Tuning Fermi-surface properties through quantum confinement in metallic metalattices: new metals from old atoms

We describe a new class of nanoscale structured metals wherein the effects of quantum confinement are combined with dispersive metallic electronic states to induce modifications to the fundamental low-energy microscopic properties of a three-dimensional metal: the density of states, the distribution of Fermi velocities, and the collective electronic response.     

Review on the properties of the ferrorelaxor polymers and some new recent developments

It has been found that by introducing defects into the P(VDF-TrFE) copolymers, it is possible to convert the polymer from a normal ferroelectric to a relaxor ferroelectric. A new class of ferroelectric polymers, i.e., the terpolymers of P(VDF-TrFE-CFE) or of P(VDF-TrFE-CTFE), was developed from the normal ferroelectric PVDF-TrFE polymer by employing proper defect modifications which eliminate detrimental effects associated with a normal first order F?CP transition while maintaining high material responses. Relevant studies show that this class of electroactive polymers offers unique properties in comparison with other ferroelectric polymers. The syntheses of these relaxor ferroelectric polymers have been done by a combination of the suspension polymerization process and an oxygen-activated initiator at a temperature of 40°C. Films from cast solution can be made in different lengths and thicknesses. Stretching of these films increases the performance as well as the mechanical properties. These relaxor-ferroelectric terpolymers P(VDF-TrFE-CFE), P(VDF-TrFE-CTFE) are multifunctional, i.e., electrostrictive material, dielectric for electric energy storage. The terpolymer exhibits high electrostrictive strain (>7%) with relatively high modulus (>0.4?GPa). Examples of devices applications using unimorphe systems are presented. Micropump and Optical device concerning a liquid-filled varifocal lens on a chip are described.     

Quantum confinement and electronic properties of silicon nanowires

We investigate the structural, electronic, and optical properties of hydrogen-passivated silicon nanowires along [110] and [111] directions with diameter d up to 4.2 nm from first principles. The size and orientation dependence of the band gap is investigated and the local-density gap is corrected with the GW approximation. Quantum confinement becomes significant for d<2.2 nm, where the dielectric function exhibits strong anisotropy and new low-energy absorption peaks start to appear in the imaginary part of the dielectric function for polarization along the wire axis.     

Optimal localization of measurements in physical and technological applications

The problem of optimal spatial arrangement of measurement detectors is considered. An approach is suggested, based on convex quality criteria and the application of effective algorithms of convex Boolean programming.V. D. Kuznetsov Siberian Physicotechnical Institute at the Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 143–148, September, 1992.     

Toward a physical basis of attention and self-regulation

The concept of self-regulation is central to the understanding of human development. Self-regulation allows effective socialization and predicts both psychological pathologies and levels of achievement in schools. What has been missing are neural mechanisms to provide understanding of the cellular and molecular basis for self-regulation. We show that self-regulation can be measured during childhood by parental reports and by self-reports of adolescents and adults. These reports are summarized by a higher order factor called effortful control, which reflects perceptions about the ability of a given person to regulate their behavior in accord with cultural norms. Throughout childhood effortful control is related to children's performance in computerized conflict related tasks. Conflict tasks have been shown in neuroimaging studies to activate specific brain networks of executive attention. Several brain areas work together at rest and during cognitive tasks to regulate competing brain activity and thus control resulting behavior. The cellular structure of the anterior cingulate and insula contain cells, unique to humans and higher primates that provide strong links to remote brain areas. During conflict tasks, anterior cingulate activity is correlated with activity in remote sensory and emotional systems, depending upon the information selected for the task. During adolescence the structure and activity of the anterior cingulate has been found to be correlated with self-reports of effortful control.Studies have provided a perspective on how genes and environment act to shape the executive attention network, providing a physical basis for self-regulation. The anterior cingulate is regulated by dopamine. Genes that influence dopamine levels in the CNS have been shown to influence the efficiency of self-regulation. For example, alleles of the COMT gene that influence the efficiency of dopamine transmission are related to the ability to resolve conflict. Humans with disorders involving deletion of this gene exhibit large deficits in self-regulation. Alleles of other genes influencing dopamine and serotonin transmission have also been found to influence ability to resolve conflict in cognitive tasks. However, as is the case for many genes, the effectiveness of COMT alleles in shaping self-regulation depends upon cultural influences such as parenting. Studies find that aspects of parenting quality and parent training can influence child behavior and the efficiency of self-regulation.During development, the network that relates to self-regulation undergoes important changes in connectivity. Infants can use parts of the self-regulatory network to detect errors in sensory information, but the network does not yet have sufficient connectivity to organize brain activity in a coherent way. During middle childhood, along with increased projection cells involved in remote connections of dorsal anterior cingulate and prefrontal and parietal cortex, executive network connectivity increases and shifts from predominantly short to longer range connections. During this period specific exercises can influence network development and improve self-regulation. Understanding the physical basis of self-regulation has already cast light on individual differences in normal and pathological states and gives promise of allowing the design of methods to improve aspects of human development.     

Quantum confinement and electronic properties of tapered silicon nanowires

Using ab initio calculations, structural tapering of silicon nanowires is shown to have a profound effect on their electronic properties. In particular, the electronic structure of small-diameter tapered silicon nanowires is found to have a strong axial dependence, with unoccupied eigenstates being substantially more sensitive to diameter. Moreover, the states corresponding to the highest occupied and the lowest unoccupied states are spatially separated along the wire axis by the tapering-induced charge transfer and a strong electrostatic potential gradient, due to an appreciable variation in quantum confinement strength with diameter.     

Load-stepping photoelasticity: new developments using temporal phase unwrapping

A new approach to load-stepping photoelasticity is presented in which the isochromatic phase is unwrapped between load steps rather than spatially. This is based on the technique known as ‘temporal phase unwrapping’ that has mainly been applied in shape measurement. The potential advantages of temporal phase unwrapping are based on the notion that each pixel is unwrapped independently of its neighbours. The benefits for load-stepping photoelasticity for reducing the effects of measurement error are discussed in this paper. New developments using temporal phase unwrapping are applied to experimental data obtained from a bimaterial interface specimen loaded in three-point bend. The results show that a modified approach to unwrapping significantly reduces the effects of systematic noise present in the data.     

First-principles investigations of the structure and physical properties for new TcN crystal structure

Using a newly developed particle swarm optimisation technique on crystal structural prediction, we have predicted an orthorhombic Imm2 structure for TcN crystal, which is energetically much superior to the previously proposed NbO-type and R-3m structures. The new phase is stable against decomposition into the mixture of Tc and N at ambient condition. The stability is confirmed by the subsequent calculations on the phonon dispersion curves and elastic constants. An analysis of the mechanical properties indicates that Imm2-TcN is incompressible and common hard material. The evidence of strong covalent bonding of Tc-N, which plays a leading role to form a hard material, is manifested by the partial densities of state analysis. In addition, the thermodynamic properties, such as Debye temperature, heat capacity, thermal expansion coefficient, and Grüneisen parameter of TcN are investigated by the quasi-harmonic Debye model.     

Quark confinement,new cosmic expansion and general Yang-Mills symmetry

We discuss a unified model of quark confinement and new cosmic expansion with linear potentials based on a general(SU_3)_(color)×(U_1)baryon symmetry. The phase functions in the usual gauge transformations are generalized to new ‘action integrals'. The general Yang-Mills transformations have group properties and reduce to usual gauge transformations in special cases. Both quarks and ‘gauge bosons' are permanently confined by linear potentials. In this unified model of particle-cosmology, physics in the largest cosmos and that in the smallest quark system appear to both be dictated by the general Yang-Mills symmetry and characterized by a universal length. The basic force between two baryons is independent of distance. However, the cosmic repulsive force exerted on a baryonic supernova by a uniform sphere of galaxies is proportional to the distance from the center of the sphere. The new general YangMills field may give a field-theoretic explanation of the accelerated cosmic expansion. The prediction could be tested experimentally by measuring the frequency shifts of supernovae at different distances.     

Synthesis, structural and physical properties of new Terbium containing Tb-Hg-Sr-Ca-Cu-O superconducting system

Samples with various nominal compositions in the Tb-Hg-Sr-Ca-Cu-O system were prepared and studied by EDX, powder X-ray diffraction including the Rietveld refinement, electrical resistivity, magnetic susceptibility and thermoelectric power measurements. EDX and powder X-ray diffraction studies showed that Tb is required for the stabilization of the 1212, (Hg_1−yTb_y)Sr₂TbCu₂O_6+δ; y≈0.5 phase. Electrical resistivity and magnetic susceptibility measurements indicated that substitution of Tb by Ca is necessary to induce superconductivity in the 1212, (Hg_0.5Tb_0.5)Sr₂(Tb_1−xCa_x)Cu₂O_6+δ samples. The Rietveld refinements of the X-ray data of two samples with x=0.0 and 0.5 were carried out on the basis of tetragonal symmetry (space group P4/mmm) and the results indicated that the phase with x=0.5 has less puckered Cu-O planes than the Ca-free (Hg_0.5Tb_0.5)Sr₂TbCu₂O_6+δ phase. Syperconductivity is observed only for samples with x>0.2 and T_c increases with increasing Ca content, x. The results of thermoelectric power measurements suggest that the samples with x<0.8 are located in the underdoped region and the x=0.8 sample is optimally doped and exhibits the highest T_c of 88 K.     

High-pressure synthesis and physical properties of new iron (nickel)-based superconductors

We have utilized a high-pressure (HP) technique to synthesize a series of newly-discovered iron (nickel)-based superconductors. For the LnFeAsO-based superconductors (Ln = lanthanide), we show that the introduction of oxygen (O)-deficiency in the LnO layers, which is achievable only through HP process, is an effective way to dope electron carriers into the system, which results in yielding the superconducting transition temperature (T_c) comparable with those for F-substituted counterpart. The effect of O-deficiency, variation of Ln ions, and the external pressure on T_c are examined. All the experimental data indicate strong correlation between the crystal structure and the superconductivity of the oxypnictide superconductors. Upper critical field measurement on single crystalline sample of PrFeAsO_1?y shows the superconducting anisotropy of 5, which is smaller than cuprates. We also demonstrate that HP technique is applicable for the so-called ‘122’ systems, by showing the results on polycrystalline (Ca, Na)Fe₂As₂, (Ba, K)Fe₂As₂, as well as single crystal BaNi₂P₂ samples.     

A new confinement potential in spherical quantum dots: Modified Gaussian potential

A new confinement potential for spherical quantum dots, called the modified Gaussian potential (MGP), is studied. In the present work, the following problems are investigated within the effective-mass approximation: (i) the one-electron energy spectra, (ii) wave functions, (iii) the problem of existence of a bound electron state, and (iv) the binding energy of center and off-center hydrogenic donor impurities. For zero angular momentum (l=0)$(l=0)$, the new confinement potential is sufficiently flexible to obtain analytically the spectral energy and wave functions. The results obtained from the present work show that (i) the new potential is suitable for predicting the spectral energy and wave functions, and (ii) the geometrical sizes of the quantum dot play the important roles on the energy levels, wave functions, the binding energy, and the existence of a bound electron state.     

Quantum confinement and disorder in porous silicon: Effects on the optical and transport properties

Summary The striking optical properties of porous silicon (PS) show a twofold aspect typical of an ordered and a disordered material, respectively. Raman, electron microscopy, and resonant photoluminescence studies indicate that the light emission originates from crystalline regions. On the contrary, several features, like the non-exponential decay of photoluminescence (PL), the broad emission spectrum, the photoluminescence fatigue under light exposure etc. are typical of a disordered material and reminiscent of similar effects founde.g. in amorphous semiconductors. These twoapparently conflicting aspects have for a long time hindered the understanding of the basic light emission mechanism. In this paper we report new optical data showing that disorder in porous silicon leads to strong carrier localisation. Light emission in PS is suggested to occur through transitions involving localized states. Paper presented at the III INSEL (Incontro Nazionale sul Silicio Emettitore di Luce), Torino, 12–13 October 1995.     

Electronic properties of SiC surfaces and interfaces: some fundamental and technological aspects

The wide band gap semiconductor silicon carbide (SiC) is the first-choice material for power electronic devices operating at high voltages, high temperatures, and high switching frequencies. Due to their importance for crystal growth, processing, and device fabrication, the electronic properties of SiC surfaces and interfaces to other materials such as metals and dielectrics are of particular interest. Unreconstructed, H-terminated SiC surfaces which are passivated in a chemical as well as an electronic sense are obtained in a thermal hydrogenation process. It is demonstrated that deposition of Al₂O₃ on H-terminated SiC(0001) leads to an interface which is lower in defects than the thermally grown SiO₂/SiC interface. Furthermore, starting from hydrogenated SiC{0001} surfaces it is possible to prepare unreconstructed (1×1) surfaces with one dangling bond per unit cell. These surfaces show indications for strong electron correlation effects. PACS 68.47.Fg; 73.20.At; 79.60.Bm; 68.35.Bs; 68.35.Dv     

Synthesis,physical properties,and annealing investigation of new layered Bi-chalcogenide LaOBiHgS_3

The transport and thermoelectric properties together with annealing of the new layered Bi-chalcogenide LaOBiHgS_3 are studied. On the transport part, the insulating behavior of the as-grown sample is evidently depressed by post annealing.A hump-like abnormality appears around 170 K. The thermoelectric performance of the sample is observably improved by the annealing, mainly because of the enhanced electrical conductance. The present results suggest that the physical properties of LaOBiHgS_3 are sensitive to post annealing and the possible micro adjustments that follow, indicating the layered Bi-chalcogenide family to be an ideal platform for designing novel functional materials.     

高分辨率X射线显微成像及其进展

介绍了高分辨率X射线显微成像产生背景和发展过程，着重分析了基于光学元件波带片的放大成像的基本原理，并简述了高分辨率三维成像的有关理论。同时给出国内外高分辨率X射线显微成像研究的最新进展，展望了高分辨率X射线显微成像的应用前景。