Diamagnetic properties of the electron-exciton bound states in charge-transfer organic solids

An effective Hamiltonian is established which describes the excitation spectrum of independent quasiparticles arising from the coherent electron-exciton pairing in molecular solids of the charge-transfer type. This Hamiltonian is then used to study the interaction of the quasiparticles with an external transverse electromagnetic field. An expression for the dielectric function is derived which describes the dielectric properties of the system. In the presence of electron-exciton pairing and in the long-wavelength limit, the diamagnetic current is finite and is of the London type, indicating the existence of the Meissner effect. The corrections to the London value for the diamagnetic current depend on the magnitudes of the binding energy of the exciton, the gap function, and the energy difference between the excited and ground states of the system. In the absence of electron-exciton pairing, the diamagnetic current vanishes and the dielectric function corresponds to that for the exciton band. Issued as NRCC No. 14528. This paper is dedicated to Professor C. A. Winkler of McGill University on the occasion of his sixty-fifth birthday by one of his former students (C.M.).     

Electron spin localisation and correlation effects for point defects in semi-ionic solids

Point defects strongly influence the electrical, catalytic and optical properties of important industrial materials such as silica, aluminosilicates and aluminophosphates. We concentrate here on the predictive ability of modern ab initio solid-state tools to describe electron localised states in these materials. We apply a periodic boundary conditions approach to investigate equilibrium configurations and electronic structures of oxygen vacancies in the form of diamagnetic V_O and paramagnetic E^′ centres. Localisation of the electronic states at the defect is studied using both Hartree–Fock (HF) and Density Functional theories. Thus, we are able to characterise the effect of electron correlation on spin polarisation and electron trapping. Geometries and energetics of defect formation are investigated, along with the one-electron spectra and spin/charge density distributions, which allow us to gather a comprehensive representation of the defect ground state.     

Atomic structure of point defects in compound semiconductor surfaces

The present work reviews the progress in the determination and understanding of the atomic structure of point defects and dopant atoms exposed in and below cleavage surfaces of III–V semiconductors during recent years. By critically evaluating the present experimental data and theoretical concepts, we discuss the methods of identification of the types of defects and dopant atoms, the determination of defect energy levels, electrical charge states, as well as lattice relaxation, and the deduction of the physical mechanisms governing the interactions between different defects and/or dopant atoms, the formation of defect complexes, the compensation of dopant atoms, the pinning of the Fermi-level, and the stability of defects. Finally, the methodology to extract the concentrations and types of bulk defects and the physics governing bulk defects is examined.     

Silicon nitride crystal structure and observations of lattice defects

In view of the considerable progress that has been made over the last 40 years on the microstructural design of silicon nitride and related materials of tailored properties for specific applications, a clear review of the current understanding of the crystal structure and crystal chemistry of silicon nitride is timely. The crystal structures, crystal chemistry, and lattice defect nature of silicon nitride are critically reviewed and discussed, with emphasis placed firstly on the structural nature of -silicon nitride (whether it is a pure silicon nitride, or should better be regarded as an oxynitride); and secondly on the space group of -silicon nitride (whether it is P6₃/m or P6₃). In conjunction with recent observations of vacancy clusters in -silicon nitride, a comprehensive view compatible with all the experimental facts with respect to the structural nature of -silicon nitride is tentatively presented.     

Effects of electrically active point defects on the structure and electrical properties of Bi-Te-Se and Sb-Bi-Te solid solutions

The effects of intrinsic point defects on the electrical properties and structure ofp-type Sb-Bi-Te and n-type Bi-Te-Se alloys were studied. The concentrations of intrinsic donors and acceptors were shown to depend not only on the alloy composition but also on the self-compensation conditions in the materials doped with Group IV (Si, Ge, Sn, Pb) and VII (CI, I) elements. The Te-rich alloys doped with Cl or Pb to optimal carrier concentration show the best thermoelectric performance.     

Site-controlled formation of InGaAs quantum nanostructures-Tailoring the dimensionality and the quantum confinement

We report on InGaAs quantum disks (QDks) controllably formed on the top (001) facet of nano-patterned GaAs pyramidal platforms. The QDks exhibit pyramidal shape with special facets and varied dimensions, depending on the GaAs pyramidal buffer and the amount of InGaAs deposited. The formation of QDks is explained by the overgrowth of an InGaAs layer and thereafter coalescence of small InGaAs islands. Photoluminescence (PL) characteristics of ensemble QDks and exciton features of individual QDks together demonstrate that we may achieve a transition from zero-dimensional (0D) to two-dimensional (2D) quantum structure with increasing QDk size. This transition provides the flexibility to continuously tailor the dimensionality and subsequently the quantum confinement of semiconductor nanostructures via site-controlled self-assembled epitaxy for device applications based on single quantum structures.      

Dynamic theory of defects and creep in solids

The dynamic equations of the field theory of defects are used to analyze the creep characteristics. A number of interesting results are obtained, of which some are consistent with the well-known experimental data, whereas others are quite new. In particular, the existence of a critical value of the applied stress and an unstable steady-state creep rate are established, which separate the stable and unstable creep modes and can be of great practical importance.     

Effects of alloying in the dilute limit on the quantum states of electrons in magnesium

The electron quantum interference phenomenon was used to determine the effects of substitutional alloying upon the quantum-state lifetime and band gapsE _g at the Fermi energy in single crystals of pure Mg. Vapor-grown alloys containing either Zn or Cd in concentrationsC0.3–15 ppm were studied. The magnetic field dependence of the interference oscillation amplitudes for these samples indicates reductions in and significant increases inE _g relative to pure Mg (impurity concentration 10^–8). Within experimental accuracy the observed quantum-state lifetime satisfies the relation ^–1=C, with 2.7×10¹⁰ sec^–1 per ppm of Cd and 2×10¹¹ sec^–1 per ppm of Zn. The band gap corresponding to Bragg reflection from the (0001) plane of the hcp structure (which in pure Mg arises solely from spin-orbit coupling) was found to increase by more than a factor of two upon the addition of only 10 ppm Cd to Mg. These results are discussed within the framework of the pseudopotential theory of alloying. It is shown that there are discrepancies of more than three orders of magnitude between the experimental and theoretical values forE _g and of about one order of magnitude for in these dilute-limit alloys.Work supported by the National Science Foundation.Submitted to the Department of Physics, the University of Chicago in partial fulfillment of the requirements for the degree of Doctor of Philosophy.Fannie and John Hertz Foundation Fellow.     

Anisotropic diffusion of point defects in a two-dimensional crystal of streptavidin observed by high-speed atomic force microscopy

The diffusion of individual point defects in a two-dimensional streptavidin crystal formed on biotin-containing supported lipid bilayers was observed by high-speed atomic force microscopy. The two-dimensional diffusion of monovacancy defects exhibited anisotropy correlated with the two crystallographic axes in the orthorhombic C 222 crystal; in the 2D plane, one axis (the a-axis) is comprised of contiguous biotin-bound subunit pairs whereas the other axis (the b-axis) is comprised of contiguous biotin-unbound subunit pairs. The diffusivity along the b-axis is approximately 2.4?times larger than that along the a-axis. This anisotropy is ascribed to the difference in the association free energy between the biotin-bound subunit-subunit interaction and the biotin-unbound subunit-subunit interaction. The preferred intermolecular contact occurs between the biotin-unbound subunits. The difference in the intermolecular binding energy between the two types of subunit pair is estimated to be approximately 0.52?kcal?mol(-1). Another observed dynamic behavior of point defects was fusion of two point defects into a larger defect, which occurred much more frequently than the fission of a point defect into smaller defects. The diffusivity of point defects increased with increasing defect size. The fusion and the higher diffusivity of larger defects are suggested to be involved in the mechanism for the formation of defect-free crystals.     

Effects of precursors on the crystal structure and photoluminescence of CdS nanocrystalline

A series of cadmium sulfide (CdS) nanocrystalline were synthesized by precipitation from a mixture of aqueous solutions of cadmium salts and sulfur salts without adding any surface-termination agent. Their crystal structures and particle sizes were determined by X-ray diffraction (XRD). The CdS nanocrystalline precipitated from different precursors exhibited three cases: cubic phase, hexagonal phase and a hybrid of cubic and hexagonal phases. The photoluminescence (PL) of cadmium salt precursors and CdS nanocrystalline is also analyzed. Similar spectral band structure of cadmium salt precursors and CdS nanocrystalline is found. The PL of 3.4, 2.4 and 2.0 nm sized CdS nanocrystalline with the same crystal structure indicated quantum confinement effect.     

晶体面缺陷对FeS2薄膜电学性能的影响

采用Fe膜热硫化技术制备了具有不同比表面积和比晶界面积的FeS2薄膜,并测定其载流子浓度和电阻率,研究了FeS2薄膜表面和晶界等面缺陷对FeS2薄膜电学性能的影响.结果表明,表面和晶界两种面缺陷对FeS2薄膜的电学性能有类似的影响规律.在一定范围内,随着薄膜比表面积和比晶界面积的增大,载流子浓度提高而电阻率下降.面缺陷数量的变化可导致FeS2晶体中点缺陷数量、禁带中缺陷能级密度、不充分相变产物比例和相变应力水平的变化,从而导致载流子浓度和电阻率的变化.     

Effects of spin-orbit interaction and crystal fields on superconductingp-wave pair states and their collective excitations in cubic systems

Superconducting pair states for systems with strong spin-orbit interaction and crystallographic anisotropy are usually assumed to transform as the basis functions of the irreducible representations _n of the appropriate point group. Transitions to pair states belonging to different irreducible representations are supposed to occur at different transition temperaturesT _c ( _n ). Here it is shown forp-wave states that, as soon as nonlinear terms in the gap equation or quartic terms in the Ginzburg-Landau free energy are taken into account, linear combinations of pair states belonging to different irreducible representations can result. This applies to all those states that are not solutions of the gap equation when the pairing interaction is of the isotropic form V₀ used in the theory of superfluid³He. The spectrum of collective excitations of those states, which are actually solutions of the gap equation, becomes much more complex when spin-orbit interaction and crystal field effects are taken into account by assuming different transition temperaturesT _c ( _n ). In particular, a number of additional modes are found whose frequencies scale as ² ln[T _c ( _n )/T _c ( _m )]. If the deviation from the isotropic interaction is not very large, these modes will have very low frequencies. They couple to density fluctuations toO(q²) due to particle-hole asymmetry. This can be a large effect in heavy fermion compounds because of the sharp peak in the density of states. These modes could, therefore, be very well responsible for the peaks in the acoustic attenuation observed just belowT _c .     

Spin diffusion in 2D and 3D quantum solids

A moment method is used to compute the anisotropic spin diffusion constant in two-dimensional (2D) adsorbed and bulk (3D) quantum solids in which the spin motion is induced by an exchange Hamiltonian. Computations are carried out in 2D for the square and triangular lattices and in 3D for the hcp lattice. It is assumed that there are pair and three-particle exchange processes only. Since, in hcp³He, exchange processes out of the basal planeJ may occur at a different rate from processes in the planeJ, comparison with experimental results on single crystals should allow the determination ofJ andJ. Our results are given as functions of the ratioy=J/J and of the angle betweenc axis and field gradient. The 2D triangular lattice is shown to correspond to the special casey=0 of the hcp lattice. Our square-lattice result compares well with that of Morita (who used a different technique), supporting the validity of our method.     

A simple quantum mechanical model for the contribution of electronic surface states to surface stress, strength and electrocapillarity of solids

Between the fatigue limit of solids and the surface stress an inherent relation is supposed. Using the simple quantum mechanical model of Kronig and Penney for the energy bands in solids and the model of Tamm for the electronic surface states, with analytical relations a close connection is demonstrated between the strength of solids and the quantities ‘surface free energy’ and ‘surface stress’. Due to the electrocapillarity, it can be deduced an influence of electric fields on the strength of solids.     

On Relaxation of point defects in metals

Methods for studying the phenomenon of relaxation of point defects (thermal vacancies) and their influence on properties of metals are reviewed. For refractory metals, an equation for the temperature dependence of the relaxation time of an equilibrium concentration of thermal vacancies and calculation of this time at the melting temperatures of metals is proposed. The possible methods for determining the contribution of vacancies to the enthalpy are considered in the case of using a pulsed electric current for bulk heating of metals. The heating rates reached in the pulsed method, at which overheating of the metal-melting onset is observed and the relaxation phenomenon in the premelting region can be studied, are estimated.     

Investigation of point defects in TiAl alloy

The many-body potentials of TiAl alloy were constructed by empirical fitting to the properties of the L1₀TiAl tetragonal phase and the pressure–volume relationship. The properties and stable configurations for point defects and their aggregates, and the activation energies for vacancy migration are simulated in TiAl intermetallic compounds using molecular dynamics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mixture of two-mode unpolarized and pure quantum light states: quantum polarization and application in quantum communication

Quantum polarization is an important property that has been extensively used for quantum communication purposes. In this work, the mixture of an unpolarized two-mode state and a two-mode pure state is analysed. This mixture is controlled by a single parameter, namely the degree of purity. The quantum polarization of the two-mode mixed state is discussed using the quantum Stokes parameters and the quantum degree of polarization. A relation between the degree of purity and the quantum degree of polarization is established. The mixture is used to model coherent light amplified by an erbium-doped fibre amplifier, which focuses the loss of polarization due to the unpolarized light resulting from spontaneous emission. Finally, the use of such a mixed state in the quantum key distribution using multiphoton pulses is analysed.     

Identifying and counting point defects in carbon nanotubes

The prevailing conception of carbon nanotubes and particularly single-walled carbon nanotubes (SWNTs) continues to be one of perfectly crystalline wires. Here, we demonstrate a selective electrochemical method that labels point defects and makes them easily visible for quantitative analysis. High-quality SWNTs are confirmed to contain one defect per 4 microm on average, with a distribution weighted towards areas of SWNT curvature. Although this defect density compares favourably to high-quality, silicon single-crystals, the presence of a single defect can have tremendous electronic effects in one-dimensional conductors such as SWNTs. We demonstrate a one-to-one correspondence between chemically active point defects and sites of local electronic sensitivity in SWNT circuits, confirming the expectation that individual defects may be critical to understanding and controlling variability, noise and chemical sensitivity in SWNT electronic devices. By varying the SWNT synthesis technique, we further show that the defect spacing can be varied over orders of magnitude. The ability to detect and analyse point defects, especially at very low concentrations, indicates the promise of this technique for quantitative process analysis, especially in nanoelectronics development.     

Ultrasonic evaluation of geometrical and surface parameters of rough defects in solids

Ultrasonic measurements from rough cracks were carried out using both broad-band and narrow-band methods. An analysis is suggested to determine parameters of the crack quantitatively such as size, shape, rms surface roughness, and distribution function of the surface roughness. Ultrasonic measurements of the parameters compare very well with the actual parameters of the defect.