculated to all orders in an external homogeneous and time-independent magnetic field, in the weak coupling limit. The free energy, obtained explicitly, exhibits the expected de Haas–van Alphen oscillations. An effective coupling at finite temperature and density is derived in a closed form and is compared with renormalization group results.

We study the role of direct (i.e., small-scale) instantons in QCD correlation functions for the nucleon. They generate sizable nonperturbative corrections to the conventional operator product expansion, which improve the quality of both QCD nucleon sum rules and cure the long-standing stability problem, in particular, of the chirally odd sum rule.

We demonstrate that CP violation in the Higgs sector, e.g., of multidoublet model, can be directly probed using gluon-gluon collisions at the Superconducting Super Collider.

We show that the transverse polarization asymmetry of the τ lepton in the decay t→bτν is extremely sensitive to CP violating phases arising from the charged Higgs boson exchange in the Weinberg model of CP nonconservation. Qualitatively, the polarization asymmetries are enhanced over rate or energy asymmetries by a factor of ≊${\mathit{m}}_{\mathit{t}}$/${\mathit{m}}_{\mathrm{\tau }}$∼100. Thus for optimal values of the parameters the method requires ≊${10}^4$ top pairs to be observable rather than ${10}^7$ needed for rate or energy asymmetries. We also examine τ polarization in b decays via b→cντ and find that it can also be very effective in constraining the CP violation parameters of the extended Higgs sector.

We derive the lepton spectrum in semileptonic b decays from a nonperturbative treatment of QCD; it is based on an expansion in 1/${\mathit{m}}_{\mathit{Q}}$ with ${\mathit{m}}_{\mathit{Q}}$ being the heavy flavor quark mass. The leading corrections arising on the 1/${\mathit{m}}_{\mathit{Q}}$ level are completely expressed in terms of the difference in the mass of the heavy hadron and the quark. Nontrivial effects appear in 1/${\mathit{m}}_{\mathit{Q}}^2$ terms affecting mainly the end-point region; they are different for meson and baryon decays as well as for bottom and charm decays.