We consider the theory of mutual friction in helium II which accounts for the coupling of the normal and superfluid flows by means of quantized vortices. Numerical simulations of roton-vortex interactions suggest a simple new expression which accounts for most mutual friction at low temperatures. By carefully separating that which can be known at the present time from that which cannot be known until the structure of quantized vortices is understood, we are able to account for mutual friction in a semiempirical way at all temperatures and pressures.

The monolayer isotherms of heneicosanoic acid show a variety of features in the range 1–8 °C; such features are often assumed to be solid-fluid or fluid-fluid transitions. Using x-ray diffraction (and searching for in-plane and off-plane peaks) we find (a) two distinct distorted-hexagonal (DH) solids at high pressures, both with essentially vertical chains; (b) two distinct tilted-DH solids at intermediate pressures; and (c) an expanded solid at the lowest pressures. All ‘‘plateaus’’ in these isotherms are in-plane structural transitions; all ‘‘kinks’’ are solid-solid tilting transitions.

We propose a solid-on-solid–model description of the dynamics of wetting, using Langevin equations. The Gaussian version, appropriate to partial wetting, is solved exactly. The general version is solved using local equilibrium and scaling arguments. We obtain the dynamical contact angle, the shape of the profile near the substrate, and, for dry spreading, the occurrence, speed, and possible layering of a precursor film.

Thermodynamic equilibrium segragation at an individual grain boundary in a Pt-3 at. % Ni alloy is studied via transmission electron microscopy, atom-probe field-ion microscopy, and Monte Carlo simulations. Ni segregation is measured at the grain boundary. The Ni concentration profile is asymmetric with respect to the grain-boundary plane; it decreases in an oscillatory fashion on one side and monotonically on the other side.

Distinct deviations from smooth trends of the Hall coefficient, thermopower, and electronic specific heat with the composition in icosahedral AlCuMg and GaZnMg alloys are observed. These arise in regions where the free-electron Fermi wave vector is predicted to lie near Bragg planes determined from strong x-ray-diffraction maxima. Our results provide the first experimental evidence for anomalies in the electronic density of states near the Fermi level in these alloys, which may be important in the stabilization of this class of icosahedral phases via Hume-Rothery considerations.

We report the first definitive identification of nitrogen dangling bonds in silicon nitride. A computer analysis of ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ hyperfine parameters shows that the unpaired electron is strongly localized on the central nitrogen atom and that the unpaired electron’s wave function is almost entirely p in character. This is only the second fundamental intrinsic electron-paramagnetic-resonance center to be identified in silicon nitride.

We investigate the effect of phonons on the cooperative radiative decay of excitons in small aggregates. Our microscopic analysis establishes the existence of a temperature-dependent coherence size, provided the exciton dephasing time scale is much shorter than the fluorescence lifetime ${\mathrm{\tau }}_{\mathrm{fl}}$. We calculate the temperature dependenc of ${\mathrm{\tau }}_{\mathrm{fl}}$ and use our results to interpret recent photon-echo and fluorescence experiments in strongly coupled J-aggregates of cyanine dye molecules.

A huge enhancement of band-to-band Auger recombination in semiconductors by electron-hole correlations is found from a quantitative calculation. In contrast to earlier attempts, our calculation of the Coulomb correlation factors is valid for all temperatures and carrier densities, including the high-temperature and low-density case. Our results nicely explain recent carrier-lifetime measurements on silicon and show that Coulomb-enhanced Auger recombination poses an intrinsic upper limit to the carrier lifetime even at room temparature, a factor of 30 lower than previously assumed.

The H-induced shift of the surface-atom core-level binding energy in W(110) is shown to arise from two distinct effects, one chemical in nature and the other structural. The structural shift supports a recently proposed (1p×1) reconstruction that turns on at ∼0.5 monolayer coverage. These new findings are used to provide a self-consistent interpretation of previously reported shifts from H-covered W(111) and W(100) surfaces.

Using synchrotron x-ray-scattering measurements on ${\mathrm{NbSe}}_3$, we show that the diffuse scattering associated with the ${\mathit{q}}_1$ charge-density-wave fluctiations are rods perpendicular to the (a,b) planes reflecting a 2D regime of fluctuations with an in-plane anisotropy ${\xi }_{\mathit{b}}^{\mathrm{*}}$/${\xi }_{\mathit{a}}^{\mathrm{*}}$=3.5±0.5. The plane of the 2D fluctuations is likely to be formed by the chains with the shortest Se-Se pairing. The 3D coupling sets in within 2 K above the transition temperature ${\mathit{T}}_1$≊150 K with an out-of-plane anisotropy ${\xi }_{\mathit{b}}^{\mathrm{*}}$/${\xi }_{\mathit{c}}^{\mathrm{*}}$=27±2. $q sub 1— has a significant temperature variation from about 0.2445(1) at ${\mathit{T}}_1$ to 0.2411(1) at about 80 K and then remians constant to the lowest temperatures. Upon cooling, the incommensurate wave vector is driven far away from the commensurate value 1/4. No anomaly in either the amplitude or the phase of ${\mathit{q}}_1$ is observed at the second charge-density-wave transition.

We have studied the Shubnikov–de Haas oscillations in n-type gray tin films in which several size-effect-quantized subbands are relevant. The observed oscillations at oblique magnetic-field angles become increasingly two-dimensional in character as the film thickness decreases. In addition, frequencies associated with nonextremal cross sections of the Fermi surface are observed.

We present calculations of femtosecond-pump, continuum-probe nonlinear absorption in GaAs, including effects of electrons and heavy, light, and split-off holes. To account for hole-band nonparabolicity and anisotropy, a 30×30 k⋅p Hamiltonian is diagonalized. Carrier dynamics are determined using an ensemble Monte Carlo method. Differential transmission spectra are obtained from the carrier distributions and directly compared with experiments. Our results show that pump–continuum-probe experiments provide the first direct evidence for a redistribution of holes on a femtosecond scale.

Inelastic resonant tunnel spectroscopy is used to directly probe the density of states of two-dimensional magnetopolarons in a GaAs quantum well over a wide range of energies and magnetic fields. In agreement with theory, Landau levels are found to be drastically renormalized near degeneracies between the longitudinal-optic-phonon and cyclotron energies.

Multiband envelope theory is reformulated in terms of a basis of total envelope and zone-center Bloch angular momentum. The k⋅p operator is scalar with respect to the total-angular-momentum operator and hence is block diagonal in its eigenbasis. The projections of the Hamiltonian into the F=1/2 and the F=3/2 subspaces of total angular momentum are derived and quantum-dot band structure is calculated in GaAs(AlGaAs) and InAs(GaSb) systems as an example application of the formalism.

We identify the boundary energy of a many-body system of fermions on a lattice under twisted boundary conditions as the inverse of the effective charge-carrying mass, or the stiffness, renormalizing nontrivially under interactions due to the absence of Galilean invariance. We point out that this quantity is a sensitive and direct probe of the metal-insulator transitions possible in these systems, i.e., the Mott-Hubbard transition or Density-wave formation. We calculate exactly the stiffness, or the effective mass, in the 1D Heisenberg-Ising ring and the 1D Hubbard model by using the ansatz of Bethe. For the Hubbard ring we also calculate a spin stiffness by extending the nested ansatz of Bethe-Yang to this case.

Vacuum tunneling of spin-polarized electrons from a ferromagnetic ${\mathrm{CrO}}_2$ tip into a Cr(001) single crystal has been observed by means of a scanning tunneling microscope (STM) operated in UHV. Topographic STM images of the Cr(001) surface using a tungsten tip confirm the model of topological antiferromagnetism between ferromagnetic terraces separated by monatomic steps of 0.144 nm height. With ${\mathrm{CrO}}_2$ tips, the measured step-height values alternate around the mean value of 0.144 nm due to an additional contribution from spin-polarized-electron tunneling.

It is shown that the nonzero quantized value of the Hall conductivity in Haldane's model Phys. Rev. Lett. 61 2015 1988 appears because the electron Green's function is a topologically nontrivial function of momentum and frequency. If the $\mathbf{n}$ field is introduced in this model then the same topological invariant, defined in the momentum-frequency space, gives a nonzero coefficient for the Hopf-Chern-Simons term of the $\mathbf{n}$ field. A Chern-Simons-like term, which mixes the electromagnetic and $\mathbf{n}$ fields and determines the electric charge of the Skyrmion, can also have a nonzero coefficient in the model.

The dynamics of a single vortex present in a ring-shaped (Corbino geometry) two-dimensional array of low-capacity Josephson junctions is studied. The vortex is treated as a macroscopic quantum particle, whose energy levels ${\mathit{E}}_{\mathit{n}}$(${\mathit{Q}}_0$) are periodic functions of the externally induced gauge charge ${\mathit{Q}}_0$ which is enclosed by the vortex, with a period 2e. This Aharonov-Bohm–type effect may manifest itself as a persistent voltage ${\mathit{V}}_{\mathit{s}}$=${\mathit{dE}}_0$(${\mathit{Q}}_0$)/${\mathit{dQ}}_0$ between interior and exterior contacts, or alternatively as Bloch oscillations. The relation with the Aharonov-Casher effect and the possibility for experimental observation are discussed.

The theory of pinning of a vortex liquid by weak disorder is developed. Two different vortex-liquid dissipative regimes are shown to exist: the flux flow above some crossover temperature ${\mathit{T}}_{\mathit{k}}$, where the vortex liquid is unpinned, and the thermally assisted flux flow below ${\mathit{T}}_{\mathit{k}}$. The activation barriers in the latter regime are those associated with the plastic motion of the vortices in the liquid.

Neutron-scattering measurements have been performed on (Nd,Pr${)}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$ (0.0<x<0.125) single crystals. All samples exhibit three-dimensional (3D) long-range magnetic order; the temperature dependence of the 2D magnetic correlation lengths in all is similar in character to that observed in pure and lightly doped ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ albeit with renormalized spin stiffness constants ${\mathrm{\rho }}_{\mathit{s}}$. Thus, in the nonsuperconducting doping regime the hole- and electron-doped systems exhibit analogous magnetic behavior except for the drastically different dopant-concentration scales for the destruction of magnetic order.