Dimensional crossover and metal-insulator transition in quasi-two-dimensional disordered conductors

We study the metal-insulator transition (MIT) in weakly coupled disordered planes on the basis of a Non-Linear Sigma Model (NL$\sigma$M). Using two different methods, a renormalization group (RG) approach and an auxiliary field method, we calculate the crossover length between a 2D regime at small length scales and a 3D regime at larger length scales. The 3D regime is described by an anisotropic 3D NL$\sigma$M with renormalized coupling constants. We obtain the critical value of the single particle interplane hopping which separates the metallic and insulating phases. We also show that a strong parallel magnetic field favors the localized phase and derive the phase diagram.Comment: 16 pages (RevTex), 4 poscript figure

Multifractal analysis of the metal-insulator transition in anisotropic systems

We study the Anderson model of localization with anisotropic hopping in three dimensions for weakly coupled chains and weakly coupled planes. The eigenstates of the Hamiltonian, as computed by Lanczos diagonalization for systems of sizes up to $48^3$, show multifractal behavior at the metal-insulator transition even for strong anisotropy. The critical disorder strength $W_c$ determined from the system size dependence of the singularity spectra is in a reasonable agreement with a recent study using transfer matrix methods. But the respective spectrum at $W_c$ deviates from the ``characteristic spectrum'' determined for the isotropic system. This indicates a quantitative difference of the multifractal properties of states of the anisotropic as compared to the isotropic system. Further, we calculate the Kubo conductivity for given anisotropies by exact diagonalization. Already for small system sizes of only $12^3$ sites we observe a rapidly decreasing conductivity in the directions with reduced hopping if the coupling becomes weaker.Comment: 25 RevTeX pages with 10 PS-figures include

Localization Transition in Multilayered Disordered Systems

The Anderson delocalization-localization transition is studied in multilayered systems with randomly placed interlayer bonds of density $p$ and strength $t$. In the absence of diagonal disorder (W=0), following an appropriate perturbation expansion, we estimate the mean free paths in the main directions and verify by scaling of the conductance that the states remain extended for any finite $p$, despite the interlayer disorder. In the presence of additional diagonal disorder ($W > 0$) we obtain an Anderson transition with critical disorder $W_c$ and localization length exponent $\nu$ independently of the direction. The critical conductance distribution $P_{c}(g)$ varies, however, for the parallel and the perpendicular directions. The results are discussed in connection to disordered anisotropic materials.Comment: 10 pages, Revtex file, 8 postscript files, minor change

Behavioral states may be associated with distinct spatial patterns in electrocorticogram

To determine if behavioral states are associated with unique spatial electrocorticographic (ECoG) patterns, we obtained recordings with a microgrid electrode array applied to the cortical surface of a human subject. The array was constructed with the intent of extracting maximal spatial information by optimizing interelectrode distances. A 34-year-old patient with intractable epilepsy underwent intracranial ECoG monitoring after standard methods failed to reveal localization of seizures. During the 8-day period of invasive recording, in addition to standard clinical electrodes a square 1 × 1 cm microgrid array with 64 electrodes (1.25 mm separation) was placed on the right inferior temporal gyrus. Careful review of video recordings identified four extended naturalistic behaviors: reading, conversing on the telephone, looking at photographs, and face-to-face interactions. ECoG activity recorded with the microgrid that corresponded to these behaviors was collected and ECoG spatial patterns were analyzed. During periods of ECoG selected for analysis, no electrographic seizures or epileptiform patterns were present. Moments of maximal spatial variance are shown to cluster by behavior. Comparisons between conditions using a permutation test reveal significantly different spatial patterns for each behavior. We conclude that ECoG recordings obtained on the cortical surface with optimal high spatial frequency resolution reveal distinct local spatial patterns that reflect different behavioral states, and we predict that similar patterns will be found in many if not most cortical areas on which a microgrid is placed

Structural studies of WC(0001) and the adsorption of benzene

We report on studies dealing with the structure of WC(0001) and the adsorption of benzene on this surface. An I(V)-low-energy electron diffraction structure analysis has been performed to elucidate the surface structure of WC(0001). These studies indicate that the surface consists of a tungsten layer covered by carbon randomly distributed on the hcp sites with a coverage of 30% that of a full carbon layer. The distance between this carbon layer and the tungsten layer beneath is enlarged by 5% with respect to the spacing between carbon and tungsten layers in the bulk. Only a small deviation from the bulk value was found for the distance between the first tungsten layer and the carbon layer below. No indications of surface reconstruction have been observed. Benzene adsorption was studied on clear oxygen covered and oxidized WC(0001). The benzene multilayer desorbs at T≤200 K. On stoichiometric WC(00001), molecular benzene of (sub)monolayer coverage is found up to temperatures of T≈230 K. After desorption of this species, small signals of fragments are visible in the photoelectron spectra up to T≈1000 K. Above this temperature, a graphite covered surface remains. On a surface covered by a thin closed oxide phase (WO) only multilayer adsorption is found; above T≈200 K no adsorption takes place under UHV conditions. Weakly oxidized WC(0001) interacts more strongly with benzene in that strong photoemission signals of a (sub)monolayer species are visible up to a temperature of T≈340 K

Discrimination between domain wall and uniformly incommensurate structures by surface extendex x-ray absorption fine structure: Absorption of chlorine on Ag{100}

A well-established problem in surface structure determination is the experimental difficulty in distinguishing a uniformly incommensurate structure from a domain wall structure. Surface extended x-ray absorption fine structure (SEXAFS), as a local structural probe, should provide an unambiguous means of making the distinction. We have studied chlorine adsorbed on Ag{110} by SEXAFS at several coverages. The Cl-Ag nearest-neighbor distance, 2.56 ± 0.04 A, is found to be substantially shorter than on the {111} and {100} faces, and is independent of coverage. At \theta = 0.75 an incommensurate phase is formed. A multishell analysis (up to 5.6 A) allowed a complete structural determination and has, for the first time, confirmed our expectation. The CI-Cl spacing unambiguously favors a domain wall structure. Structural models have been tested by laser simulation experiments, and by kinematic analysis of the low-energy electron diffraction data