Insulator-to-metal transition driven by clustering of solitons in polyacetylene

It's believed that the simple Su-Schrieffer-Heeger Hamiltonian can not predict the insulator to metal transition of transpolyacetylene (t-PA). The soliton lattice configuration at a doping level y=6% still has a semiconductor gap. Disordered distributions of solitons close the gap, but the electronic states around the Fermi energy are localized. However, within the same framework, it is possible to show that a cluster of solitons can produce dramatic changes in the electronic structure, allowing an insulator-to-metal transition.

Ferromagnetism, paramagnetism, and a Curie-Weiss metal in an electron-doped Hubbard model on a triangular lattice

Motivated by the unconventional properties and rich phase diagram of NaxCoO2 we consider the electronic and magnetic properties of a two-dimensional Hubbard model on an isotropic triangular lattice doped with electrons away from half-filling. Dynamical mean-field theory (DMFT) calculations predict that for negative intersite hopping amplitudes (t < 0) and an on-site Coulomb repulsion, U, comparable to the bandwidth, the system displays properties typical of a weakly correlated metal. In contrast, for t > 0 a large enhancement of the effective mass, itinerant ferromagnetism, and a metallic phase with a Curie-Weiss magnetic susceptibility are found in a broad electron doping range. The different behavior encountered is a consequence of the larger noninteracting density of states (DOS) at the Fermi level for t > 0 than for t < 0, which effectively enhances the mass and the scattering amplitude of the quasiparticles. The shape of the DOS is crucial for the occurrence of ferromagnetism as for t > 0 the energy cost of polarizing the system is much smaller than for t < 0. Our observation of Nagaoka ferromagnetism is consistent with the A-type antiferromagnetism (i.e., ferromagnetic layers stacked antiferromagnetically) observed in neutron scattering experiments on NaxCoO2. The transport and magnetic properties measured in NaxCoO2 are consistent with DMFT predictions of a metal close to the Mott insulator and we discuss the role of Na ordering in driving the system towards the Mott transition. We propose that the Curie-Weiss metal phase observed in NaxCoO2 is a consequence of the crossover from a bad metal with incoherent quasiparticles at temperatures T > T-* and Fermi liquid behavior with enhanced parameters below T-*, where T-* is a low energy coherence scale induced by strong local Coulomb electron correlations. Our analysis also shows that the one band Hubbard model on a triangular lattice is not enough to describe the unusual properties of NaxCoO2 and is used to identify the simplest relevant model that captures the essential physics in NaxCoO2. We propose a model which allows for the Na ordering phenomena observed in the system which, we propose, drives the system close to the Mott insulating phase even at large dopings.

Atomic level phenomena on transition metal surfaces

In this study we discuss the atomic level phenomena on transition metal surfaces. Transition metals are widely used as catalysts in industry. Therefore, reactions occuring on transition metal surfaces have large industrial intrest. This study addresses problems in very small size and time scales, which is an important part in the overall understanding of these phenomena. The publications of this study can be roughly divided into two categories: The adsorption of an O2 molecule to a surface, and surface structures of preadsorbed atoms. These two categories complement each other, because in the realistic case there are always some preadsorbed atoms at the catalytically active surfaces. However, all transition metals have an active d-band, and this study is also a study of the in uence of the active d-band on other atoms. At the rst part of this study we discuss the adsorption and dissociation of an O2 molecule on a clean stepped palladium surface and a smooth palladium surface precovered with sulphur and oxygen atoms. We show how the reactivity of the surface against the oxygen molecule varies due to the geometry of the surface and preadsorbed atoms. We also show how the molecular orbitals of the oxygen molecule evolve when it approaches the di erent sites on the surface. In the second part we discuss the surface structures of transition metal surfaces. We study the structures that are intresting on account of the Rashba e ect and charge density waves. We also study the adsorption of suphur on a gold surface, and surface structures of it. In this study we use ab-initio based density functional theory methods to simulate the results. We also compare the results of our methods to the results obtained with the Low-Energy-Electron-Difraction method.

Phase diagram of the one-dimensional Holstein model of spinless fermions

The one-dimensional Holstein model of spinless fermions interacting with dispersionless phonons is studied using a new variant of the density matrix renormalization group. By examining various low-energy excitations of finite chains, the metal-insulator phase boundary is determined precisely and agrees with the predictions of strong coupling theory in the antiadiabatic regime and is consistent with renormalization group arguments in the adiabatic regime. The Luttinger liquid parameters, determined by finite-size scaling, are consistent with a Kosterlitz-Thouless transition.

Low-voltage driven ZnS:Mn MIS and MISIM thin film electroluminescent devices with Eu2O3 insulator layer

AC thin film electroluminescent devices of MIS and MISIM have been fabricated with a novel dielectric layer of Eu2O3 as an insulator. The threshold voltage for light emission is found to depend strongly on the frequency of excitation source in these devices. These devices are fabricated with an active layer of ZnS:Mn and a novel dielectric layer of Eu2O3 as an insulator. The observed frequency dependence of brightness-voltage characteristics has been explained on the basis of the loss characteristic of the insulator layer. Changes in the threshold voltage and brightness with variation in emitting or insulating film thickness have been investigated in metal-insulator-semiconductor (MIS) structures. It has been found that the decrease in brightness occurring with decreasing ZnS layer thickness can be compensated by an increase in brightness obtained by reducing the insulator thickness. The optimal condition for low threshold voltage and higher stability has been shown to occur when the active layer to insulator thickness ratio lies between one and two.

Ac and dc magnetotransport properties of the phase-separated La(0.6)Y(0.1)Ca(0.3)MnO(3) manganite

The physical properties of the La(0.6)Y(0.1)Ca(0.3)MnO(3) compound have been investigated, focusing on the magnetoresistance phenomenon studied by both dc and ac electrical transport measurements. X-ray diffraction and scanning electron microscopy analysis of ceramic samples prepared by the sol-gel method revealed that specimens are single phase and have average grain size of similar to 0.5 mu m. Magnetization and 4-probe dc electrical resistivity rho(T,H) experiments showed that a ferromagnetic transition at T(C) similar to 170 K is closely related to a metal-insulator (MI) transition occurring at essentially the same temperature T(MI). The magnetoresistance effect was found to be more pronounced at low applied fields (H <= 2.5 T) and temperatures close to the MI transition. The ac electrical transport was investigated by impedance spectroscopy Z(f,T,H) under applied magnetic field H up to 1 T. The Z(f,T,H) data exhibited two well-defined relaxation processes that exhibit different behaviors depending on the temperature and applied magnetic field. Pronounced effects were observed close to T (C) and were associated with the coexistence of clusters with different electronic and magnetic properties. In addition, the appreciable decrease of the electrical permittivity epsilon`(T,H) is consistent with changes in the concentration of e(g) mobile holes, a feature much more pronounced close to T (C).

Loss of superfluidity in a Bose-Einstein condensate on an optical lattice via a novel classical phase transition

We predict the loss of superfluidity in a Bose-Einstein condensate (BEC) trapped in a combined optical and axially-symmetric harmonic potentials during a resonant collective excitation initiated by a periodic modulation of the atomic scattering length a, when the modulation frequency equals twice the radial trapping frequency or multiples thereof. This classical dynamical transition is marked by a loss of superfluidity in the BEC and a subsequent destruction of the interference pattern upon free expansion. Suggestion for future experiment is made. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Magnetophotoresistance in Pr 1–x CaxMnO3 Thin Films

In this work, a colossal magnetoresistive (CMR) Pr1−xCaxMnO3 (PCMO) man- ganite thin films and polycrystalline samples were studied with the main focus on the properties of the insulator to metal Mott phase transition. The polycrystalline PCMO samples were fabricated with the solid state reaction method. The polycrystalline sam- ples were further processed into the epitaxial thin films with the pulsed laser deposition method (PLD). The structural and magnetic properties of the samples were systemat- ically investigated and the thin films samples were further investigated with magneto- transport measurement where the thin films response to illumination was also studied. After the successful synthesis of polycrystalline PCMO samples with varying x = between 0.0 - 0.5, the magnetic characterization of the samples showed rich magnetic properties having the signatures of the magnetic phase coexistence of antiferromag- netic (AFM) and ferromagnetic (FM) ordering and cluster glass behaviour. With the increased doping concentration from x = 0.3 to 0.5, the AFM charge-order (CO) phase started to form up being strongest on the sample x = 0.5. This AFM CO phase could be melted with the high external magnetic field at temperatures below Neel’s tempera- ture TN inducing an irreversible first order metamagnetic AFM to FM phase transition. The strength of the AFM CO phase decreased with decreasing Ca concentration and increasing temperature. The polycrystalline PCMO samples with Ca concentrations of 0.3 - 0.5, showing metamagnetic behaviour, were selected for the fabrication of the thin film samples. The films were grown using two different in situ oxygen treatment temperatures at 500 ◦C and 700 ◦C in the PLD system. The films with x = 0.4 and 0.5 showed weaker AFM CO phase with greatly reduced melting fields when compared to polycrystalline samples. Also, the robustness of the AFM CO phase was further decreased in thin films with the lower oxygen treatment temperature of 500 ◦C. The magneto-transport measurements made on the thin films showed that the melting of AFM CO phase was connected to CMR effect where the increasing magnetic field induced an insulator to metal phase transition, which reduces the resistivity of the film around nine orders of magnitude. The use of illumination during the magneto-transport measurements showed a vari- ety of intriguing phenomena including magnetophotoresistance. The illumination had a huge effect to the insulator to metal transition (IMT) reducing the transition magnetic field significantly. Moreover, by magnetically biasing the thin films with the constant external magnetic field, the IMT could be induced by switching on the illumination.

Design principles for maximizing photovoltage in metal-oxide-protected water-splitting photoanodes

Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis, but the poor photovoltages often reported so far will severely limit their performance. Here we report a novel observation of photovoltage loss associated with a charge extraction barrier imposed by the protection layer, and, by eliminating it, achieve photovoltages as high as 630mV, the maximum reported so far for water-splitting silicon photoanodes. The loss mechanism is systematically probed in metal-insulator-semiconductor Schottky junction cells compared to buried junction p(+) n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A leaky-capacitor model related to the dielectric properties of the protective oxide explains this loss, achieving excellent agreement with the data. From these findings, we formulate design principles for simultaneous optimization of built-in field, interface quality, and hole extraction to maximize the photovoltage of oxide-protected water-splitting anodes.

A novel subwavelength plasmon-polariton optical filter based on tilted coupled structures

This work proposes a completely new approach for the design of resonant structures aiming at wavelength-filtering applications. The structure consists of a subwavelength metal-insulator-metal (MIM) waveguide presenting tilted coupled structures transversely arranged in the midpoint between the input and output ports. The cavity-like response of this device has shown that this concept can be particularly attractive for optical filter design for telecom applications. The extra degree of freedom provided by the tilting of the cavity has proved to be not only very effective on improving the quality factor of these structures, but also to be an elegant way of extending the range of applications for tuning multiple wavelengths, if necessary.

Electrical and Mechanical Properties of Post-annealed SiC(x)N(y) Films

Amorphous SiC(x)N(y) films have been deposited on (100) Si substrates by RF magnetron sputtering of a SiC target in a variable nitrogen-argon atmosphere. The as-deposited films were submitted to thermal anneling in a furnace under argon atmosphere at 1000 degrees C for 1 hour. Composition and structure of unannealed and annealed samples were investigated by RBS and FTIR. To study the electrical characteristics of SiC(x)N(y) films, Metal-insulator-semiconductor (MIS) structures were fabricated. Elastic modulus and hardness of the films were determined by nanoindentation. The results of these studies showed that nitrogen content and thermal annealing affect the electrical, mechanical and structural properties of SiC(x)N(y) films.

Driving Scheme Using MIS Photosensor for Luminance Control of AMOLED Pixel

This paper presents a new driving scheme utilizing an in-pixel metal-insulator-semiconductor (MIS) photosensor for luminance control of active-matrix organic light-emitting diode (AMOLED) pixel. The proposed 3-TFT circuit is controlled by an external driver performing the signal readout, processing, and programming operations according to a luminance adjusting algorithm. To maintain the fabrication simplicity, the embedded MIS photosensor shares the same layer stack with pixel TFTs. Performance characteristics of the MIS structure with a nc-Si : H/a-Si : H bilayer absorber were measured and analyzed to prove the concept. The observed transient dark current is associated with charge trapping at the insulator-semiconductor interface that can be largely eliminated by adjusting the bias voltage during the refresh cycle. Other factors limiting the dynamic range and external quantum efficiency are also determined and verified using a small-signal model of the device. Experimental results demonstrate the feasibility of the MIS photosensor for the discussed driving scheme.

Relation for the nonequilibrium population of the interface states: effects on the bias dependence of the ideality factors

By an analysis of the exchange of carriers through a semiconductor junction, a general relationship for the nonequilibrium population of the interface states in Schottky barrier diodes has been derived. Based on this relationship, an analytical expression for the ideality factor valid in the whole range of applied bias has been given. This quantity exhibits two different behaviours depending on the value of the applied bias with respect to a critical voltage. This voltage, which depends on the properties of the interfacial layer, constitutes a new parameter to complete the characterization of these junctions. A simple interpretation of the different behaviours of the ideality factor has been given in terms of the nonequilibrium charging properties of interface states, which in turn explains why apparently different approaches have given rise to similar results. Finally, the relevance of our results has been considered on the determination of the density of interface states from nonideal current-voltage characteristics and in the evaluation of the effects of the interfacial layer thickness in metal-insulator-semiconductor tunnelling diodes.

Relation for the nonequilibrium population of the interface states: effects on the bias dependence of the ideality factors

By an analysis of the exchange of carriers through a semiconductor junction, a general relationship for the nonequilibrium population of the interface states in Schottky barrier diodes has been derived. Based on this relationship, an analytical expression for the ideality factor valid in the whole range of applied bias has been given. This quantity exhibits two different behaviours depending on the value of the applied bias with respect to a critical voltage. This voltage, which depends on the properties of the interfacial layer, constitutes a new parameter to complete the characterization of these junctions. A simple interpretation of the different behaviours of the ideality factor has been given in terms of the nonequilibrium charging properties of interface states, which in turn explains why apparently different approaches have given rise to similar results. Finally, the relevance of our results has been considered on the determination of the density of interface states from nonideal current-voltage characteristics and in the evaluation of the effects of the interfacial layer thickness in metal-insulator-semiconductor tunnelling diodes.

The finite bandwidth model for spin fluctuations in Pd

The frequency dependence of the electron-spin fluctuation spectrum, P(Q), is calculated in the finite bandwidth model. We find that for Pd, which has a nearly full d-band, the magnitude, the range, and the peak frequency of P(Q) are greatly reduced from those in the standard spin fluctuation theory. The electron self-energy due to spin fluctuations is calculated within the finite bandwidth model. Vertex corrections are examined, and we find that Migdal's theorem is valid for spin fluctuations in the nearly full band. The conductance of a normal metal-insulator-normal metal tunnel junction is examined when spin fluctuations are present in one electrode. We find that for the nearly full band, the momentum independent self-energy due to spin fluctuations enters the expression for the tunneling conductance with approximately the same weight as the self-energy due to phonons. The effect of spin fluctuations on the tunneling conductance is slight within the finite bandwidth model for Pd. The effect of spin fluctuations on the tunneling conductance of a metal with a less full d-band than Pd may be more pronounced. However, in this case the tunneling conductance is not simply proportional to the self-energy.