We have observed in superfluid ${}_{}{}^3{}_{}{}^{}\mathit{B}$ two-phonon absorption, which is one of the nonlinear effects recently predicted by McKenzie and Sauls. The J=$2^{+}$ real squashing mode was excited with two coincident sound pulses at the temperature ${\mathit{T}}^{+}$, ${\mathrm{f}}_{\mathrm{signal}}$+${\mathit{f}}_{\mathrm{pump}}$=${\mathit{f}}_{\mathrm{rsq}}$(${\mathit{T}}^{+}$), under 0, 2.7, and 3.5 bars pressure. The heights of the attenuation peaks scale almost linearly with the energy of the higher-intensity wave, in qualitative agreement with the theory. Furthermore, a satellite peak appears even in zero magnetic field, which originates from the reflected wave.

We consider a nonlinear version of the Edwards-Wilkinson interface model for the growth in the presence of a collimated particle flux. We include both surface-tension and surface-diffusion relaxation. In a range of positive small values of the surface tension, the interface develops a state which fulfills standard criteria for spinodal decomposition. This suggests the existence of a phase transition from the ordinary state with the interface perpendicular to the incoming flux to a novel, noise-induced state with the interface spontaneously tilted with respect to the direction of the flux.

Helium-atom time-of-flight spectra taken at 69-meV incident energy in scattering from Pt(111) show energy-loss peaks that cannot be assigned to one-phonon transitions, but are qualitatively explained by simple formulas for the multiphonon processes in the classical limit. This assignment is supported by trajectory-approximation calculations that correctly reproduce the experimentally determined density of states of Pt(111).

We investigate the behavior of hydrogen on the Pd(111) surface by elastic scattering of a monoenergetic thermal He beam. Dramatic changes in the intensities of the diffraction peaks are observed as a function of sample temperature in the range of 140–350 K, and are attributed to changes in H coverage. A consistent explanation of the diffraction data is achieved when we consider the H system as a correlated quantum fluid where particle delocalization occurs at medium and low coverages.

We report Monte Carlo simulations of a system of two-dimensional, hard, homonuclear dimers. The equation of state and the Gibbs free energy were computed for the fluid phase and several crystalline and noncrystalline (aperiodic) solid structures. We observe that the differences in Gibbs free energy between the various solid structures are much less than the contribution to the entropy due to degeneracy of the ‘‘ground state’’ of the aperiodic solid. Hence, the thermodynamically stable solid structure of the system corresponds to an aperiodic arrangement of the molecular centers of mass and orientations. The melting point determined for this aperiodic solid is located within the observed narrow hystersis region.

A new algorithm for the quantum dynamical simulation of a mixed classical-quantum system that rigorously includes nonadiabatic quantum transitions is applied to the problem of the solvation dynamics of an initially energetic excess electron in liquid water. Computed results reveal a major channel associated with the appearance of a relatively long-lived solvated excited state postulated earlier; this state is identified as a distorted form of the equilibrium first excited state. The transient spectra evaluated directly from the simulation compare well with experimental data.

A metal-semiconductor transition has been observed in the range 230–260 K for the first time in a slightly superstoichiometric rare-earth dihydride, ${\mathrm{YH}}_{2.10}$ (as compared to the x-rich systems ${\mathrm{CeH}}_{2+\mathit{x}}$ and ${\mathrm{LaH}}_{2+\mathit{x}}$ with 0.7≲x≲0.9), implying an order-disorder transformation in the octahedral sublattice of the excess hydrogen atoms as the driving mechanism. Furthermore, a resistivity minimum in the range 60–80 K was observed, indicating for the first time in such systems an additional metal-semiconductor transition towards lower temperatures.

We discuss theoretical calculations of ballistic-electron-emission-microscopy spectra based in part on a first-principles computation of the transmission across the interfaces. We propose a way of presenting experimental data that highlights the transmission process with respect to contributions from the tunneling distribution. We present a specific application to A- and B-type ${\mathrm{NiSi}}_2$/Si(111) interfaces showing a factor three difference between them at low voltages.

Scanning tunneling microscopy has been used to characterize the effects of Nm impurities on the incommensurate charge-density-wave (CDW) phase in 1T-${\mathrm{TaS}}_2$. Real- and reciprocal-space data indicate that disorder in the CDW is due to dislocations and small random rotations of the CDW. The dislocations destroy translational order; however, calculations show that the orientational order is long range. These results are consistent with weak pinning and suggest the possibility of a hexatic CDW phase. The similarity of our data to results obtained for other two-dimensional systems is discussed.

The effect of a magnetic field on the tunneling probability of strongly localized electrons is calculated in the presence of spin-orbit scattering. Numerical (transfer matrix) and theoretical (replica analysis) arguments indicate a positive magnetoconductance, but no increase in the localization length. Universal and nonuniversal aspects of the tunneling probability distribution, and their relation to the symmetries of the Hamiltonian, are discussed.

It is shown that a sufficiently pure mesoscopic metallic ring will exhibit a magnetic phase transition to a low-temperature state with a persistent orbital current in zero external field (to a flux-phase state). The transition temperature and the spontaneous persistent current depend strongly on the geometry of the ring and of the Fermi surface.

Specific-heat (${\mathit{C}}_{\mathit{p}}$) measurements have been made near the H=0 phase transition in ${\mathrm{CsMnBr}}_3$, an XY stacked triangular antiferromagnet, and Ho, a helical antiferromagnet. Previous neutron-scattering experiments indicate both to be prime candidates for chiral universality. ${\mathit{C}}_{\mathit{p}}$ vs T data for ${\mathrm{CsMnBr}}_3$ yield α=0.39±0.09, in agreement with Monte Carlo results; however, data for Ho show quite a different behavior.

The magnetic behavior of the bordering Ni and Cu monolayers at the interface of ultrathin Cu films on Ni(111) has been investigated. ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$ probes have been used, utilizing perturbed γγ angular correlations, to study strength, orientation, and temperature dependence of the magnetic hyperfine field at the probe nuclear site. This magnetic hyperfine field is strongly reduced to about 50% in the first Ni layer covered by Cu; it vanishes in the adjacent Cu layer. The results are compared to hyperfine fields at free Ni(111) surfaces, to magnetometry measurements, and to theoretical predictions.

We show that the recently discovered η-pairing scheme combined with the Nagaoka-Thouless picture for itinerant ferromagnetism leads to a new type of superconductivity in the attractive-U Hubbard model in the sector 0<(${\mathit{N}}_{\mathrm{\uparrow }}$-${\mathit{N}}_{\mathrm{\downarrow }}$)/(${\mathit{N}}_{\mathrm{\uparrow }}$+${\mathit{N}}_{\mathrm{\downarrow }}$)≪1, where ${\mathit{N}}_{\mathrm{\uparrow }}$ and ${\mathit{N}}_{\mathrm{\downarrow }}$ are the occupation numbers of up and down spins. This results holds for all dimensionalities d≥2.

The existence of a novel incompressible quantum liquid for spinless fermions at ν=1/2 in the Hall effect is suggested. This state is plausibly related by smooth extrapolation in quantum statistics to a strong p-wave pairing state of fermions in zero magnetic field, and reduces to a state previously proposed by Halperin in the (unrealistic) limit of tightly bound pairs. It supports unusual excitations, including neutral fermions and charge-e/4 anyons with statistical parameter θ=π/8. Numerical experiments are presented, which provide evidence for several aspects of our theory.

Despite their small occupancy, the apical oxygen $p_z$ states have the indirect effect of modifying the relative occupancy of holes in states with different local symmetries in the Cu${\mathrm{O}}_2$ planes. A well-defined correlation is found between the maximum critical temperature $T_c^{max}$ of various cuprates and the excess of holes in states with local $a_1$ symmetry with respect to the doping ${\delta }^{max}$ at which $T_c^{max}$ is obtained for each compound. Implications for superconductivity models are discussed.

In highly anisotropic or layered high-${\mathit{T}}_{\mathit{c}}$ superconductors with magnetic field along the c axis, both pinning-caused and thermal fluctuations of the highly flexible vortex cores, or of chains of vortex dots, threading the cuperconducting layers may drastically reduce the variance σ of the magnetic-field fluctuation. In contrast, the fluctuations of the averaged flux lines (smoothed over the in-plane penetration depth λ) increase σ. As a consequence, muon-spin-rotation lines may be sharpened not only the thermal motional narrowing, but also by pinning-caused static disorder.

The variation of the phonon spectrum of ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ upon oxygen isotope substitution is determined by inelastic neutron scattering (INS). By combining the INS results with molecular-dynamics computer simulation, a value of 0.42±0.05 is obtained for the reference isotope-effect exponent. This is close to the isotope-effect exponent determined from the variation of ${\mathit{T}}_{\mathit{c}}$ upon oxygen substitution, indicating that ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ is a weak-coupling superconductor with large matrix elements coupling the electrons to high-energy phonons involving oxygen vibrations.

The electron-beam-breakup transit-time oscillator has been investigated via theoretical analysis and computer simulation as a high-power microwave generator. Coupling the beam-breakup instability with the transit-time effect of the electron beam in the cavity, rapid energy exchange between the electrons and cavity modes can occur. Furthermore, the transit-time resonance naturally leads to narrow-band output. Driven by a high-current electron beam, the beam-breakup transit-time oscillator can be a compact source for high-power (>1 GW) operation. Good agreements have been obtained between linear theory and simulation.

We show that multifractal measures arising from the symbolic dynamics of chaotic systems can be reproduced using iterated-functions system involving Möbius maps. This powerful approximation scheme is exact for hyperbolic billiards: The coding measures of zero-angle N-sided polygonal billiards are exactly rendered by N-1 Möbius maps. We also approximate with success the coding measures for the anisotropic Kepler system.