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A molecular cluster approach is used to investigate the optical spectrum of superconducting YBa 2 Cu 3 O 7 . The calculations are performed through a semiempirical LCAO technique and correlation effects are taken into account by a configuration interaction procedure (INDO/CI). In selected clusters, representing planes and chains, four characteristic optical transitions are obtained: around 0.6, 1.4, 2.4 and 5.0 eV. These values are in good agreement with experimental data and our calculations help to elucidate the origin of the corresponding peaks in the optical spectrum. In particular, we find that none of the transitions should be assigned to pure intra-d excitations at Cu sites.

We investigated the interaction of point defects (vacancy and self-interstitials) with an intrinsic stacking fault in silicon. The calculations were carried out using ab initio total energy methods. The results show that point defects at an intrinsic stacking fault display a different behavior as compared to the same defect in the crystalline environment. This is evidenced by differences in formation energies and electronic structures. These results suggest that there is migration of point defects from the bulk to a stacking fault. This could affect the dislocation mobility in the crystal.

Here we summarize our investigation of the effect of a recently observed phase transformation on ruby's optical transitions. This study involved a first principles calculation of the electronic and structural properties of a chromium impurity in alumina host lattices and a subsequent calculation of the multiple structure using eigenvalues and eigenvectors derived from the first principles calculation. This investigation is relevant to clarify the behavior of the fluorescent optical transitions which are used as pressure sensor in diamond anvil experiments across the structural transformation.

Extended defects like stacking faults (SFs) can originate topologically protected metallic states in bulk topological insulators (TIs). These induced topological states are a response to the weakening of the inter-layer van der Waals interactions due to the SF defect. In TI thin films, the degeneracy of Dirac bands of opposite surfaces can be lifted upon the formation of SF defects. Such slab asymmetry can promote a net spin current, in the absence of backscattering processes, in thin film made of TIs. These results have been obtained by fully relativistic first principles calculations.

Carbon nanotubes have been at the forefront of nanotechnology, leading not only to a better understanding of the basic properties of charge transport in one dimensional materials, but also to the perspective of a variety of possible applications, including highly sensitive sensors. Practical issues, however, have led to the use of bundles of nanotubes in devices, instead of isolated single nanotubes. From a theoretical perspective, the understanding of charge transport in such bundles, and how it is affected by the adsorption of molecules, has been very limited, one of the reasons being the sheer size of the calculations. A frequent option has been the extrapolation of knowledge gained from single tubes to the properties of bundles. In the present work we show that such procedure is not correct, and that there are qualitative differences in the effects caused by molecules on the charge transport in bundles versus isolated nanotubes. Using a combination of density functional theory and recursive Green's function techniques we show that the adsorption of molecules randomly distributed onto the walls of carbon nanotube bundles leads to changes in the charge density and consequently to significant alterations in the conductance even in pristine tubes. We show that this effect is driven by confinement which is not present in isolated nanotubes. Furthermore, a low concentration of dopants randomly adsorbed along a two-hundred nm long bundle drives a change in the transport regime; from ballistic to diffusive, which can account for the high sensitivity to different molecules.

The use of nanoscale low-dimensional systems could boost the sensitivity of gas sensors. In this work we simulate a nanoscopic sensor based on carbon nanotubes with a large number of binding sites using ab initio density functional electronic structure calculations coupled to the Non-Equilibrium Green's Function formalism. We present a recipe where the adsorption process is studied followed by conductance calculations of a single defect system and of more realistic disordered system considering different coverages of molecules as one would expect experimentally. We found that the sensitivity of the disordered system is enhanced by a factor of 5 when compared to the single defect one. Finally, our results from the atomistic electronic transport are used as input to a simple model that connects them to experimental parameters such as temperature and partial gas pressure, providing a procedure for simulating a realistic nanoscopic gas sensor. Using this methodology we show that nitrogen-rich carbon nanotubes could work at room temperature with extremely high sensitivity.

We show, using ab initio density functional theory calculations, that Mn dimers adsorbed on graphene nanoribbons (Mn2/GNRs) present a magnetic bistability, as does the isolated Mn dimer. Our total energy results indicate that Mn dimers lying along the edge sites of zigzag GNRs represent the most likely configuration. We find that similar to the isolated Mn2 molecule, the antiferromagnetic coupling represents the ground state for Mn2/GNR, and the spin density configuration of the GNR does not play an important role on the net magnetic moment of Mn2, which makes GNRs an ideal substrate for adsorption of these molecules. The ground state and the excited state configuration of the Mn dimer, viz., low-spin (LS) and high-spin (HS), are maintained in the face of changes in the spin density configuration of the substrate. Here we find that the Mn2/GNR systems exhibit a LS ↔ HS binary behavior, which can be considered as a useful property in the development of nanomemories based upon metallic clusters.

Carbon nanotubes rank amongst potential candidates for a new family of nanoscopic devices, in particular for sensing applications. At the same time that defects in carbon nanotubes act as binding sites for foreign species, our current level of control over the fabrication process does not allow one to specifically choose where these binding sites will actually be positioned. In this work we present a theoretical framework for accurately calculating the electronic and transport properties of long disordered carbon nanotubes containing a large number of binding sites randomly distributed along a sample. This method combines the accuracy and functionality of ab initio density functional theory to determine the electronic structure with a recursive Green's functions method. We apply this methodology on the problem of nitrogen-rich carbon nanotubes, first considering different types of defects and then demonstrating how our simulations can help in the field of sensor design by allowing one to compute the transport properties of realistic nanotube devices containing a large number of randomly distributed binding sites.

The use of the spin of the electron as the ultimate logic bit - in what has been dubbed spintronics - can lead to a novel way of thinking about information flow. At the same time single layer graphene has been the subject of intense research due to both its fundamental properties as well as its potential application in nanoscale electronics. While defects can significantly alter the electronic properties of nanoscopic systems, the lack of control can lead to seemingly deleterious effects arising from the random arrangement of such impurities. Here we demonstrate, using {\it ab initio} density functional theory and non-equilibrium Green's functions calculations, that it is possible to obtain perfect spin selectivity in doped graphene nanoribbons to produce a perfect spin filter. We show that initially unpolarized electrons entering the system give rise to 100% polarization of the current due to random disorder. This effect is explained in terms of different localization lengths for each spin channel which together with the well know exponential dependence of the conductance on the length of the device leads to a new mechanism for the spin filtering effect that is enhanced by disorder.

An ab initio study of Ge nanowires (GeNWs) oriented along [110] and [111] directions is performed for H-passivated and non-passivated wires with diameters up to 3.5 nm. GeNWs grown in the [110] direction present a direct energy gap and in the [111] direction the energy gap is indirect, even for small diameters. The highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO–LUMO) gap scales with diameter, d, as and d−1.1 for the [110] and [111] growth directions, respectively. Consequently for the same diameter GeNWs grown in the [110] direction present a smaller gap than nanowires grown in the [111] direction.

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A molecular cluster model of the tetrahedrally coordinated semiconductors is discussed within the framework of the self-consistent-field multiple-scattering method. A method to avoid the dangling bond effects is discussed and the model is particularly investigated with respect to gallium arsenide. It is shown that the model leads to a fairly consistent description of the bulk electronic structure.

The multiple-scattering Xalpha molecular cluster model is used to carry out self-consistent calculations of the electronic states of an isolated P vacancy in GaP. A fully occupied singly degenerate a1 level and a threefold-degenerate t2 level occupied by one electron are found within the band gap. This configuration indicates that the centre is unstable with respect to Jahn-Teller distortions. The vacancy states a1 and t2 are quite extended showing an appreciable charge distribution in the next-nearest neighbours of the vacancy (P atoms). The role played by these results in the explanation of the properties of the GaP:O centre is discussed.

We present a new multiplet theory that separates mean-field from multiplet effects in the excitation and donor-acceptor ionization spectra of localized impurities. Analysis of the experimental data for all 3d impurities in ZnO, ZnS, ZnSe, and GaP for which sufficient data exist and for the bulk Mott insulators CoO, MnO, and NiO reveals, for the first time, regular chemical trends in many-electron effects with the impurity and the host-crystal covalency and delineates the regime where one-electron theory is applicable from the region where it is not.

A new theoretical approach to the multiplet structure of localized states in solids is applied to the Mott insulators NiO, CoO, and MnO. This method circumvents the major approximations underlying the conventional Tanabe-Sugano approach to the multiplet problem in that differential hybridization for e and t 2 states is incorporated directly, the Racah parameter A is allowed to depend on the one-electron configurations, and the crystal-field parameter incorporates both bare-ion effects (as traditionally done) and average multiplet corrections. Analysis of the observed optical spectra of NiO, CoO, and MnO in light of this approach produces (i) an excellent fit to the spectra and new assignments for some of the transitions, (ii) the values of the mean-field parameters for these materials, (iii) a direct measure for the amount by which an ideal, spin-restricted mean-field electronic structure theory would fail to reproduce the excitation energies, and (iv) analysis of the optical transitions above the interband onset.

We use multiple scattering-Xα molecular cluster models to study complex defects in Si, the E center (Si : PV o ) and the divacancy, this latter in different charge states (Si : V o 2 , V + 2 , V - 2 ). For both systems we find indications of the occurence of Jahn-Teller effects, and these are included in the study of the Si : V + 2 system. Our results are compared to experimental data.

Using a multiple-scattering X alpha molecular cluster model, the authors studied the electronic structure of GaAs:OAs, including +or-5% symmetric relaxations of the nearest-neighbour atoms. Their results indicate that the oxygen impurity introduces a non-degenerate a1 energy level in the crystal band gap. Their calculation leads to the conclusion that the oxygen impurity provokes an inward relaxation of the neighbouring atoms, and that Jahn-Teller distortions are not to be expected.

An exchange-correlation potential model to be used in connection with the multiple scattering method is presented. Retaining the main advantages of the multiple scattering method with the Xα potential, particularly its low computational requirements, this new formalism does not require any adjustable parameter. Test calculations on the NiF64− system are reported and compared with experimental and ab initio results.

The molecular cluster model, within the framework of the self-consistent field multiple scattering Xα method, is applied to calculate the electronic structure of a Mn substitutional impurity in GaAs. The charge states Mn3+, with spin configurations S = 0 and 2, and Mn2+, with S = 5 2 , were analysed. The theoretical results compare fairly well with the experimental data obtained from ESR and optical spectroscopic experiments.

The hamiltonian eigenvalue problem is solved for a diva cancy in silicon within the framework of the defect-molecule model. A direct connection between the solution of this problem and a self-consistent multiple-scattering cluster calculation is established in order to determine the parameters of the model. The obtained results provide a quantitative confirmation of the currently accepted microscopic picture of the divacancy.

Using clusters with 17 atoms the electronic structure of Ga and Sb single vacancies is studied in GaSb, using the MS-Xα method. For the Ga vacancy an impurity level of t2 symmetry is obtained, occupied with three electrons, and in the case of the Sb vacancy two impurity levels are obtained, one a fully occupied a1 level and a t2 level occupied with one electron. La structure electronique des lacunes uniques de Ga et Sb dans GaSb a ete etudiee par la methode MS-Xα avec l'usage des agreges avec 17 atomes. Pour les lacunes de Ga nous avons obtenu un niveau d'impurite avec une symetrie t2 et occupe par 3 electrons. Dans le cas des lacunes de Sb nous avons obtenu deux niveaux d'impurite: un niveau a1 completement occupe et un outre t2 qui contient un electron.

We consider the effects of a non-muffin-tin charge density in the multiple-scattering method. The study is carried out by means of a parametrized electronic density, which is used in a variational expression for the energy of the molecular system. We show that, at least in two situations, the parametrized density reproduces the true density very well. In a numerical study of the electronic configuration of the CH4 molecule, we show that the non-muffin-tin density may shift the one-electron energies by as much as 1 eV and the total energy by as much as 1 Ry.

Using the Haldane–Anderson formalism we study the possibility of obtaining a negative U system for transition metal impurities in semiconductors. We propose, even without including electron-lattice interaction, the occurrence of negative-U as a consequence of the impurity-host hybridization to explain why some oxidation states would be missing in these systems.

The multiple scattering-X alpha cluster model was applied to the study of the electronic structure of vacancies and copper and selenium substitutional impurities in GaAs. In order to eliminate the 'dangling bond' effects, a suitable boundary condition at the cluster surface was assumed. Calculations lead to the conclusion that As neutral vacancies are donors, whereas Ga vacancies are acceptors. Cu atoms as substitutional impurities replacing Ga in the GaAs lattice form acceptor levels, quite similar to those generated by Ga neutral vacancies. This explains the experimental difficulties concerning the identification of these centres in GaAs. Se atoms replacing As in the lattice form a donor level near the bottom of the conduction band. The promising agreement with experimental observations leads to the conclusion that the proposed cluster model is an accurate technique for studying the electronic structure of locally perturbed covalent semiconductors.

In this work we present ab initio calculations of the formation energies and stability of different types of multi-vacancies in carbon nanotubes. We demonstrate that, as in the case of graphene, the reconstruction of the defects has drastic effects on the energetics of the tubes. In particular, the formation of pentagons eliminates the dangling bonds thus lowering the formation energy. This competition leads to vacancies having an even number of carbon atoms removed to be more stable. Finally the appearance of magic numbers indicating more stable defects can be represented by a model for the formation energies that is based on the number of dangling bonds of the unreconstructed system, the pentagons and the relaxation of the final form of the defect formed after the relaxation.

The self-consistent field multiple-scattering Xα molecular cluster model is applied to calculate the electronic states of a divacancy plus two oxygen complex in silicon. The calculations were carried out for the undistorted configuration of the defect. The obtained results, which are in fairly good agreement with EPR measurements, confirm the main features of the accepted microscopic model for the Si - P2 center.

We study the Cr impurity in GaAs at the anion and cation tetrahedral interstitial sites, in several charge states. Lattice relaxation was not included. Our results indicate the charge state 2+ is unstable, i. e., the system presents negative-U characteristics as was also proposed for the impurity in Si by Yoshida and Zunger.

A straighforward way to treat the exchange potential within the framework of the multiple scattering method is suggested. The procedure, which eliminates the difficulties related with the Xα potential, is applied to the study of the molecular clusters of Ga As.

The internal luminescence under pressure of Cr2+ in GaAs is studied theoretically using the multiplet method. The results agree with the experimental values obtained by Zigone et al concerning an important modification of the spectra occuring arrund 4.0 GPa, interpreted as an evidence for the crossing of the 5E and 1A1 states.

Self-consistent calculations of the electronic states of V0, V- and V2- gallium vacancies in GaP have been carried out within the framework of the multiple-scattering X alpha molecular cluster model. The results obtained are used to interpret a recent electron paramagnetic resonance experiment which was assumed to be related to the V2- centre.

A multiple-scattering-cluster model of IV and III-V covalent semiconductors is proposed. The feasibility of the model is tested by carrying out calculations on GaAs. The valence band width, the energy band gap and the location of the cationic and anionic d levels obtained are in fairly good agreement with the experimental results for the bulk crystal.

Whereas the conventional practice of referring binding energies of deep donors and acceptors to the band edges of the host semiconductor does not produce transparent chemical trends when the same impurity is compared in different crystals, referring them to the vacuum level through the use of the photothreshold reveals a remarkable material invariance of the levels in III-V and II-VI semiconductors. It is shown that this is a consequence of the antibonding nature of the deep gap level with respect to the impurity atom-host orbital combinations.

Variational cellular method (VCM) calculations for F2 have been carried out at several internuclear distances. The ground and excited state potential curves are presented. The overall agreement between the VCM results and ab initio calculations is fairly good.

We study the electronic structure of the Mo 4d and W 5d impurities in GaAs, using the cluster approach within the multiple-scattering X\ensuremath{\alpha} formalism; results are also given for the Cr 3d isovalent impurity. Multiplet structures for the systems are evaluated through the approach of Fazzio, Caldas, and Zunger. Our results indicate that the Mo and W impurities will present a low-spin ground state in the 2+ oxidation state (acceptor), and that the observed luminescence lines are not related to the $^{5}\mathrm{}^{5}$${\mathrm{T}}_{2}$ internal transition of the isolated substitutional Mo and W impurities.