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During the course of this grant, research focused on using available theoretical tools to explore and elucidate complex dynamics of impurities in Si. In particular, we studied clustering of As in heavily doped Si, segregation of As in Si grain boundaries and the migration of oxygen in Si. In addition we carried out atomic scale nonequilibruim heat flow simulations and mesoscopic calculations of void growth in polycrystalline metal lines.

During the course of this grant, research focused on using available theoretical tools to explore and elucidate complex dynamics of impurities in Si. In particular, we studied clustering of As in heavily doped Si, segregation of As in Si grain boundaries and the migration of oxygen in Si. In addition we carried out atomic scale nonequilibruim heat flow simulations and mesoscopic calculations of void growth in polycrystalline metal lines.

A surface potential model is developed to explain dopant effects on chemical vapor deposition. Auger analysis of the interaction between allotropic forms of carbon and silicon films has shown Si-C formation for all forms by glassy carbon. LEED intensity measurements have been used to determine the mean square displacement of surface atoms of silicon single crystals, and electron loss spectroscopy has shown the effect of structure and impurities on surface states located within the band gap. A thin film of Al has been used to enhance film crystallinity at low temperature.

A surface potential model is developed to explain dopant effects on chemical vapor deposition. Auger analysis of the interaction between allotropic forms of carbon and silicon films has shown Si-C formation for all forms by glassy carbon. LEED intensity measurements have been used to determine the mean square displacement of surface atoms of silicon single crystals, and electron loss spectroscopy has shown the effect of structure and impurities on surface states located within the band gap. A thin film of Al has been used to enhance film crystallinity at low temperature.

We give a detailed description of a recently proposed first principles approach to the electronic structure of strongly correlated materials. The method combines the GW approximation with dynamical mean field theory. It is designed to describe Coulomb interactions and screening effects without adjustable parameters, thus avoiding the conceptual problems inherent to LDA+DMFT techniques.

It is known from self-energy calculations in the electron gas and sp materials based on the GW approximation that a typical quasiparticle renormalization factor (Z factor) is approximately 0.7-0.8. Band narrowing in electron gas at rs = 4 due to correlation effects, however, is only approximately 10%, significantly smaller than the Z factor would suggest. The band narrowing is determined by the frequency-dependent self-energy, giving the Z factor, and the momentum-dependent or nonlocal self-energy. The results for the electron gas point to a strong cancellation between the effects of frequency- and momentum-dependent self-energy. It is often assumed that for systems with a nar- row band the self-energy is local. In this work we show that even for narrow-band materials, such as SrVO3, the nonlocal self-energy is important.

It is known from self-energy calculations in the electron gas and sp materials based on the GW approximation that a typical quasiparticle renormalization factor (Z factor) is approximately 0.7-0.8. Band narrowing in electron gas at rs = 4 due to correlation effects, however, is only approximately 10%, significantly smaller than the Z factor would suggest. The band narrowing is determined by the frequency-dependent self-energy, giving the Z factor, and the momentum-dependent or nonlocal self-energy. The results for the electron gas point to a strong cancellation between the effects of frequency- and momentum-dependent self-energy. It is often assumed that for systems with a nar- row band the self-energy is local. In this work we show that even for narrow-band materials, such as SrVO3, the nonlocal self-energy is important.

The surface structure of phosphorene crystals materials is determined using surface sensitive dynamical micro-spot low energy electron diffraction ({\mu}LEED) analysis using a high spatial resolution low energy electron microscopy (LEEM) system. Samples of (\textit{i}) crystalline cleaved black phosphorus (BP) at 300 K and (\textit{ii}) exfoliated few-layer phosphorene (FLP) of about 10 nm thicknes, which were annealed at 573 K in vacuum were studied. In both samples, a significant surface buckling of 0.22 {\AA} and 0.30 {\AA}, respectively, is measured, which is one order of magnitude larger than previously reported. Using first principle calculations, the presence of surface vacancies is attributed not only to the surface buckling in BP and FLP, but also the previously reported intrinsic hole doping of phosphorene materials.

Ab initio electronic structure calculations of two-dimensional layered structures are typically performed using codes that were developed for three-dimensional structures, which are periodic in all three directions. The introduction of a periodicity in the third direction (perpendicular to the layer) is completely artificial and may lead in some cases to spurious results and to difficulties in treating the action of external fields. In this paper we develop a new approach, which is "native" to quasi-2D materials, making use of basis function that are periodic in the plane, but atomic-like in the perpendicular direction. We show how some of the basic tools of ab initio electronic structure theory -- density functional theory, GW approximation and Bethe-Salpeter equation -- are implemented in the new basis. We argue that the new approach will be preferable to the conventional one in treating the peculiarities of layered materials, including the long range of the unscreened Coulomb interaction in insulators, and the effects of strain, corrugations, and external fields.

Layered transition metal dichalcogenide (LTMD) compounds constitute a class of materials with unique and unusual properties based on the extreme degree of anisotropy in their layered crystal structures. LTMDs are formed by stacking sandwiches consisting of a layer of transition metal between two layers of chalcogen. There is strong covalent bonding within the sandwiches but weak (van der Waals') bonding between them. Details of the exact crystal type within this general structure vary and result in some LTMDs being very good lubricants and others being poor lubricants or abrasives. Coincidentally, the electrical properties vary greatly also, ranging from insulators to semiconductors, and finally to metals with evidence of superconductivity in some of the class members. Keywords: Electronic structure; Molybdenum disulfide; Solid lubricants; Tribology; Materials properties; Thin films.

Layered transition metal dichalcogenide (LTMD) compounds constitute a class of materials with unique and unusual properties based on the extreme degree of anisotropy in their layered crystal structures. LTMDs are formed by stacking sandwiches consisting of a layer of transition metal between two layers of chalcogen. There is strong covalent bonding within the sandwiches but weak (van der Waals') bonding between them. Details of the exact crystal type within this general structure vary and result in some LTMDs being very good lubricants and others being poor lubricants or abrasives. Coincidentally, the electrical properties vary greatly also, ranging from insulators to semiconductors, and finally to metals with evidence of superconductivity in some of the class members. Keywords: Electronic structure; Molybdenum disulfide; Solid lubricants; Tribology; Materials properties; Thin films.

X-ray absorption spectroscopy (XAS) is one of the most widely used experimental techniques to study the electronic and spatial structure of materials. Fluorescence yield mode is bulk-sensitive, but has several serious problems coming from saturation effects. In this study, we show the usefulness of partial fluorescence yields in addressing these problems. We discuss the different behaviors of La2NiMnO6 and LiMnO2 at the Mn 2p absorption edges. The total fluorescence yield produces misleading spectra for LiMnO2 due to the absence of high-Z (Z: atomic number) elements. We conclude that the measurement of the inverse partial fluorescence yield is essential in studies of LiMnO2, which is a hotly debated Li-ion battery material.

This article is from Chemical Communications (Cambridge, England) , volume 49 . Abstract A new modified paullone ligand bearing a TEMPO free-radical unit (HL2) and its copper(ii) complex have been prepared. The compounds demonstrate high cytotoxicity in vitro and strongly inhibit cell-free hR2 RNR activity.

This article is from Chemical Communications (Cambridge, England) , volume 49 . Abstract A new modified paullone ligand bearing a TEMPO free-radical unit (HL2) and its copper(ii) complex have been prepared. The compounds demonstrate high cytotoxicity in vitro and strongly inhibit cell-free hR2 RNR activity.

We present a method for calculating the electronic structure of correlated materials based on a truly first-principles LDA+U scheme. Recently we suggested how to calculate U from first-principles, using a method which we named constrained RPA (cRPA). The input is simply the Kohn-Sham eigenfunctions and eigenvalues obtained within the LDA. In our proposed self-consistent LDA+U scheme, we calculate the LDA+U eigenfunctions and eigenvalues and use these to extract U. The updated U is then used in the next iteration to obtain a new set of eigenfunctions and eigenvalues and the iteration is continued until convergence is achieved. The most significant result is that our numerical approach is indeed stable: it is possible to find the effective exchange and correlation interaction matrix in a self-consistent way, resulting in a significant improvement over the LDA results, regarding both the bandgap in NiO and the f-band exchange spin-splitting in Gd, but some discrepancies still remain.

The electronic structures of the ground state for several different superconducting materials, such as cuprates, conventional 3-dimensional superconductors, doped semiconductors and low-dimensional systems, are quite different and sometimes in contrast to what is supposed to make a superconductor. Properties like the Fermi-surface (FS) topology, density-of-states (DOS), stripes, electron-phonon coupling ($\lambda_{ep}$) and spin fluctuations ($\lambda_{sf}$) are analyzed in order to find clues to what might be important for the mechanism of superconductivity. A high DOS at $E_F$ is important for standard estimates of $\lambda's$, but it is suggested that superconductivity can survive a low DOS if the FS is simple enough. Superconducting fluctuations are plausible from coupling to long wave length modes in underdoped cuprates, where short coherence length is a probable obstacle for long-range superconductivity. Thermal disorder is recognized as a limiting factor for large $T_C$ independently of doping.

We review a recent proposal of a first principles approach to the electronic structure of materials with strong electronic correlations. The scheme combines the GW method with dynamical mean field theory, which enables one to treat strong interaction effects. It allows for a parameter-free description of Coulomb interactions and screening, and thus avoids the conceptual problems inherent to conventional ``LDA+DMFT'', such as Hubbard interaction {\it parameters} and double counting terms. We describe the application of a simplified version of the approach to the electronic structure of nickel yielding encouraging results. Finally, open questions and further perspectives for the development of the scheme are discussed.

Reduced dimensionality and strong electronic correlations, which are among the most important ingredients for cupratelike high-Tc superconductivity, characterize also the physics of nickelate-based heterostructures. Starting from the local-density approximation we arrive at a simple two-band model for quasi-two-dimensional 2D LaNiO3 /LaAlO3 heterostructures and extend it by introducing an appropriate hopping in the z direction to describe the dimensional crossover to three dimensions 3D. Using dynamical mean-field theory, we study the effects of electronic correlations with increasing interaction strength along the crossover from 2D to 3D. Qualitatively, the effects of electronic correlations are surprisingly similar, albeit quantitatively larger interaction strengths are required in three dimensions for getting a Mott-Hubbard insulating state. The exchange parameters of an effective Kugel-Khomskii-type spin-orbital model are also derived and reveal strong antiferromagnetic tendencies.

We present a comprehensive first-principles study of the electronic structure of 51 semiconducting monolayer transition metal dichalcogenides and -oxides in the 2H and 1T hexagonal phases. The quasiparticle (QP) band structures with spin-orbit coupling are calculated in the $G_0W_0$ approximation and comparison is made with different density functional theory (DFT) descriptions. Pitfalls related to the convergence of $GW$ calculations for 2D materials are discussed together with possible solutions. The monolayer band edge positions relative to vacuum are used to estimate the band alignment at various heterostructure interfaces. The sensitivity of the band structures to the in-plane lattice constant is analysed and rationalized in terms of the electronic structure. Finally, the $q$-dependent dielectric functions and effective electron/hole masses are obtained from the QP band structure and used as input to a 2D hydrogenic model to estimate exciton binding energies. Throughout the paper we focus on trends and correlations in the electronic structure rather than detailed analysis of specific materials. All the computed data is available in an open database.

In this work we have studied the properties of Rydberg states of amines and ethers with a view to their application to and importance for the chemistry of energetic materials. We have learned that in clusters these Rydberg states can be quite reactive and, in fact, they dominate the excited state chemistry and dynamics through electron transfer reactions. This unexpected and new phenomenon can be understood based on available excited state orbitals (Rydberg, sigma*, pi*) for the solvent molecules. If the solvent molecule for the amine or ether is another amine, ether, or aromatic, a low lying electron transfer state is generated, due to the exchange interaction, which is the first step in a chain of chemical reactions. We conclude that these electron transfer states generated from Rydberg state-solvent interactions are quite general and can represent the initial step in electron transfer oxidation-reduction chemistry for energetic materials.

A versatile method for combining density functional theory (DFT) in the local density approximation (LDA) with dynamical mean-field theory (DMFT) is presented. Starting from a general basis-independent formulation, we use Wannier functions as an interface between the two theories. These functions are used for the physical purpose of identifying the correlated orbitals in a specific material, and also for the more technical purpose of interfacing DMFT with different kinds of band-structure methods (with three different techniques being used in the present work). We explore and compare two distinct Wannier schemes, namely the maximally-localized-Wannier-function (MLWF) and the $N$-th order muffin-tin-orbital (NMTO) methods. Two correlated materials with different degrees of structural and electronic complexity, SrVO3 and BaVS3, are investigated as case studies. SrVO3 belongs to the canonical class of correlated transition-metal oxides, and is chosen here as a test case in view of its simple structure and physical properties. In contrast, the sulfide BaVS3 is known for its rich and complex physics, associated with strong correlation effects and low-dimensional characteristics. New insights into the physics associated with the metal-insulator transition of this compound are provided, particularly regarding correlation-induced modifications of its Fermi surface. Additionally, the necessary formalism for implementing self-consistency over the electronic charge density in a Wannier basis is discussed.

The use of ion implantation in silicon device fabrication is known to give rise to defects both in the silicon and oxide. Some of these defects may be capable of withstanding even high temperature (1000C) anneals. Renewed interest in ion implantation in Si/SiO2 device structures in recent years has led to the use of a variety of experimental techniques to investigate the nature of damage produced by implantation of ions in the energy and dose ranges of technological interest. Since such defects may be paramagnetic, EPR has proved to be a very useful technique for studying radiation damage on an atomic scale. Ion irradiation of amorphous SiO2 produces two predominant paramagnetic centres, one associated with an oxygen vacancy (E '1) and the other with a non-bridging oxygen atom (oxygen hole centre). There is a marked difference between the EPR spectra of these centres. The E'1 centre is characterised by an almost axial g-tensor which in thermal oxides gives rise to a glassy lineshape with apparent g-value of 2.0005 and linewidth of approx. 2G (at X-band). The ESR signal saturates at low levels of microwave power. The OHC also exhibits a glass lineshape but the spread of g-values is much larger giving rise to a very extended spectrum.

We present a systematic assessment of the structural properties, the electronic density of states, the charge densities, and the phase stabilities of AgInSe$_2$ and AuInSe$_2$ using screened exchange hybrid density functional theory, and compare their properties to those of CuInSe$_2$. For AgInSe$_2$, hybrid density functional theory properly captures several experimentally measured properties, including the increase in the band gap and the change in the direction of the lattice distortion parameter $u$ in comparison to CuInSe$_2$. While the electronic properties of AuInSe$_2$ have not yet been experimentally characterized, we predict it to be a small gap ($\approx 0.15$ eV) semiconductor. We also present the phase stability of AgInSe$_2$ and AuInSe$_2$ according to screened-exchange density functional theory, and compare the results to predictions from conventional density functional theory, results tabulated from several online materials data repositories, and experiment (when available). In comparison to conventional density functional theory, the hybrid functional predicts phase stabilities of AgInSe$_2$ in better agreement with experiment: discrepancies in the calculated formation enthalpies are reduced by approximately a factor of three, from $\approx$ 0.20 eV/atom to $\approx$ 0.07 eV/atom, similar to the improvement observed for CuInSe$_2$. We further predict that AuInSe$_2$ is not a stable phase, and can only be present under non-equilibrium conditions.

Various technological problems in the development of GaAs microwave devices are covered in this report. The work reported here includes: epitaxial growth of ultra-thin and high-resistivity films, study of the relationship between device performance and material parameters using IMPATT and LSA Gunn diodes, growth of semi-insulating substrates and characterization techniques, and study of ion implantation as a tool for microwave device fabrication.

The intent of this preliminary programme was to deposit langmuir-Blodgett (LB) films on semiconductor surfaces, and assess their structure, thickness and perfection using a variety of experimental techniques. In doing this we have contributed to rehousing Professor Pethrick's L-B trough in a 'clean room' in the Chemistry Laboratory. This followed advice from Dr. M. Petty at Durham University, largely because we have experienced difficulties in obtaining reproducible results with many samples. To date our work has been concerned with GaAs, GaP and Si, all in the form of (111) wafers, which had been mechanically polished to an 0.25 micrometer finish using diamond paste, prior to chemical polishing and refluxing for several hours in isopropyl alcohol vapour. Such samples were assumed to be clean, and were kept in an evacuated chamber until ready to be treated in the L B trough.

A surface potential model is developed to explain dopant effects on chemical vapor deposition. Auger analysis of the interaction between allotropic forms of carbon and silicon films has shown Si-C formation for all forms by glassy carbon. LEED intensity measurements have been used to determine the mean square displacement of surface atoms of silicon single crystals, and electron loss spectroscopy has shown the effect of structure and impurities on surface states located within the band gap. A thin film of Al has been used to enhance film crystallinity at low temperature.

The funding from this grant was utilized to further develop a new approach for the electronic structure of materials. The method formulates the Kohn-Sham equations of Density Functional Theory directly in real space with a high order Finite Difference approach. The resulting equations were solved using the linear scaling multigrid algorithm developed by Brandt and coworkers. Multigrid techniques were used to solve both the self consistent eigenvalue equations and the Poisson equation for the electrostatic potential at each step of iterations. Accurate numerical results were obtained for finite and periodic electrostatic problems and for the eigenvalue equations for many electron atoms and simple molecules. Recently, conservative grid equations have been developed so that grid refinement strategies can be employed. This allows one to perform extensive numerical work selectively in regions of high electron density. The new method should have wide applications for numerical studies of complex and disordered materials which require a quantum mechanical treatment for many atoms.

To fulfill the great potential of intermetallic alloys for high temperature structural applications, it is essential to understand the mechanisms controlling their mechanical behavior on the microscopic level. We focus on the mechanical behavior of homogeneous intermetallics with LI 2 and B2 structures, two-phase ly' alloys, and bcc transition metals and their alloys. Based on highly-accurate total energy calculations and large-scale Peierls-Nabarro and atomistic modeling, we investigated the relation between electronic structure, dislocation properties, and brittle-ductile behavior in these metals and alloys. We demonstrated that Ir- and Rh-based alloys are intrinsically brittle and established the dislocation structure in Ir and Rh-based LI2 alloys. We determined the temperature dependence of the lattice misfit parameter in y/y' superalloys, and established its simple relation with the alloy phase diagram. We studied the structure of dislocations in bcc Nb, Ta, Mo and W alloys, analyzed the effect of alloying with 3d and Sd transition metal elements on dislocation energetics, and established the electronic origins of the solid solution softening/hardening phenomena. We investigated the effect of topologically close packed phase formation on the solubility of interstitial impurities in Cr- and Mo-based alloys and demonstrated their effect in preventing the formation of embrittling phases. We showed that Co- and Y-based B2 intermetallics are intrinsically ductile, established the dislocation structure in Co-based alloys, analyzed the origins of their unusual mechanical behavior, and identified the microscopic origins of the yield stress anomaly.