Advances in Condensed Matter Physics
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Open Access journal
ISSN (Print) 1687-8108 - ISSN (Online) 1687-8124
Published by Hindawi [334 journals]
- Micromagnetic Simulation of Strain-Assisted Current-Induced Magnetization
Abstract: We investigated the effect of substrate misfit strain on the current-induced magnetization switching in magnetic tunnel junctions by combining micromagnetic simulation with phase-field microelasticity theory. Our results indicate that the positive substrate misfit strain can decrease the critical current density of magnetization switching by pushing the magnetization from out-of-plane to in-plane directions, while the negative strain pushes the magnetization back to the out-of-plane directions. The magnetic domain evolution is obtained to demonstrate the strain-assisted current-induced magnetization switching.
PubDate: Thu, 22 Sep 2016 13:45:40 +000
- Pressure Prediction of Electronic, Anisotropic Elastic, Optical, and
Thermal Properties of Quaternary (M2/3Ti1/3)3AlC2 (M = Cr, Mo, and Ti)
Abstract: The electronic, mechanical, anisotropic elastic, optical, and thermal properties of quaternary (M2/3Ti1/3)3AlC2 (M = Cr, Mo, and Ti) under different pressure are systematically investigated by first-principles calculations. The bonding characteristics of these compounds are the mixture of metallic and covalent bonds. With an increase of pressure, the heights of total density of states (TDOS) for these compounds decrease at Fermi level. The highest volume compressibility among three compounds is Mo2TiAlC2 for its smallest relative volume decline. The relative bond lengths are decreasing when the pressure increases. The bulk and shear modulus of the one doped with Cr or Mo are larger than those of Ti3AlC2 with pressure increasing. With an increase of pressure, the anisotropy of these compounds also increases. Moreover, Mo2TiAlC2 has the biggest anisotropy among the three compounds. The results of optical functions indicate that the reflectivity of the three compounds is high in visible-ultraviolet region up to ~10.5 eV under ambient pressure and increasing constantly when under pressure. Mo2TiAlC2 has the highest loss function. The calculated sound velocity and Debye temperature show that they all increase with pressure. of the three compounds is also calculated.
PubDate: Mon, 19 Sep 2016 07:32:54 +000
- Electronic and Magnetic Properties Studies on Mn and Oxygen Vacancies
Codoped Anatase TiO2
Abstract: The electronic and magnetic properties of Mn and oxygen vacancies codoped anatase TiO2 were investigated. The calculated results showed that the TiO2 codoped with Mn and oxygen vacancies have a magnetic moment value of 3.415 per Ti31MnO63 supercell. Furthermore, Ti31MnO63 gets the lowest energy with a geometrical optimization where the Mn ions locate at the nearest-neighbor sites of the oxygen vacancy. And experimental results indicated the magnetism is associated with the defects of Mn ions and oxygen vacancies induced by the Mn doping, which is consistent with the calculation results.
PubDate: Sun, 18 Sep 2016 11:47:27 +000
- Origins of the Exchange-Bias Phenomenology, Coercivity Enhancement, and
Asymmetric Hysteretic Shearing in Core-Surface Smart Nanoparticles
Abstract: We have used a spin-1 Ising model Hamiltonian with dipolar (bilinear, ), quadrupolar (biquadratic, ), and dipolar-quadrupolar (odd, ) interactions in pair approximation to investigate the exchange-bias (EB), coercive field, and asymmetric hysteretic shearing properties peculiar to core/surface () composite nanoparticles (NPs). Shifted hysteresis loops with an asymmetry and coercivity enhancement are observed only in the presence of the odd interaction term in the Hamiltonian expression and their magnitudes show strong dependence on the value of . The observed coercivity and EB in NPs originated from nonzero odd coupling energies and their dependence on temperature () and particle size () are also discussed in relation to experimental findings.
PubDate: Sun, 04 Sep 2016 06:29:57 +000
- Impact of Pressure and Brine Salinity on Capillary Pressure-Water
Saturation Relations in Geological CO2 Sequestration
Abstract: Capillary pressure-water saturation relations are required to explore the CO2/brine flows in deep saline aquifers including storage capacity, relative permeability of CO2/brine, and change to stiffness and volume. The study on capillary pressure-water saturation curves has been conducted through experimentation and theoretical models. The results show that as the pressure increases up to 12 MPa, (1) capillary pressure-water saturation curves shift to lower values at given water saturation, (2) after the drainage process, residual water saturation decreases, and (3) after the imbibition process, capillary CO2 trapping increases. Capillary pressure-water saturation curves above 12 MPa appear to be similar because of relatively constant contact angle and interfacial tension. Also, as brine salinity increases from 1 M to 3 M NaCl, (1) capillary pressure-water saturation curves shift to lower capillary pressure, (2) residual water saturation decreases, and (3) capillary CO2 trapping increases. The results show that pressure and brine salinity have an influence on the capillary pressure-water saturation curves. Also, the scaled capillary CO2 entry pressure considering contact angle and interfacial tension is inconsistent with atmospheric conditions due to the lack of wettability information. Better exploration of wettability alteration is required to predict capillary pressure-water saturation curves at various conditions that are relevant to geological CO2 sequestration.
PubDate: Mon, 29 Aug 2016 08:59:18 +000
- Dosimetric UV Exposure Effect on the Optical Properties of Ag2O Doped
Abstract: Silver phosphate glass types within composition 60P2O5-30ZnO-10CuO-100000 ppm Ag2O were prepared by melt-quenching technique. The optical properties of these glass types were studied under UV exposure at different times, 0, 20, 80, 105, and 115 minutes. The optical absorbance spectra were measured in the range of wavelength from 190 to 3200 nm. The absorbance bandwidth decreases with increasing the time of UV exposure. The optical energy gap, , linear refractive index, , ratio between molar refraction, , and molar volume, , and metallization criterion () were estimated. The value of decreases from 2.132 to 1.91 eV with increasing the time of UV exposure from 0 to 115 min. Otherwise value and metallization increase with increase in the time of UV exposure. The results indicated that these glass types are promising for using an ultraviolet radiation dosimeter.
PubDate: Wed, 24 Aug 2016 15:11:15 +000
- Corrigendum to “Molecular Dynamics Simulation of Aggregates in the
Dodecane/span80 System and Their Behaviour in an Electric Field”
PubDate: Wed, 17 Aug 2016 12:04:35 +000
- The Effect of Indium Concentration on the Structure and Properties of
Zirconium Based Intermetallics: First-Principles Calculations
Abstract: The phase stability, mechanical, electronic, and thermodynamic properties of In-Zr compounds have been explored using the first-principles calculation based on density functional theory (DFT). The calculated formation enthalpies show that these compounds are all thermodynamically stable. Information on electronic structure indicates that they possess metallic characteristics and there is a common hybridization between In-p and Zr-d states near the Fermi level. Elastic properties have been taken into consideration. The calculated results on the ratio of the bulk to shear modulus (B/G) validate that InZr3 has the strongest deformation resistance. The increase of indium content results in the breakout of a linear decrease of the bulk modulus and Young’s modulus. The calculated theoretical hardness of α-In3Zr is higher than the other In-Zr compounds.
PubDate: Sun, 17 Jul 2016 13:11:47 +000
- A Self-Consistent Model for Thermal Oxidation of Silicon at Low Oxide
Abstract: Thermal oxidation of silicon belongs to the most decisive steps in microelectronic fabrication because it allows creating electrically insulating areas which enclose electrically conductive devices and device areas, respectively. Deal and Grove developed the first model (DG-model) for the thermal oxidation of silicon describing the oxide thickness versus oxidation time relationship with very good agreement for oxide thicknesses of more than 23 nm. Their approach named as general relationship is the basis of many similar investigations. However, measurement results show that the DG-model does not apply to very thin oxides in the range of a few nm. Additionally, it is inherently not self-consistent. The aim of this paper is to develop a self-consistent model that is based on the continuity equation instead of Fick’s law as the DG-model is. As literature data show, the relationship between silicon oxide thickness and oxidation time is governed—down to oxide thicknesses of just a few nm—by a power-of-time law. Given by the time-independent surface concentration of oxidants at the oxide surface, Fickian diffusion seems to be neglectable for oxidant migration. The oxidant flux has been revealed to be carried by non-Fickian flux processes depending on sites being able to lodge dopants (oxidants), the so-called DOCC-sites, as well as on the dopant jump rate.
PubDate: Tue, 19 Apr 2016 08:29:42 +000
- Normal Stress Differences and Yield Stresses in Attractive Particle
Abstract: The nature of attractive particulate networks, yield stresses, and normal stress differences is systematically reviewed, each in terms of the relevant definitions, underlying mechanisms, and current measurement techniques. With this foundation, experimental observations of normal stress differences in some suspensions and colloidal systems are surveyed, along with constitutive models that allow for normal stress differences to arise prior to yielding. Yield stresses are a hallmark of attractive colloidal systems and vital in their processing. In contrast, little attention has been given to the role of normal stress differences in these systems. The presence or absence of normal stress differences necessarily affects the isotropy of the normal stress field through the solid particulate phase (treated as a continuum), in turn affecting estimation of yield stress. Given the importance of yield stresses in dealing with practical industrial problems, and in understanding fundamental behaviours, it is important to ensure that yield measurements can be relied upon.
PubDate: Tue, 12 Apr 2016 10:26:15 +000
- Electronic Origin of Defect States in Fe-Doped LiNbO3 Ferroelectrics
Abstract: We investigate the role of Fe in the electronic structure of ferroelectric LiNbO3 by density-functional theory calculations. We show that Fe2+ on the Li site () features a displacement opposite to the direction of spontaneous polarization and acts as a trigger for the bulk photovoltaic (PV) effect. In contrast to Fe3+ on the Li site that forms the defect states (1e, a, and 2e) below the conduction band minimum, the reduction from Fe3+ to Fe2+ accompanied by a lattice relaxation markedly lowers only the state () owing to a strong orbital hybridization with Nb-4d. The state of provides the highest electron-occupied defect state in the middle of the band gap. A reduction treatment of Fe-LN is expected to increase the concentration of Fe2+ and therefore to enhance the PV effect under visible light illumination.
PubDate: Wed, 23 Mar 2016 12:20:16 +000
- Ni-Based Ohmic Contacts to n-Type 4H-SiC: The Formation Mechanism and
Abstract: The fabrication of low-resistance and thermal stable ohmic contacts is important for realization of reliable SiC devices. For the n-type SiC, Ni-based metallization is most commonly used for Schottky and ohmic contacts. Many experimental studies have been performed in order to understand the mechanism of ohmic contact formation and different models were proposed to explain the Schottky to ohmic transition for Ni/SiC contacts. In the present review, we summarize the last key results on the matter and post open questions concerning the unclear issues of ohmic contacts to n-type SiC. Analysis of the literature data and our own experimental observations have led to the conclusion that the annealing at high temperature leads to the preferential orientation of silicide at the heterointerface (0001)SiC//(013)-Ni2Si. Moreover, we may conclude that only δ-Ni2Si grains play a key role in determining electrical transport properties at the contact/SiC interface. Finally, we show that the diffusion barriers with free diffusion path microstructure can improve thermal stability of metal-SiC ohmic contacts for high-temperature electronics.
PubDate: Wed, 16 Mar 2016 09:02:14 +000
- Theoretical Investigation on Structural and Electronic Properties of InN
Growth on Ce-Stabilized Zirconia (111) Substrates
Abstract: The structural and electronic properties of InN on Ce-stabilized zirconia (CeSZ) (111) substrates are investigated using first-principles calculations based on density functional theory with GGA + method. Surface energy calculations indicate that the structure of Ce-segregated surface is more energetically stable than that of Ce-segregation-free surface. Adsorption energies of indium and nitrogen atoms on both Ce-segregated and Ce-segregation-free CeSZ (111) surfaces at the initial growth stage have been studied. The results suggest that the first layer of InN films consists of a nitrogen layer, which leads to epitaxial relationships between InN (0001) // CeSZ (111) and InN // CeSZ . In addition, density of states (DOS) analysis revealed that the hybridization effect plays a crucial role in determining the interface structure for the growth of InN on CeSZ (111) surfaces. Furthermore, adsorption energies of indium atoms on the nitrogen layer have also been evaluated in order to investigate the lattice polarity determination for InN films. It was found that an indium atom preferentially adsorbs at the center of three nitrogen atoms stacked on the CeSZ substrate, which results in the formation of In-polarity InN.
PubDate: Sun, 13 Mar 2016 14:20:13 +000
- Effect of In Situ Thermal Annealing on Structural, Optical, and Electrical
Properties of CdS/CdTe Thin Film Solar Cells Fabricated by Pulsed Laser
Abstract: An in situ thermal annealing process (iTAP) has been introduced before the common ex situ cadmium chloride (CdCl2) annealing to improve crystal quality and morphology of the CdTe thin films after pulsed laser deposition of CdS/CdTe heterostructures. A strong correlation between the two annealing processes was observed, leading to a profound effect on the performance of CdS/CdTe thin film solar cells. Atomic force microscopy and Raman spectroscopy show that the iTAP in the optimal processing window produces considerable CdTe grain growth and improves the CdTe crystallinity, which results in significantly improved optoelectronic properties and quantum efficiency of the CdS/CdTe solar cells. A power conversion efficiency of up to 7.0% has been obtained on thin film CdS/CdTe solar cells of absorber thickness as small as 0.75 μm processed with the optimal iTAP at 450°C for 10–20 min. This result illustrates the importance of controlling microstructures of CdTe thin films and iTAP provides a viable approach to achieve such a control.
PubDate: Mon, 29 Feb 2016 09:30:11 +000
- High Dielectric Constant Study of TiO2-Polypyrrole Composites with Low
Contents of Filler Prepared by In Situ Polymerization
Abstract: TiO2/polypyrrole composites with high dielectric constant have been synthesized by in situ polymerization of pyrrole in an aqueous dispersion of low concentration of TiO2, in the presence of small amount of HCl. Structural, optical, surface morphological, and thermal properties of the composites were investigated by X-ray diffractometer, Fourier transform infrared spectroscopy, field-emission scanning electron microscopy, and thermogravimetric analysis, respectively. The data obtained from diffractometer and thermal gravimetric analysis confirmed the crystalline nature and thermal stability of the prepared composites. The dielectric constant of 5 wt% TiO2 increased with filler content up to 4.3 × 103 at 1 kHz and then decreased to 1.25 × 103 at 10 kHz.
PubDate: Thu, 11 Feb 2016 11:22:32 +000
- The First Observation of Metallic Behaviour in Nd3.5Sm0.5Ni3O8
Abstract: In Nd3.5Sm0.5Ni3O8 which has basically the same crystal structure and the similar electrical configuration (Ni+ 3d9/Ni2+ 3d8 mix valence state) with high- cuprate, it has been found that this material shows metallic behaviour down to about 20 K by intercalation and subsequent deintercalation with sulfur. This is the first observation of the metallic state in this system. It is unclear why sulfur-intercalation and deintercalation induce the metallic state. We speculate that sulfur works as an effective getter for removing the interstitial apical oxygen which impedes the metallic conduction. However, the weak localization of carriers in the NiO2 planes still remains below 20 K and the localization may be one of the obstacles to occurrence of possible superconductivity.
PubDate: Tue, 02 Feb 2016 08:08:46 +000
- Amphiphiles Self-Assembly: Basic Concepts and Future Perspectives of
Abstract: Amphiphiles are synthetic or natural molecules with the ability to self-assemble into a wide variety of structures including micelles, vesicles, nanotubes, nanofibers, and lamellae. Self-assembly processes of amphiphiles have been widely used to mimic biological systems, such as assembly of lipids and proteins, while their integrated actions allow the performance of highly specific cellular functions which has paved a way for bottom-up bionanotechnology. While amphiphiles self-assembly has attracted considerable attention for decades due to their extensive applications in material science, drug and gene delivery, recent developments in nanoscience stimulated the combination of the simple approaches of amphiphile assembly with the advanced concept of supramolecular self-assembly for the development of more complex, hierarchical nanostructures. Introduction of stimulus responsive supramolecular amphiphile assembly-disassembly processes provides particularly novel approaches for impacting bionanotechnology applications. Leading examples of these novel self-assembly processes can be found, in fact, in biosystems where assemblies of different amphiphilic macrocomponents and their integrated actions allow the performance of highly specific biological functions. In this perspective, we summarize in this tutorial review the basic concept and recent research on self-assembly of traditional amphiphilic molecules (such as surfactants, amphiphile-like polymers, or lipids) and more recent concepts of supramolecular amphiphiles assembly which have become increasingly important in emerging nanotechnology.
PubDate: Tue, 29 Dec 2015 14:28:39 +000
- Synthesis and Characterization of Bifunctional α-Fe2O3-Ag
Abstract: The synthesis of α-Fe2O3-Ag bimetallic nanoparticles using a novel and simplified route is presented in this work. These hybrid nanoparticles were produced using a modification of the chemical reduction method by sodium borohydride (NaBH4). Fe(III) chloride hexahydrate (FeCl3·6H2O) and silver nitrate (AgNO3) as precursors were employed. Particles with semispherical morphology and dumbbell configuration were observed. High-resolution transmission electron microscopy (HRTEM) technique reveals the structure of the dumbbell-like α-Fe2O3-Ag nanoparticles. Some theoretical models further confirm the formation of the α-Fe2O3-Ag structures. Analysis by cyclic voltammetry reveals an interesting catalytic behavior which is associated with the combination of the individual properties of the Ag and α-Fe2O3 nanoparticles.
PubDate: Mon, 14 Dec 2015 08:51:21 +000
- Nonlinear Tunneling of Surface Plasmon Polaritons in Periodic Structures
Containing Left-Handed Metamaterial Layers
Abstract: The transmission of surface plasmon polaritons through a one-dimensional periodic structure is considered theoretically by using the transfer matrix approach. The periodic structure is assumed to have alternate left-handed metamaterial and dielectric layers. Both transverse electric and transverse magnetic modes of surface plasmon polaritons exist in this structure. It is found that, for nonlinear wave propagation, tunneling structures are formed to transform nontransmitting frequencies into transmitting frequencies and hence transmission bistability is observed. It is further observed that the structure shows sensitivity with respect to the polarization of the electromagnetic field for this phenomenon.
PubDate: Sun, 13 Dec 2015 07:48:54 +000
- Molecular Dynamics Simulation of Aggregates in the Dodecane/span80 System
and Their Behaviour in an Electric Field
Abstract: Molecular dynamics simulation of self-assembly of surfactant span80 molecules to form reverse micelles in nonpolar liquid dodecane is carried out. Simulations are performed using a united atom model for dodecane and a hybrid model for span80 molecules. Various physical characteristics of reverse micelle are measured, and the same are compared with available experimental results. Presence of charge carriers in the form of solvated ions in the core of reverse micelles is confirmed by the simulation. Movement of reverse micelles under the effect of uniform external electric field is also discussed.
PubDate: Mon, 23 Nov 2015 09:42:29 +000
- Analysis for the Sorption Kinetics of Ag Nanoparticles on Natural
Abstract: The kinetic adsorption behavior of silver nanoparticles deposited on a natural zeolite from Oaxaca is presented. Theoretical models as Lagergren first-order, pseudo-second-order, Elovich, and intraparticle diffusion were employed and compared with experimental data obtained by atomic absorption spectrophotometry technique. Correlation factors of the order of 0.99 were observed. Analysis by transmission electron microscopy revealed that the silver nanoparticles were homogeneously distributed on the zeolite. Additionally, chemical characterization of the material was carried out through a dilution process with lithium metaborate. An average value of 9.3 in the Si/Al ratio was observed and related to the kinetic adsorption behavior of the zeolite.
PubDate: Sun, 22 Nov 2015 09:53:46 +000
- Ultrasonic Studies of Emulsion Stability in the Presence of Magnetic
Abstract: Pickering emulsions are made of solid particle-stabilized droplets suspended in an immiscible continuous liquid phase. A magnetic emulsion can be obtained using magnetic particles. Solid magnetic nanoparticles are adsorbed strongly at the oil-water interface and are able to stabilize emulsions of oil and water. In this work emulsions stabilized by magnetite nanoparticles were obtained using high-energy ultrasound waves and a cavitation mechanism and, next, their stability in time was tested by means of acoustic waves with a low energy, without affecting the structure. An acoustic study showed high stability in time of magnetic emulsions stabilized by magnetite particles. The study also showed a strong influence of an external magnetic field, which can lead to changes of the emulsion properties. It is possible to control Pickering emulsion stability with the help of an external stimulus—a magnetic field.
PubDate: Thu, 12 Nov 2015 11:09:04 +000
- Noise and Electrical Oscillations Generation during the Investigation of
the Resistive Switching in the Yttria Stabilized Zirconia Films by
Conductive Atomic Force Microscopy
Abstract: The effect of resistive switching in the yttria stabilized zirconia (YSZ) thin films on Si substrates has been studied by Conductive Atomic Force Microscopy (CAFM). The resistive switching of the YSZ films from the low conductive state to the highly conductive one has been found to be associated with the increasing of the noise with broad frequency spectrum related to the redistribution of the oxygen vacancies in YSZ. The electrical oscillations in oscillation loop connected in series to the CAFM probe, the sample, and the bias source related to the excitation of the oscillation loop by the noise in the probe-to-sample contact film have been observed. The effect discovered is promising for application in the memristor devices of new generation.
PubDate: Thu, 29 Oct 2015 12:41:16 +000
- A Process for Modelling Diffuse Scattering from Disordered Molecular
Crystals, Illustrated by Application to Monoclinic
Abstract: Diffuse scattering from a crystal contains valuable information about the two-body correlations (related to the nanoscale order) in the material. Despite years of development, the detailed analysis of single crystal diffuse scattering (SCDS) has yet to become part of the everyday toolbox of the structural scientist. Recent decades have seen the pair distribution function approach to diffuse scattering (in fact, total scattering) from powders become a relatively routine tool. However, analysing the detailed, complex, and often highly anisotropic three-dimensional distribution of SCDS remains valuable yet rare because there is no routine method for undertaking the analysis. At present, analysis requires significant investment of time to develop specialist expertise, which means that the analysis of diffuse scattering, which has much to offer, is not incorporated thorough studies of many compounds even though it has the potential to be a very useful adjunct to existing techniques. This article endeavours to outline in some detail how the diffuse scattering from a molecular crystal can be modelled relatively quickly and largely using existing software tools. It is hoped this will provide a template for other studies. To enable this, the entire simulation is included as deposited material.
PubDate: Tue, 27 Oct 2015 09:08:41 +000
- Rare Gas Adsorption to Silver-Exchanged Zeolites
Abstract: The adsorption of rare gas atoms to silver aluminosilicate has been investigated using density functional theory (DFT) with the local density approximation, generalized gradient approximation, and dispersion correction. The adsorption energies of rare gas atoms to the honeycomb lattice of silver aluminosilicate were calculated, and the results are discussed. The relationship between the electric charge density distribution and the adsorption energy is discussed. It indicates that the xenon atom has the most electrons to affect the van der Waals dispersion, so it has the highest minimum charge density, strongest polarization, most spacious spherical scope, and most favorable adsorption on silver zeolites.
PubDate: Thu, 22 Oct 2015 12:31:44 +000
- Influence of Electric Field in the Adsorption of Atomic Hydrogen on
Abstract: The influence of external electric field (EF) in the adsorption of atomic hydrogen on graphene (H/G) was studied by means of electronic structure calculations based on spin-polarized density functional theory with generalized gradient approximation (GGA). The changes in atomic hydrogen physisorption-chemisorption on graphene owed to EF (which ranged between −1.25 V/Å and 0.75 V/Å) were determined. Analysis of the electronic charge density for an H/G system explained the EF influences on the adsorption properties (analyzing changes in electronic charge density for H/G system). A decrease of more than 100% in the chemisorption barrier for an EF of −1.25 V/Å was found. The changes in the electronic charge density confirm the possibility of manipulating the physical-chemical adsorption of hydrogen on graphene by applying electric fields.
PubDate: Thu, 22 Oct 2015 08:28:20 +000
- Precipitation Behavior and Magnetic Properties of Cu-Fe-Co Alloys
Containing Nanogranular Ferromagnetic-Element Particles
Abstract: This work investigates the evolution of microstructures and magnetic properties during isothermal annealing of Cu-Fe-Co alloys, using electron microscopy and superconducting quantum interference device (SQUID) magnetometry. Small coherent granular precipitates composed of iron and cobalt formed in the copper matrix in the early stage of precipitation. As annealing proceeded, the precipitates lost coherency to the matrix after reaching a size of 15–20 nm and twin-like structures were consecutively introduced in the particles. The SQUID measurements revealed that the magnetic properties of the specimens correlated with the microstructural evolution. The coercive force initially increased with annealing time but decreased after reaching a peak. Lorentz Microscopy suggested that the initial large increase of magnetization was invoked by a structural transition from fcc to B2 in the precipitates.
PubDate: Tue, 20 Oct 2015 12:00:18 +000
- Superconductivity, Antiferromagnetism, and Kinetic Correlation in Strongly
Correlated Electron Systems
Abstract: We investigate the ground state of two-dimensional Hubbard model on the basis of the variational Monte Carlo method. We use wave functions that include kinetic correlation and doublon-holon correlation beyond the Gutzwiller ansatz. It is still not clear whether the Hubbard model accounts for high-temperature superconductivity. The antiferromagnetic correlation plays a key role in the study of pairing mechanism because the superconductive phase exists usually close to the antiferromagnetic phase. We investigate the stability of the antiferromagnetic state when holes are doped as a function of the Coulomb repulsion . We show that the antiferromagnetic correlation is suppressed as is increased exceeding the bandwidth. High-temperature superconductivity is possible in this region with enhanced antiferromagnetic spin fluctuation and pairing interaction.
PubDate: Tue, 13 Oct 2015 16:42:52 +000
- The Electronic Structure of Short Carbon Nanotubes: The Effects of
Abstract: This paper presents a tight binding and ab initio study of finite zig-zag nanotubes of various diameters and lengths. The vertical energy spectra of such nanotubes are presented, as well as their spin multiplicities. The calculations performed using the tight binding approach show the existence of quasi-degenerate orbitals located around the Fermi level, thus suggesting the importance of high-quality ab initio methods, capable of a correct description of the nondynamical correlation. Such approaches (Complete Active Space SCF and Multireference Perturbation Theory calculations) were used in order to get accurate ground and nearest excited-state energies, along with the corresponding spin multiplicities.
PubDate: Mon, 12 Oct 2015 07:35:37 +000
- Recent Approaches for Broadening the Spectral Bandwidth in Resonant Cavity
Abstract: Resonant cavity optoelectronic devices, such as vertical cavity surface emitting lasers (VCSELs), resonant cavity enhanced photodetectors (RCEPDs), and electroabsorption modulators (EAMs), show improved performance over their predecessors by placing the active device structure inside a resonant cavity. The effect of the optical cavity, which allows wavelength selectivity and enhancement of the optical field due to resonance, allows the devices to be made thinner and therefore faster, while simultaneously increasing the quantum efficiency at the resonant wavelengths. However, the narrow spectral bandwidth significantly reduces operating tolerances, which leads to severe problems in applications such as optical communication, imaging, and biosensing. Recently, in order to overcome such drawbacks and/or to accomplish multiple functionalities, several approaches for broadening the spectral bandwidth in resonant cavity optoelectronic devices have been extensively studied. This paper reviews the recent progress in techniques for wide spectral bandwidth that include a coupled microcavity, asymmetric tandem quantum wells, and high index contrast distributed Bragg-reflectors. This review will describe design guidelines for specific devices together with experimental considerations in practical applications.
PubDate: Wed, 07 Oct 2015 13:40:32 +000