Importance Of Disorder Research Articles

Chemical Biology: Powerful Synergy Between Two Cultures

Chemical Biology: Powerful Synergy Between Two Cultures

Site-specific C13 NMR study on the locally distorted triangular lattice of the organic conductor κ−(BEDT−TTF)2Cu2(CN)3

To verify the effect of geometrical frustration, we artificially distort the triangular lattice of quasi-two-dimensional organic conductor $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ [BEDT-TTF: bis(ethylenedithio)terathiofulvalene] by analogous-molecular substitution and apply $^{13}$C NMR of bulk and substituted sites, electric conductivity, and static magnetic susceptibility measurements. The results indicate that the magnetic characteristics of the substituted sample are quantitatively similar to those of the pure sample. Moreover the magnetic characteristics at the substituted sites are also the same as in the bulk. These results suggest that the observed magnetic properties may not be due to the geometrical frustration but the importance of disorder.

Dimensional crossover and incipient quantum size effects in superconducting niobium nanofilms

Superconducting and normal state properties of Niobium nanofilms have been systematically investigated as a function of film thickness, on different substrates. The width of the superconducting-to-normal transition for all films is remarkably narrow, confirming their high quality. The superconducting critical current density exhibits a pronounced maximum for thickness around 25 nm, marking the 3D-to-2D crossover. The magnetic penetration depth shows a sizeable enhancement for the thinnest films. Additional amplification effects of the superconducting properties have been obtained with sapphire substrates or squeezing the lateral size of the nanofilms. For thickness close to 20 nm we measured a doubled perpendicular critical magnetic field compared to its large thickness value, indicating shortening of the correlation length and the formation of small Cooper pairs. Our data analysis indicates an exciting interplay between quantum-size and proximity effects together with strong-coupling effects and the importance of disorder in the thinnest films, placing these nanofilms close to the BCS-BEC crossover regime.

Tailoring Disorder and Dimensionality: Strategies for Improved Solid Oxide Fuel Cell Electrolytes

Reducing the operation temperature of solid oxide fuel cells is a major challenge towards their widespread use for power generation. This has triggered an intense materials research effort involving the search for novel electrolytes with higher ionic conductivity near room temperature. Two main directions are being currently followed: the use of doping strategies for the synthesis of new bulk materials and the implementation of nanotechnology routes for the fabrication of artificial nanostructures with improved properties. In this paper, we review our recent work on solid oxide fuel cell electrolyte materials in these two directions, with special emphasis on the importance of disorder and reduced dimensionality in determining ion conductivity. Substitution of Ti for Zr in the A(2)Zr(2-) (y)Ti(y)O(7) (A = Y, Dy, and Gd) series, directly related to yttria stabilized zirconia (a common fuel cell electrolyte), allows controlling ion mobility over wide ranges. In the second scenario we describe the strong enhancement of the conductivity occurring at the interfaces of superlattices made by alternating strontium titanate and yttria stabilized zirconia ultrathin films. We conclude that cooperative effects in oxygen dynamics play a primary role in determining ion mobility of bulk and artificially nanolayered materials and should be considered in the design of new electrolytes with enhanced conductivity.

Pressure-induced tuning of phase transition and role of disorder in electrical transport properties ofβ−SrxV6O15

We report the resistivity and thermoelectric power of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Sr}}_{x}{\mathrm{V}}_{6}{\mathrm{O}}_{15}$, for various stoichiometries, $0.6\ensuremath{\leqslant}x\ensuremath{\leqslant}1$, and under pressures up to $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The pristine system $(x=1)$ exhibits a semiconductor-insulator transition at $155\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is evidenced in both resistivity and thermopower and is probably induced by charge ordering. We observe a pronounced change in the nature of the phase transition under pressure and we attribute it to the tuning of the nearest neighbor Coulomb interaction $V$. At ambient pressure, as the system moves away from stoichiometry to $x<1$, disorder is introduced into the strontium sublattice and the phase transition is immediately suppressed. The temperature dependence of the thermoelectric power gradually weakens as the system moves away from $x=1$, indicating the importance of disorder. While for $x<1$ compound thermoelectric power shows evidence of a localized contribution to the conduction, which may involve polaronic effects, the activation energies speak against small polarons in the pristine $x=1$ compound. We explain our results in a model of conduction through localized states in the off-stoichiometric systems and of thermally activated conduction in the pristine system.

The Importance of Disorder and Mixed Electron/Proton Conductivity in Fuel Cell Electrocatalysis: Strategies toward an Ionomer- and Carbon-Free Electrode

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Model of Fe nanostripes on Cu(111)

Magnetization as a function of temperature calculated with Monte Carlo techniques is compared to experimental results of Fe stripes grown on vicinal Cu(111) surfaces. The stripes are step decorations grown with molecular beam epitaxy (MBE), are 1–2 monolayers thick, and display perpendicular magnetization. The atomic parameters are determined from fully relativistic electronic structure calculations. The moments are found to be 2.57 μB, with some variation due to film thickness, and uniaxial anisotropy of 40 μRy/atom for Fe atoms facing vacuum. The Heisenberg model extended to include crystalline anisotropy as well as dipole–dipole interactions is considered for two different values of the exchange constant: J=20 and 2 meV. Under a large applied field (4000 G), the calculated saturation magnetization falls slowly from 507 emu/cm3 with an increase in temperature until it falls rapidly around 600 K, after which a more modest falloff with an increase in temperature is observed. For larger J the rapid change occurs for higher temperatures. The importance of disorder in the height and width of the stripes is investigated by generating stripe geometries with a model that incorporates nucleation and growth of Fe particles at step edges under the constraint of constant deposition from MBE. The primary effect of disorder in the stripes is to reduce the saturated magnetization at lower temperatures.

The Importance of Disorder in Very High Quality Semiconductor Microcavities

We investigate a semiconductor microcavity containing a quantum well, a sample of exceptional quality. We observe a four-wave mixing response which resembles that of an inhomogeneously broadened exciton system, while the linear transmission measurements are affected by the homogeneous part of the broadening and have a completely different decay time. We show that the four-wave mixing response cannot be calculated using the excitonic parameters (linewidth and relaxation time) extracted from the linear transmission measurements, as the coupling of exciton and photon states deeply modifies the scattering channels.

Dynamic interaction of bulk acoustic waves with a two-dimensional electron gas at an AlxGa1-xAs/GaAs heterojunction in strong magnetic fields.

The results of a theoretical and experimental study of the interaction of longitudinal coherent bulk acoustic waves at 9.3 GHz with two-dimensional electron gas (2DEG) at a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterojunction in a strong magnetic field are given. The theory of the linear 2DEG response to an incident bulk acoustic wave is developed, based on a system of quantum kinetic equations. The importance of disorder in the 2DEG and the coupling of the acoustic waves to the localized states is demonstrated. A comparison of the theoretical results with experiment is given.

Many-body effects in polymers: especially polyacetylene

Many-electron effects in polymers are considered, with emphasis on the way electron-electron interactions are reflected in ground-state properties. After a survey of results on graph theory, built up from valence-bond structures, in which the importance of size consistency again emerges, some specific results of ground-state electron pair correlations in polyacetylene and polydiacetylene are considered, motivated by the linear dispersion of the π-electron plasmon, as measured in the optical experiments of Ritsko et al. The way in which such collective excitations can affect ground-state pair correlations is discussed as is the influence of low dimensionality. A brief summary is given of the way in which the energy gap in polyacetylene and polydiacetylene depends on electron-electron repulsion although unlike the plasmon problem where the long-range Coulomb repulsion e 2/ r ij is essential, this problem is usefully tackled using the on-site Hubbard repulsion. Again, there can be no doubt that correlation is important in the ground states of polyacetylene and polydiacetylene. However, recent work on bond-charge repulsion shows that whether correlation enhances or suppresses dimerization depends strongly on the particular thermodynamic state in question: this is illustrated by specific examples relating to the metal-insulator transition in low-dimensional solids. A quite different aspect of many-body phenomena will then be considered, pertaining especially to AC electrical transport in biopolymers. Here, the importance of disorder in the experimental results currently available is stressed. A power law behaviour of the frequency-dependent conductivity, i.e. σ(ω) proportional to ω s, is a characteristic feature at suitably high frequencies. This focusses attention on the power, s, and a brief discussion of simple models shows that, for those considered to date s< 1, with possible values of 1 4 and 1 2 being exhibited. A possible influence of a compensation effect on the high-frequency conductivity is suggested. It is argued, following Kreuzer and March, that this is a many-particle phenomenon ands = 1-( T/T c) for T < T c, where T c is the compensation temperature; a result apparently due to Dyre. The possibility of further experimental and theoretical studies related to the many-body phenomena is referred to.

Importance of disorder in the conductivity of packings under compression

In granular media under compression, the forces are transmitted in a very heterogeneous way as can be seen from photoelasticity experiments. This strong heterogeneity has consequences not only for mechanical properties: for instance, the conductance of a granular 2-, or 3-dimensional conducting medium as a function of stress follows a power law with an exponent which is different from the microscopic Hertz exponent and depends on the disorder of the structure. We propose a simple explanation.

Surface enhanced and disorder induced Raman scattering from silver films

For silver films evaporated at 120 K, disorder induced Raman scattering from phonons, a structureless inelastic background and surface enhanced Raman scattering from adsorbed pyridine is observed. All three phenomena disappear irreversibly by heating up to room temperature. This indicates the importance of disorder on an atomic scale.

Professional Wrestling as Moral Order

This paper treats structuralist and interactionist perspectives as complementary in an approach to the dynamics of professional wrestling as a social scene. Both directive or formulating aspects of culture as it is specified for social life and the creativity of human agents in negotiating such constraints from the viewpoint of their particularistic interests are emphasized in an examination of the confirmation and negotiation of identities, the clash and resolution of moral meanings, the calculated transgression of legal tradition, the location of the charismatic hero, and the ritual importance of disorder and crisis in wrestling. As such, the paper is offered as a contribution to an understanding of the sources and movement of meanings in ritual and dramatic forms.