Transport and structural properties of the charge density wave material ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_3$ have been measured in situ. After cooling to low temperatures in zero electric field, a dc electric field was applied, and simultaneous measurements of the time evolution of the electric current and x-ray scattering peak profiles were carried out. A correlation was observed between the time dependence of the charge transport properties and the superlattice peak intensity.

The exchange splitting of image-induced surface states on Fe(110) is calculated by spin-polarized near-surface embedding. The splitting is 55 meV for the n=1 state and is primarily a result of coupling to the spin-polarized substrate potential. The effect of the spin-polarized surface barrier is relatively small and of opposite sign to the substrate contribution. This surprising result is a direct consequence of the negative spin density of surface electrons at the Fermi energy, illustrating the sensitivity of spin-split image states to surface magnetic properties.

We construct the effective Hamiltonian describing the motion of electrons in compositionally graded crystals which is valid throughout a given energy band and part way into the gaps. Near the edges of a simple or degenerate band this effective Hamiltonian reduces to an effective mass Hamiltonian with position dependent effective mass. Next, we examine more general states—not restricted to the vicinity of a band edge—in crystals with composition and applied potential variation in one direction. We obtain a WKB-type solution for the envelope functions, as well as the appropriate turning point connection rules.

We show that a quantum dot in the fractional Hall regime exhibits mesoscopic oscillations in its thermodynamic properties with a period which is a multiple of the period for free electrons. Our calculations are performed for parabolic quantum dots with hard-core electron-electron interactions and are exact in the strong field limit for ${\mathit{k}}_{\mathit{B}}$T smaller than the fractional Hall gap. Explicit expressions are given for the temperature dependence of the amplitude of the oscillations.

Angle-resolved photoemission spectroscopy has been employed to study in situ intercalation of the layered compound ${\mathrm{VSe}}_2$ with Cs. The results show that the valence band structure of ${\mathrm{VSe}}_2$, which initially is of 3D character, is transformed to become essentially 2D upon Cs intercalation. The changes, which go far beyond the rigid-band model, are successfully reproduced by self-consistent linear-augmented-plane-wave calculations. This is the first report of a UHV compatible system where the dimensionality of the electronic structure can be controlled in situ.

The observation of many body shakeup in the photoluminescence spectra of InGaAs-InP quantum wells, in the quantum Hall regime, is reported. The occurrence of this many body effect is demonstrated from the observation of low energy satellites, corresponding to inter-Landau-level excitations of the degenerate electron gas. Comparison of the zero and finite magnetic field spectra shows that the low energy tail at B=0 also arises from Fermi sea shakeup. The strength of the shakeup is controlled by the localization of the recombining hole, in agreement with recent theoretical predictions.

The Meissner effect is analyzed in the Josephson junction predicted recently for a certain double-layer quantum Hall system. As a result of the Josephson current the magnetic field parallel to the layers is squeezed into a sine-Gordon vortex with the flux quantized in the unit of 2π/e. A distinctive feature of the vortex is the appearance of an electric potential well perpendicular to the magnetic field within the layers. We also estimate the maximum strength of the parallel magnetic field which can be expelled outside the junction.

We calculate the average persistent current in a mesoscopic metal ring threaded by a magnetic flux in the diffusive regime. It is shown that the classical electromagnetic energy leads to a universal average current of the order of αec/${\mathit{C}}_0$, where α is the fine structure constant, -e is the charge of the electron, c is the velocity of light, and ${\mathit{C}}_0$ is the classical capacitance of the ring. Striking similarities between persistent currents and universal conductance fluctuations are discovered. We suggest a simple experiment to test our theory.

Data on the ${}_{}{}^{205}{}_{}{}^{}\mathrm{Tl}$ NMR frequency shift and linewidth due to the magnetic field variation in the mixed state of ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10+\mathrm{\delta }}$ are presented. These data, which are commonly used to extract the London penetration depth, show a near linear temperature dependence at T≪${\mathit{T}}_{\mathit{c}}$. We offer an explanation of this effect as being caused by thermal fluctuations of vortices in weakly coupled layered systems. We argue that the fluctuations cannot be ignored in interpretation of NMR and Muon-Spin-resonance data for materials with large penetration depths even at low T.

${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ NMR Knight shifts were measured on ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ for x=0.10, 0.16, and 0.20 in the normal and superconducting states. These results, combined with magnetic susceptibility measurements, allow the separation of the carrier spin and orbital contributons to the susceptibility. At temperatures just above the superconducting transition, the conduction electron spin susceptibility increases dramatially with strontium doping. The analysis of the Knight shift at low temperature suggests that the spin susceptibility is zero and the superconducting state is a spin singlet.

Cryogenic scanning tunneling microscopy has been used to measure the structure and the local electronic density of states of thin gold islands in metallic contact with a bulk superconducting ${\mathrm{NbSe}}_2$ crystal. Peaks are observed in the tunneling conductance below the energy gap corresponding to discrete bound quasiparticle states in the gold. The energies and spatial dependence of the bound states imply that the superconducting pair potential falls off abruptly at the Au-${\mathrm{NgSe}}_2$ interface.

We present the first theoretical study of oxygen ordering for ${\mathrm{Y}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_7$${\mathrm{O}}_{14+\mathit{x}}$ compound. The proposed model takes into account the asymmetry, induced by the orthorhombic strain of double completely oxidized planes, which leads to a phase diagram very different from that for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$. In addition to phases with oxygen chains directed along the b axis, phases with oxygen chains directed along the a axis occur. Our model describes the change in relative occupancy of the oxygen sites in the two sublattices of the ${\mathrm{CuO}}_{\mathit{x}}$ plane [the so-called O(8) and O(9) sites] in agreement with the experimental data.

We study an extended Hubbard model in the limit of infinite dimensions. The local correlation functions of this model are those of a generalized asymmetric Anderson model. The impurity model displays a Fermi liquid phase, a phase with neither the spin nor the charge of the impurity quenched, and an intermediate phase with the spin but not the charge of the impurity quenched. This analysis establishes the existence of metallic non-Fermi-liquid phases of the lattice model over a wide range of parameters and electron densities. The non-Fermi-liquid phases describe metals with incoherent spin and/or charge excitations and self-similar local correlation functions.

We examine the quantum phase transition at zero temperature between paramagnetic and spin-glass-ordered phases as the strength of a uniform transverse field, Γ, is varied. At the critical point, ${\mathrm{\Gamma }}_{\mathit{c}}$, the spin autocorrelation function decays with time, t, as ${\mathit{t}}^{\mathrm{-}2}$. As the transition is approached from the paramagnetic phase the nonlinear susceptibility diverges as {‖ln(Γ-${\mathrm{\Gamma }}_{\mathit{c}}$)‖/(Γ-${\mathrm{\Gamma }}_{\mathit{c}}$)${\mathrm{\}}}^{1/2}$.

The short-ranged correlations associated with magneitc ordering in the rare earth antiferromagnet holmium have been characterized in high-resolution x-ray and neutron scattering studies. We find that within about 2 K of ${\mathit{T}}_{\mathit{c}}$, the magnetic fluctuations exhibit two length scales, instead of one as expected in an ideal system. This result is reminiscent of behavior observed at the cubic-to-tegragonal structural phase transitions of the perovskites.

We present a theory of stripe domain structures in a thin ferromagnetic film with easy-axis magnetic anisotropy and long-range dipole interactions, for a wide range of temperatures below the reorientational phase transition. Spatial anisotropy generated by the exchange energy within domain walls pins the orientation of the domains. The domain structure can be described as a smectic liquid crystal with positional order destroyed by both thermal meandering of domain walls and by proliferated dislocations. The theory is in good agreement with experiment.

Using angle-resolved photoemission, we have mapped out the Fermi surface (FS) of single crystal ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ when doped as a superconductor (x=0.15) and overdoped as a metal (x=0.22). The measured FS agrees very well with local-density-approximation calculations and appears to shift with electron doping as expected by a band-filling scenario. The observed FS shape suggests that a model Hamiltonian with only nearest-neighbor interactions is not sufficient to describe the electronic structure near ${\mathit{E}}_{\mathit{F}}$; next-nearest-neighbor interactions should be considered.

Angle-resolved photoemission measurements for bulk single crystals of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ with x=0.15 show that the spectral weight which builds in the gap of the x=0 insulator displays a Luttinger Fermi surface, the first such observation for a material which is electron doped and which must be driven metallic by alloying. This finding greatly strengthens our previous ocnclusion that the metal’s chemical potential lies in gap-filling spectral weight, and sharpens the distinction between theories in which this does or does not occur.

The effect of the structure of the input distribution on the complexity of learning a pattern classification task is investigated. Using statistical mechanics, we study the performance of a winner-take-all machine at learning to classify points generated by a mixture of K Gaussian distributions (‘‘clusters’’) in ${\mathit{R}}^{\mathit{N}}$ with intercluster distance u (relative to the cluster width). In the separation limit u≫1, the number of examples required for learning scales as ${\mathit{NKu}}^{\mathrm{-}\mathit{p}}$, where the exponent p is 2 for zero-temperature Gibbs learning and 4 for the Hebb rule.

We discuss a model of repeated measurements of position in a quantum system which is monitored for a finite amount of time with a finite instrumental error. In this framework we recover the optimum monitoring of a harmonic oscillator proposed in the case of an instantaneous collapse of the wave function into an infinite-accuracy measurement result. We also establish numerically the existence of an optimal measurement strategy in the case of a nonlinear system. This optimal strategy is completely defined by the spectral properties of the nonlinear system.