Using the scanning method we carry out for the first time extensive simulations of trails and self-avoiding walks (SAWs) terminally attached to an adsorbing linear boundary on a square lattice. A bulk attraction energy is also defined for a self-intersection of a trail and a pair of nonbonded nearest-neighbor monomers of a SAW. The chains are simulated at the special point. Our critical exponents differ significantly from the exact values of Vanderzande, Stella, and Seno [Phys. Rev. Lett. 67, 2757 (1991)] for the θ’ model. Thus, their conjecture, that the θ and θ’ points belong to the same universality class, is not supported.

We have modified multiple-wavelength anomalous dispersion, a powerful direct method, and applied it for the first time to an interface. This allows us to separate heavy and light atoms, and so deduce the structure. We find that at the Si/B(√3 × √3 )R30°/${\mathrm{Ge}}_{\mathit{x}}$${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$(111) interface Ge and Si atoms occupy separate sites. Boron is in a substitutional site with four Si nearest neighbors and the other positions are dominated by Ge. This ordering may be due to chemical (binding-energy) effects combined with a kinetic, surface-strain mechanism related to one proposed for the ordered GeSi(001) system.

We have recorded real-space images of Xe atoms adsorbed in a 4-K Pt{111} surface. In the dilute coverage limit, we find nearly all the Xe atoms at step edges. From this observation we deduce a lower limit of hundreds of angstroms that Xe atoms scatter across the surface before becoming thermally accommodated. This result is in sharp contrast to previous studies of metal-atom adsorption on metal surfaces in which it is believed that a large fraction of the impinging atoms remain at their point of impact at low surface temperature. For Xe, once the step sites are nearly saturated, point defects on the Pt{111} surface nucleate the growth of compact, incommensurate, rotationally ordered Xe islands. Thus, we have imaged the initiation of the growth of a surface overlayer, and have identified the sites at which this growth begins.

We derive an extension of the Hohenberg-Kohn (HK) theorem which applies to the valence electrons only, excluding core states. The energy functional of the valence electron density is minimized giving a variational bound on the total ground-state energy. A Kohn-Sham-type approximation suggests treating valence states by the local density approximation (LDA) and the core states by Hartree-Fock. The functional can be used to generate pseudopotentials or to perform all-electron calculations beyond conventional LDA. Our formalism also yields a simple derivation of the HK theorem for Dirac electrons.

The photoinduced ir absorption spectra of the halogen-bridged metal complexes have been measured on single crystals with different dimensionalities of chain structures. A decrease of the interchain coupling via the hydrogen bonds leads to the appearance of a new band, which is attributable to photogeneration of solitons. The structures assigned to photogenerated polarons are rather insensitive to the intechain coupling. From the excitation spectra of the photoinduced absorption signals, it has been found that the excitonic effect completely quenches photogeneration of these gap states.

The Hall conductivity of a high-mobility two-dimensional electron gas (2DEG) has been investigated in a distribution of quantized magnetic flux tubes (vortices) formed at a type-II superconducting ‘‘gate’’ layer. A pronounced suppression of the Hall effect was observed for long-Fermi wavelengths (as compared to the submicron vortex size) indicating a situation where electrons are diffracted by the flux quanta. In contrast, for shorter Fermi wavelengths the Hall conductivity has been found to be insensitive to the extreme inhomogeneity of the magnetic field and determined by the average field.

${\mathit{D}}^0$ and ${\mathit{D}}^{\mathrm{-}}$ states in GaAs quantum wells are studied simultaneously by far-infrared magnetophotoconductivity. In high magnetic fields, the observed sharp transitions are attributed to transitions between ground and excited states of ${\mathit{D}}^0$ and ${\mathit{D}}^{\mathrm{-}}$ centers, respectively. In low magnetic fields, the spectra are dominated by free-electron response. The changes in spectral responses are explained in terms of a magnetic-field-induced metal-insulator transition of the ${\mathit{D}}^{0\mathrm{-}}$${\mathit{D}}^{\mathrm{-}}$ band.

The valence subband dispersion of GaAs/AlGaAs quantum wells is measured with meV accuracy using the recombination of hot electrons at neutral acceptors. This precision, in combination with polarization-dependent matrix elements, allows direct observation and measurement of the warping. The measurements, for in-plane wave vectors which extend over a substantial fraction of the Brillouin zone, are shown to be in very good agreement with k⋅p calculations.

Recent theories of the NMR in the CuO superconductors are based on a spin-singlet ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ order parameter. Since this state has nodal lines on the Fermi surface, nonlinear effects associated with low-energy quasiparticles become important, particularly at low temperatures. We show that the field dependence of the supercurrent, below the nucleation field for vortices, can be used to locate the positions of the nodal lines of an unconventional gap in momentum space, and hence test the proposed ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ state.

We discuss a state of an orbital glass which may exist in granular superconductors. The orbital glass is related to the existence of the special loops of Josephson junctions with positive and negative Josephson couplings. With an odd number of negative couplings on the loop, a spontaneous orbital moment is created. Similar to anyon superconductivity, the new state with orbital moments is characterized by broken fundamental symmetries (C and P). There the orbital paramagnetism coexists with the superconductivity and at small magnetic field and low temperatures the Meissner effect may disappear.

Detailed neutron scattering measurements have been made for selected phonons for ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$. The temperature dependence of the linewidths indicates large anharmonic effects particularly for the oxygen modes. The superconductivity induced change in width below ${\mathit{T}}_{\mathit{c}}$ of the two highest energy oxygen modes is found to be nearly 2% of the phonon energy suggesting a large electron phonon coupling.

New magnetic torque measurements on a high-quality ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{x}}$ single crystal show that only a lower bound for the anisotropy parameter γ>150 can be given. For misorientations of the field H from the (a,b) plane larger than 0.5°, the results can be rescaled to magnetization M versus H. The behavior of M can be consistently described by a recent theory of Bulaevskii, Ledvij, and Kogan.

We construct a ‘‘phase diagram’’ of the vortex-solid phase of Y-Ba-Cu-O single crystals by making a first overall connection of the shape of the magnetic hysteresis M(H,T) with the single-vortex (1D) and collective (3D) pinning regimes. The crossovers between different regimes are visualized by drawing contours of constant critical current density ${\mathit{J}}_{\mathit{c}}$ in the H-T plane. We identify the transition from 1D to 3D pinning, and from the nonlocal into a local behavior of vortex bundles in the collective pinning regime. A direct correlation between the shape of M(H) and the field-dependent thermal relaxation rate is demonstrated. We also identify a signature of the thermal softening boundary at which thermal fluctuations on the scale of the coherence length ξ are relevant.

Optical reflectivity spectra were measured for the superconducting fulleride ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and for pristine ${\mathrm{C}}_{60}$ single crystals in the wavelength region from infrared to ultraviolet. High-quality samples of ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ with flat surfaces were grown for this purpose by cosublimation of both ${\mathrm{C}}_{60}$ and potassium metal. A sharp plasma edge was found at 0.73 eV in the reflection spectra of ${\mathrm{K}}_3$${\mathrm{C}}_{60}$. A quantitative discussion is given on the conduction band structures based on the band parameters derived from a spectral shape analysis in terms of the Drude-Lorentz model.

The occurrance of ferromagnetism for the t-J model on a square lattice is investigated using high-temperature series for the Helmholtz free energy F and the uniform magnetic susceptibility ${\mathrm{\chi }}_0$. For J=0 fourteenth-order series have been calculated for F and ${\mathrm{\chi }}_0$ and we estimate F(T=0) to find the ground-state energy. A region of fully polarized ferromagnetism is found for J<0. A ferrimagnetic region is observed for larger J/t where ${\mathrm{\chi }}_0$ is divergent, but F(T=0) is below that of spinless fermions. Our results do not support Nagaoka’s theorem at a thermodynamic density of holes.

We present a recursive procedure to calculate the parameters of the recently introduced multicanonical ensemble and explore the appraoch for spin glasses. Temperature depenedence of the energy, the entropy, and other physical quantities are easily calculable and we report results for the zero-temperature limit. Our data provide evidence that the large L increase of the ergodicity time is greatly improved. The multicanonical ensemble seems to open new horizons for simulations of spin glasses and other systems which have to cope with conflicting constraints.

Local-spin-density theory and a Green’s-function technique based on the linear muffin-tin orbitals method have been used to calculate the surface energy of the 3d metals. The theory explains the variation of the values derived from measurements of the surface tension of liquid metals including the pronounced anomaly occurring between vanadium and nickel in terms of a decrease in the d contribution caused by spin polarization.

The first direct measurement of the modulation of the magnetic superexchange parameter J by optical phonons is reported. High-pressure Raman scattering studies of the model compound ${\mathrm{FeBO}}_3$ reveal an unexpectedly large interaction between a (${\mathrm{BO}}_3$${)}^{3\mathrm{-}}$ internal mode and spin-pair excitations. By pressure tuning J, resonant decay of the phonon into the two-magnon continuum and associated mixing were observed yielding a phonon-magnon-pair coupling constant g≊(0.15–0.20)J.

We report new results on subpicosecond optical rectification near the semiconductor GaAs band gap at room temperature. We discriminate between radiation produced by real carriers and that produced by optical rectification by using normal and off-normal incident optical illumination. Under normal incident optial illumination of the 〈111〉 GaAs surface, the amplitude of the nonlinear susceptibility varies dramatically (2 orders of magnitude) and the sign of the susceptibility reverses when the photon energy is tuned above the band gap. This resonance enhancement and the sign reversal are explained by the dispersion of the second-order nonlinear susceptibility at the band gap.

Magnetic circular x-ray dichroism measurements at the Co ${\mathit{L}}_3$ and ${\mathit{L}}_2$ edges in Co/Pd multilayers give first experimental evidence for a greatly enhanced orbital moment compared to that in bulk Co, confirming recent theoretical predictions. Our results also show that, in general, there is no simple relationship between the dichroism signal and either the spin moment or the total moment. This is verified by a simple model calculation.