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An electrically active defect has been observed at a level position of ∼ 0.70 eV below the conduction band edge (Ec) with an extrapolated capture cross section of ∼ 5×10−14 cm2 in epitaxial layers ...

Darwin-Foldy nuclear-size corrections in electronic atoms and nuclear radii are discussed from the nuclear-physics perspective. Interpretation of precise isotope-shift measurements is formalism dependent, and care must be exercised in interpreting these results and those obtained from relativistic electron scattering from nuclei. We strongly advocate that the entire nuclear-charge operator be used in calculating nuclear-size corrections in atoms, rather than relegating portions of it to the non-radiative recoil corrections. A preliminary examination of the intrinsic deuteron radius obtained from isotope-shift measurements suggests the presence of small meson-exchange currents (exotic binding contributions of relativistic order) in the nuclear charge operator, which contribute approximately 1/2%., Comment: 17 pages, latex, 1 figure -- Submitted to Phys. Rev. A -- epsfig.sty required

The structural and electronic properties of Cu2O have been investigated using the periodic Hartree-Fock method and a posteriori density-functional corrections. The lattice parameter, bulk modulus, and elastic constants have been calculated. The electronic structure of and bonding in Cu2O are analyzed and compared with x-ray photoelectron spectroscopy spectra, showing a good agreement for the valence-band states. To check the quality of the calculated electron density, static structure factors and Compton profiles have been calculated, showing a good agreement with the available experimental data. The effective electron and hole masses have been evaluated for Cu2O at the center of the Brillouin zone. The calculated interaction energy between the two interpenetrated frameworks in the cuprite structure is estimated to be around -6.0 kcal/mol per Cu2O formula. The bonding between the two independent frameworks has been analyzed using a bimolecular model and the results indicate an important role of d10-d10 type interactions between copper atoms.

Existence of collective effects in magnetic coupling in ionic solids is studied by mapping spin eigenstates of the Heisenberg and exact nonrelativistic Hamiltonians on cluster models representing KNiF{sub 3}, K{sub 2}NiF{sub 4}, NiO, and La{sub 2}CuO{sub 4}. {ital Ab initio} techniques are used to estimate the Heisenberg constant J. For clusters with two magnetic centers, the values obtained are about the same for models having more magnetic centers. The absence of collective effects in J strongly suggests that magnetic interactions in this kind of ionic solids are genuinely local and entangle only the two magnetic centers involved. {copyright} {ital 1997} {ital The American Physical Society}

The synthesis and structural characterization at room temperature and 130 K of the organic charge-transfer salt bis(thiodimethylene)-tetrathiafulvalene tetracyanoquinodimethane (BTDMTTF-TNCQ) is reported. X-ray diffuse scattering as well as ir and Raman measurements show that the charge transfer in this salt is ∼0.5. BTDMTTF-TCNQ remains metallic down to very low temperatures but exhibits an anomaly at 175 K in the conductivity and thermopower which could be related to some unusual structural modification of the unit cell. Band-structure calculations show that this salt is a narrow-band one-dimensional metal. The high value of the spin susceptibility, the low value of the conductivity, and the observation of a 4 k F charge-density-wave (CDW) instability around room temperature prove that it is a highly correlated metal. In contrast with other salts of the TTF-TCNQ family, BTDMTTF-TCNQ does not exhibit critical 2 k F structural fluctuations. It is proposed that the absence of the 2 k F CDW transition is due to the stronger interlocking of the donor and acceptor molecules due to the presence of the sulfur atoms in the central position of the outer five-membered rings of the BTDMTTF donor.

The nature of the chemical bond in three titanium oxides of different crystal structure and different formal oxidation state has been studied by means of the ab initio cluster-model approach. The covalent and ionic contributions to the bond have been measured from different theoretical techniques. All the analysis is consistent with an increasing of covalence in the TiO, ${\mathrm{Ti}}_{2}$${\mathrm{O}}_{3}$, and ${\mathrm{TiO}}_{2}$ series as expected from chemical intuition. Moreover, the use of the ab initio cluster-model approach combined with different theoretical techniques has permitted us to quantify the degree of ionic character, showing that while TiO can approximately be described as an ionic compound, ${\mathrm{TiO}}_{2}$ is better viewed as a rather covalent oxide.

Systematic trends in the properties of a linear split-gate heterojunction are studied by solving iteratively the Poisson and Schr\"odinger equations for different gate potentials and temperatures. A two-dimensional approximation is presented that is much simpler in the numerical implementation and that accurately reproduces all significant trends. In deriving this approximation, we provide a rigorous and quantitative basis for the formulation of models that assumes a two-dimensional character for the electron gas at the junction.

The magnetic behavior of the reduced Nd 1.8 Sr 0.2 NiO 3.72 nickelate has been investigated by means of neutron-powder diffraction, magnetic susceptibility, and isothermal magnetization measurements. The most noticeable result is that, in spite of the large concentration of oxygen vacancies, both the Ni and Nd sublattices become antiferromagnetically ordered below T N 1 ≊320 K and T N 2 ≊13.5 K, respectively. Moreover, two spin reorientations of the Ni magnetic moments have been observed, one around T 1 ≊130 K and the other simultaneously to the cooperative ordering of the Nd sublattice. The existence of a weak ferromagnetic component below 70 K has also been clearly established from the analysis of the macroscopic magnetic measurements. Finally, a set of effective exchange parameters have been derived, and their values compared with those reported for stoichiometric Nd 2 NiO 4 .

In this work, we demonstrate that conductive atomic force microscopy (C-AFM) is a very powerful tool to investigate, at the nanoscale, metal-oxide-semiconductor structures with silicon nanocrystals (Si-nc) embedded in the gate oxide as memory devices. The high lateral resolution of this technique allows us to study extremely small areas ( ~ 300nm2) and, therefore, the electrical properties of a reduced number of Si-nc. C-AFM experiments have demonstrated that Si-nc enhance the gate oxide electrical conduction due to trap-assisted tunneling. On the other hand, Si-nc can act as trapping centers. The amount of charge stored in Si-nc has been estimated through the change induced in the barrier height measured from the I-V characteristics. The results show that only ~ 20% of the Si-nc are charged, demonstrating that the electrical behavior at the nanoscale is consistent with the macroscopic characterization.

An analytical approximation, depending on five parameters, for the atomic screening function is proposed. The corresponding electrostatic potential takes a simple analytical form (superposition of three Yukawa potentials) well suited to most practical applications. Parameters in the screening function, determined by an analytical fitting procedure to Dirac-Hartree-Fock-Slater (DHFS) self-consistent data, are given for Z=1--92. The reliability of this analytical approach is demonstrated by showing that (a) Born cross sections for elastic scattering of fast charged particles by the present analytical field and by the DHFS field practically coincide and (b) one-electron binding energies computed from the independent-particle model with our analytical field (corrected for exchange and electrostatic self-interaction) agree closely with the DHFS energy eigenvalues.

ResumenSe presenta una revisión de los métodos que se han desarrollado dentro de la Química Teórica para estudiar a la materia en diversas escalas de espacio y tiempo. La trayectoria de un conocido químico teórico se utiliza para mostrar cómo, en 4 décadas, los desarrollos de teorías, programas (algoritmos) y de la tecnología computacional nos permiten en la actualidad el estudio confiable de la estructura electrónica de sistemas cuyos tamaños los hacían prohibitivos hace 40 años. Se señala y enfatiza que la dificultad central en el estudio teórico de la Química reside en que, en todas los escalas, se trata de un problema de muchos cuerpos cuya solución y comprensión siguen siendo un reto científico no resuelto y fascinante.AbstractA review of the methods developed within Theoretical Chemistry to study matter in many spatial and temporal scales is presented. The scientific career of a well known theoretical chemist is used to show how the astonishing advances that have been achieved in four decades in the development of theories, codes (algorithms) and computer technology allow nowadays the reliable study of the electronic structure of systems whose sizes were unmanageable forty years ago. It is emphasized that the central difficulty in studying Chemistry from a theoretical perspective is that, in every scale, one deals with a many-body problem whose solution and comprehension are still an unsolved and fascinating scientific challenge.

La variación de la energía de transición de un elemento en función de su número de coordinación, es un parámetro que proporciona información sobre la estructura electrónica de un elemento en diferentes compuestos, por lo que ha sido estudiado con especial interés. En este trabajo se demuestra que la relación de intensidades entre las líneas características de un elemento en diferentes tipos de enlace, es un parámetro sensible con respecto a la estructura electrónica. Esto se prueba en los casos del Si y Al. The variation of the transiction energy of an element in function of bis coordination number, is a parameter that gives information about the electronic structure of an element in different compounds. This aspect has been studied with special interest. In this work it has been outlined that the relation of intensities between the characteristic lines of an element in different chemical bounds is a sensitive parameter comported with the electronic structure. It has been proved in the cases of silicium and aluminium.

Filiació URV: SI The key role of the molecular orbitals in describing electron transfer processes is put in evidence for the intervalence charge transfer (IVCT) of a synthetic non-haem binuclear mixed-valence Fe3+/Fe2+ compound. The electronic reorganization induced by the IVCT can be quantified by controlling the adaptation of the molecular orbitals to the charge transfer process. We evaluate the transition energy and its polarization e¿ects on the molecular orbital by means of ab initio calculations. The resulting energetic profile of the IVCT shows strong similarities to the Marcus¿ model, suggesting a response behaviour of the ensemble of electrons analogue to that of the solvent. We quantify the extent of the electronic reorganization induced by the IVCT process to be 11.74 eV, a very large e¿ect that induces the crossing of states reducing the total energy of the transfer to 0.89 eV.

We report on a growth study of MgO films deposited on Al2O3(0001) substrates by magnetron sputtering. The films exhibited a preferred rocksalt MgO(111) orientation. Surprisingly, depending on the O2 gas flow ratio, a structure of graphiticlike wurtzite MgO(0001) has been revealed. The observed Mg-O perpendicular bond length reduction is accompanied by an atomically flat surface morphology for the development of MgO(111) films; the transition to the bulk rocksalt structure occurs in the 3-6 nm coverage range. Previously, relaxation of the electrostatic instability of MgO(111) films accompanied by an in-plane lattice increase has been suggested theoretically [Phys. Rev. Lett. 98, 205701 (2007)]. Here, relying on ab initio calculations, we infer that Mg vacancies facilitate the lattice match with the substrate. This mechanism suggests methods to engineer oxide heterostructures.

Filiació URV: SI Density functional theory, Complete Active Space Self-Consistent Field (CASSCF) and perturbation theory (CASPT2) methodologies have been used to explore the electronic structure of a series of trichromium Extended Metal Atom Chains (EMACS) with different capping ligands. The study is motivated by the very different structural properties of these systems observed in X-ray experiments: the CN−-capped example has a symmetric Cr3 unit while for the NO3−-capped analogue the same unit has two very different Cr–Cr bond lengths. Density functional theory fails to locate an unsymmetric minimum for any of the systems studied, although the surface corresponding to the asymmetric stretch is very flat. CASPT2, in contrast, does locate an unsymmetric minimum only for the NO3−-capped system, although again the surface is very flat. We use the Generalized active space (GASSCF) technique and effective Hamiltonian theory to interrogate the multi-configurational wavefunctions of these systems, and show that the increase in the σ–σ* separation as the chain becomes unsymmetric plays a defining role in the stability of the ground state and its expansion in terms of configuration state functions.

Zinc indium tin oxide (ZITO) transparent conductive oxide layers were deposited via radio frequency (RF) magnetron co-sputtering at room temperature. A series of samples with gradually varying zinc content was investigated. The samples were characterized with x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) to determine the electronic structure of the surface. Valence and conduction bands maxima (VBM, CBM), and work function were determined. The experiments indicate that increasing Zn content results in films with a higher defect rate at the surface leading to the formation of a degenerately doped surface layer if the Zn content surpasses ∼50%. Furthermore, the experiments demonstrate that ZITO is susceptible to ultraviolet light induced work function reduction, similar to what was earlier observed on ITO and TiO2 films.

The electronic structure of an isolated oxygen vacancy in SrTiO3 has been investigated with a variety of ab initio quantum mechanical approaches. In particular we compared pure density functional theory ~DFT! approaches with the Hartree-Fock method, and with hybrid methods where the exchange term is treated in a mixed way. Both local cluster models and periodic calculations with large supercells containing up to 80 atoms have been performed. Both diamagnetic ~singlet state! and paramagnetic ~triplet state! solutions have been considered. We found that the formation of an O vacancy is accompanied by the transfer of two electrons to the 3d(z 2 ) orbitals of the two Ti atoms along the Ti-Vac-Ti axis. The two electrons are spin coupled and the ground state is diamagnetic. New states associated with the defect center appear in the gap just below the conduction band edge. The formation energy computed with respect to an isolated oxygen atom in the triplet state is 9.4 eV.

The performance of density-functional theory to solve the exact, nonrelativistic, many-electron problem for magnetic systems has been explored in a new implementation imposing space and spin symmetry constraints, as in ab initio wave function theory. Calculations on selected systems representative of organic diradicals, molecular magnets and antiferromagnetic solids carried out with and without these constraints lead to contradictory results, which provide numerical illustration on this usually obviated problem. It is concluded that the present exchange-correlation functionals provide reasonable numerical results although for the wrong physical reasons, thus evidencing the need for continued search for more accurate expressions.

Filiació URV: SI Electronic structure calculations have been performed on four dierent Mn corrole and corrolazine complexes to clarify the role of the imide axial ligand on the relative stability of the dierent spin states and the stabilization of the high valent Mn ion in these complexes. Multicongurational perturbation theory energy calculations on the DFT optimized geometries shows that all complexes have a singlet ground state except the complex with the strongest electron with- drawal axial ligand, which is found to have a triplet ground state. The analysis of the sigma and pi interaction between metal and imide ligand shows that this spin crossover is caused by a subtle interplay of geometrical factors (Mn-N distance and coordination angle) and the electron withdrawal character of the imide. The analysis of the multicongurational wave functions reveals that the formally MnV ion is stabilized by an important electron transfer from both the equatorial cor-role/corrolazine ligand and the axial imide. The macrocycle donates roughly half an electron, being somewhere between the closed-shell trianionic and the dian-ionic radical form. The imide ligand transfers 2.5 electrons to the metal center, resulting in an eective d-electron count close to ve in all complexes.

CuF2 is known to be an antiferromagnetic compound with a weak ferromagnetism due to the anisotropy of its monoclinic unit cell ~Dzialoshinsky-Moriya mechanism!. We investigate the magnetic ordering of this compound by means of ab initio periodic unrestricted Hartree-Fock calculations and by cluster calculations which employ state-of-the-art configuration interaction expansions and modern density functional theory techniques. The combined use of periodic and cluster models permits us to firmly establish that the antiferromagnetic order arises from the coupling of one-dimensional subunits which themselves exhibit a very small ferromagnetic coupling between Cu neighbor cations. This magnetic order could be anticipated from the close correspondence between CuF2 and rutile crystal structures. @S0163-1829~99!15301-3#

Ab initio cluster model wave functions are used to predict the existence of localized excited states in MgO, Al2O3, and SiO2 arising from metal 2p core-level excitations. Theoretical values obtained at different levels of theory result in a quantitative agreement with experiment, and the use of different models permits us to quantify the different contributions to the final excitation energy. The most important contribution is atomic in nature; a meaningful zero-order approximation is that in MgO and Al2O3 the exciton can be assigned to a M(2p6)→M(2p53s1)-like excitation, where M5Mg or Al. For the atomic models, the singlet-triplet exchange in the excited configuration is in good agreement with experiment. In addition, the solid-state effects on this exchange energy predicted by experiment are well reproduced by the cluster models representing MgO and SiO2, whereas a less clear situation appears in Al2O3. The open-shell orbital in the final state has, however, important contributions from the ions near the atomic site where excitation occurs. Nevertheless, the final state appears to be localized in space without any a priori assumption, the localization following from the holeparticle interaction implicitly induced in the final-state wave function. The Madelung field reduces the excitation energy with respect to the atomic value; the effect of neighboring atoms, mainly Pauli repulsion, acts in the opposite way; and electronic correlation effects decrease it again. In agreement with the covalent nature of SiO2, the exciton cannot be simply understood as arising from a Si(2p6)→Si(2p53s1) in a fully oxidized Si cation.

The electronic structure of A2CuO2X2 (A5Ca, Sr and X5F ,Cl) compounds is investigated by means of the periodic unrestricted Hartree-Fock ~UHF! method using the linear combination of atomic orbitals approach ~LCAO!. The relative stability of different alternative structures is discussed. All the systems are described as insulators with strong ionic character and well localized spin moments on the Cu atoms. The O 2p nature of the highest occupied bands is clear, supporting the charge transfer nature of this kind of narrow band systems; optical gaps are however, overestimated. The magnetic ordering of these materials is two dimensional in nature with almost independent antiferromagnetic CuO2 planes with Jc /J1;1023. The in–plane nearest (J1) and next-nearest (J2) magnetic coupling constants are antiferromagnetic and much larger than the interplane interactions (Jc), with J2 /J1;0.02. The relative values of the magnetic constants are in qualitative agreement with available experimental results. None of the analyzed properties provide differences between the various compounds so relevant to limit the possible development of a superconducting phase. On the contrary, the close similarity between them suggest that kinetic limitations in the doping process can be responsible of the fact that no superconducting transition has been observed in Sr2CuO2Cl2 and in the T8 phase of Sr2CuO2F2 and Ca2CuO2F2.

In the previous Comment, Forker and co-workers claim that perturbed angular correlation (PAC) data leave no alternative to the conclusion that the spontaneous magnetization of $\mathrm{Pr}{\mathrm{Co}}_{2}$ and $\mathrm{Nd}{\mathrm{Co}}_{2}$ undergoes a discontinuous, first-order phase transition at ${T}_{C}$. We show here that their claim is in clear contradiction with a wealth of experimental evidence, including our own. Finally, we propose a possible origin for the disagreement between their interpretation of the PAC results and the literature on this subject.

The electronic structure and chemical bonding in donor-acceptor complexes formed by group 13 element adamantine and perfluorinated adamantine derivatives EC9Rʹ15 (E = B, Al; R´= H, F) with Lewis bases XR3 and XC9H15 (X=N, P; R= H, CH3) have been studied using energy decomposition analysis (EDA) at the BP86/TZ2P level of theory. Larger stability of complexes with perfluorinated adamantine derivatives is mainly due to better electrostatic and orbital interactions. Deformation energies of the fragments and Pauli repulsion are of less importance, with exception for the boron-phosphorus complexes. The MO analysis reveals that LUMO energies of EC9Rʹ15 significantly decrease upon fluorination (by 4.7 and 3.6 eV for E = B and Al, respectively) which results in an increase of orbital interaction energies by 27-38 (B) and 15-26 (Al) kcal mol-1. HOMO energies of XR3 increase in order PH3 < NH3 < PMe3 < PC9H15 < NMe3 < NC9H15. For the studied complexes, there is a linear correlation between the dissociation energy of the complex and the energy difference between HOMO of the donor and the LUMO of the acceptor molecules. The fluorination of the Lewis acid significantly reduces standard enthalpies of the heterolytic hydrogen splitting H2 + D + A = [HD]+ + [HA]-. Analysis of the several types of the [HD]+ ··[HA]- ion pair formation reveals that orientation with additional H···F interactions is the most favorable energetically. Taking into account the ion pair formation, hydrogen splitting is predicted to be highly exothermic in case of the perfluorinated derivatives. Thus, fluorinated adamantine-based Lewis superacids are attractive synthetic targets and good candidates for the construction of the donor-acceptor cryptands This work was financially supported by St. Petersburg State University research grant 12.50.1194.2014. Excellent service of the Centre de Serveis Científiics i Acadèmmics de Catalunya (CESCA) and computer cluster at St. Petersburg State University is gratefully acknowledged. J.P. thanks the Netherlands Organization for Scientific Research (NWO-CW, NWO-EW, NWO-ALW) for financial support. M. S. thanks the following organizations for financial support: the Spanish government (MINECO, project number CTQ2014-54306-P), the Generalitat de Catalunya (project number 2014SGR931, ICREA Academia 2014 prize for excellence in research, and Xarxa de Referència en Química Teòrica i Computacional), and the FEDER fund (European Fund for Regional Development) for the grant UNGI10-4E-801

The results are presented of a combined periodic and cluster model approach to the electronic structure and magnetic interactions in the spin-chain compounds Ca2CuO3 and Sr2CuO3. An extended t-J model is presented that includes in-chain and interchain hopping and magnetic interaction processes with parameters extracted from ab initio calculations. For both compounds, the in-chain magnetic interaction is found to be around -240 meV, larger than in any of the other cuprates reported in the literature. The interchain magnetic coupling is found to be weakly antiferromagnetic, -1 meV. The effective in-chain hopping parameters are estimated to be ~650 meV for both compounds, whereas the value of the interchain hopping parameter is 30 meV for Sr2CuO3 and 40 meV for Ca2CuO3, in line with the larger interchain distance in the former compound. These effective parameters are shown to be consistent with expressions recently suggested for the Néel temperature and the magnetic moments, and with relations that emerge from the t-J model Hamiltonian. Next, we investigate the physical nature of the band gap. Periodic calculations indicate that an interpretation in terms of a charge-transfer insulator is the most appropriate one, in contrast to the suggestion of a covalent correlated insulator recently reported in the literature.

The interface of MgO/Ag(001) has been studied with density functional theory applied to slabs. We have found that regular MgO films show a small adhesion to the silver substrate, the binding can be increased in off-stoichiometric regimes, either by the presence of O vacancies at the oxide film or by a small excess of O atoms at the interface between the ceramic to the metal. By means of theoretical methods, the scanning tunneling microscopy signatures of these films is also analyzed in some detail. For defect free deposits containing 1 or 2 ML and at low voltages, tunnelling takes place from the surface Ag substrate, and at large positive voltages Mg atoms are imaged. If defects, oxygen vacancies, are present on the surface of the oxide they introduce much easier channels for tunnelling resulting in big protrusions and controlling the shape of the image, the extra O stored at the interface can also be detected for very thin films.

The electronic structure of the molecular solid Ni(tmdt)2, the only well characterized neutral molecular metal to date, has been studied by means of first-principles density functional calculations. It is shown that these calculations correctly describe the metallic vs semiconducting behavior of molecular conductors of this type. The origin of the band overlap leading to the metallic character and the associated Fermi surfaces has been studied.

We have characterized the RT structure of Bi0.75Sr0.25MnO3 by means of electron diffraction, synchrotron x-ray, and neutron powder diffraction. These data evidence that a;b;A2ap and c;2ap average structure presents a modulation that doubles the a and c lattice parameters. The temperature evolution of this compound above RT has been investigated using magnetization, calorimetry measurements, and neutron powder diffraction. These data reveal that the superstructure disappears above ;575 K where a structural transition takes place. Susceptibility data show that, concomitant to the formation of the superstructure ~on cooling!, there is an enhancement of the effective paramagnetic moment as has also been found in other Bi-(Sr,Ca)MnO3 bismuth manganites. The observed features and cell evolution signals that a new type of charge and polaron ordering occurs at this composition.

We illustrate how to apply modern effective field theory techniques and dimensional regularization to factorise the various scales which appear in QED bound states at finite temperature. We focus here on the muonic hydrogen atom. Vacuum polarization effects make the physics of this atom at finite temperature very close to that of heavy quarkonium states. We comment on the implications of our results for these states in the quark gluon plasma. In particular, we estimate the effects of a finite charm quark mass in the dissociation temperature of bottomonium., 22 pages, 8 figures. Journal version, reference added

The ab initio cluster model approach has been used to study the electronic structure and magnetic coupling of KCuF3 and K2CuF4 in their various ordered polytype crystal forms. Due to a cooperative Jahn-Teller distortion these systems exhibit strong anisotropies. In particular, the magnetic properties strongly differ from those of isomorphic compounds. Hence, KCuF3 is a quasi-one-dimensional (1D) nearest neighbor Heisenberg antiferromagnet whereas K2CuF4 is the only ferromagnet among the K2MF4 series of compounds (M=Mn, Fe, Co, Ni, and Cu) behaving all as quasi-2D nearest neighbor Heisenberg systems. Different ab initio techniques are used to explore the magnetic coupling in these systems. All methods, including unrestricted Hartree-Fock, are able to explain the magnetic ordering. However, quantitative agreement with experiment is reached only when using a state-of-the-art configuration interaction approach. Finally, an analysis of the dependence of the magnetic coupling constant with respect to distortion parameters is presented.

We investigate the injection of degenerate Fermi-Dirac electrons into a multimode ballistic conductor under the space-charge limited regime. The nonequilibrium current fluctuations were found to be suppressed by both Coulomb and Fermi correlations. We show that the Fermi shot-noise suppression factor is limited below by the value 2kT/epsilon_F, where T is the temperature and epsilon_F the Fermi energy of the injected electrons. The Coulomb noise suppression factor may attain much lower values epsilon_F/2qU, because of its dependence on the applied bias U >> kT/q. The asymptotic behaviour of the overall shot-noise suppression factor in a high degenerate limit was found to be kT/qU, independently of the material parameters., 8 pages, 4 figures, minor changes, published version

The physical contributions to the KNiF 3 magnetic exchange coupling integral have been obtained from specially designed ab initio cluster model calculations. Three important mechanisms have been identified. These are the delocalization of the magnetic orbitals into the anion «p» band, the variational contribution of the second-order interactions, and the many-body terms «hidden» in the two-body operator of the Heisenberg Hamiltonian

The electronic structures of the skutterudite-type phosphides ${\mathrm{CoP}}_{3}$ and ${\mathrm{NiP}}_{3}$ have been investigated by means of first-principles linear muffin-tin orbital--atomic sphere approximation band-structure calculations. The presence of ${\mathrm{P}}_{4}$ rings in the skutterudite structure is of great importance in determining the nature of the electronic bands around the Fermi level, composed mainly of \ensuremath{\pi}-type molecular orbitals of these units. The metallic character found for ${\mathrm{NiP}}_{3}$ should be ascribed to the phosphorus framework rather than to the metal atoms. \textcopyright{} 1996 The American Physical Society.

The electronic structure of the wurtzite-type phase of aluminum nitride has been investigated by means of periodic ab initio Hartree-Fock calculations. The binding energy, lattice parameters (a,c), and the internal coordinate (u) have been calculated. All structural parameters are in excellent agreement with the experimental data. The electronic structure and bonding in AlN are analyzed by means of density-of-states projections and electron-density maps. The calculated values of the bulk modulus, its pressure derivative, the optical-phonon frequencies at the center of the Brillouin zone, and the full set of elastic constants are in good agreement with the experimental data.

The formation energy, geometry, and electronic structure of isolated oxygen and aluminum vacancies in bulk and on the (0001) surface of corundum ( α − Al 2 O 3 ) have been investigated by means of periodic calculations in the framework of density functional theory within the generalized gradient approximation and large supercells. The energy cost to form an oxygen vacancy in the bulk is estimated to be of the order of 10 eV, whereas that corresponding to the formation of Al vacancies is found to be at least a 30% larger. The relaxation of the material is rather small for both defects. The removal of an oxygen atom in bulk α − Al 2 O 3 is accompanied by the appearance of an impurity level in the gap, which is a strong indication of electron localization. This has been further confirmed by integration of the density of states in the energy interval corresponding to the impurity level and by several other theoretical analyses. For the α − Al 2 O 3 ( 0001 ) surface, the formation of oxygen and aluminum vacancies exhibits many similarities with the bulk; the energy cost to form Al vacancies is much larger than for O vacancies and, in both cases, it is accompanied by rather small atomic displacements. However, there are also significant differences between bulk and surface oxygen and aluminum vacancies. Thus the formation energy of one of these point defects in the surface is rather smaller as expected and, more importantly, the degree of electronic delocalization is also larger.

The first observation of the elusive Fe4+ charge state coming from the nuclear decay of 57Co3+ has been found in the Mössbauer emission spectra of 57Co:La2Li0.5Co0.5O4. A Ti-doped sample was prepared in order to show that the Fe4+ fraction can be conveniently monitored. Both results were predicted on the basis of the electronic energy-band scheme of these oxides.