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Over recent decades, much effort has been made to lengthen spin relaxation/decoherence times of single-molecule magnets and molecular qubits by following different chemical design rules such as maximizing the total spin value, controlling symmetry, enhancing the ligand field or inhibiting key vibrational modes. Simultaneously, electronic structure calculations have been employed to provide an understanding of the processes involved in the spin dynamics of molecular systems and served to refine or introduce new design rules. This review focuses on contemporary theoretical approaches focused on the calculation of spin relaxation/decoherence times, highlighting their main features and scope. Fundamental aspects of experimental techniques for the determination of key Single Molecule Magnet/Spin Qubit properties are also reviewed.

In this work, using DFT + U formalism, we investigate the effect of order–disorder in the A-site occupation by La3+ and Sr2+ on the stability of the ferromagnetic order in La2/3Sr1/3MnO3 with R 3 - c symmetry. To date, a detailed theoretical discussion of such phenomenon, using a combination of different representations of the electronic structure, is still missing in the Literature. We employed structural models consisting of 120 atom supercells constructed according to the precise stoichiometry of the compound. Two configurations, describing randomized and ordered occupation of the La3+/Sr2+ ions, were evaluated. We demonstrate that the ferromagnetic arrangement of La2/3Sr1/3MnO3 with randomly distributed La3+ and Sr2+ ions is more stable. In such configuration we find that the Mn3+ and Mn4+ ions are not distinguished, favoring the double-exchange mechanism, enhanced by the higher degree of covalence in the Mn O bonds near the Fermi level between the spin-up Mn-eg orbitals and the O-p orbitals.

Newton trajectories are used for the Frenkel-Kontorova model of a finite chain with free-end bound- ary conditions. We optimise stationary structures, as well as barrier breakdown points for a critical tilting force were depinning of the chain happens. These special points can be obtained straight for- wardly by the tool of Newton trajectories. We explain the theory and add examples for a finite-length chain of a fixed number of 2, 3, 4, 5 and 23 particles.

Producción Científica, Putative global minimum structures for neutral CdN and singly charged Cd+ N and Cd− N clusters in the small size regime up to N = 21 atoms are reported. A global optimization approach based on the basin hopping method and a Gupta potential fitted to cluster properties is employed to generate a diverse databank of trial structures, which are then re-optimized at the densityfunctional level of theory. Novel, previously unreported, structures are found for many sizes. Our results successfully reproduce and interpret the size-dependent stabilities known from mass spectrometry, and strongly suggest that experiments aimed at determining the relative stabilities of neutral cadmium clusters are really measuring cation stabilities. We provide an in-depth analysis of electronic structure and use it to explain the gradual emergence of metallic-like behaviour as the cluster size increases., Junta de Castilla y León (Project VA124G18), Ministerio de Ciencia, Innovación y Universidades (Project PGC2018-093745- B-100)

En el presente trabajo se utilizó el código Wien2k, en el marco de la teoría del funcional de la densidad de Kohn-Sham, aplicando el método de ondas planas aumentadas y linealizadas (full-potential linearized augmented plane wave, FP- LAPW) y adoptando la aproximación de gradiente generalizado (GGA) de Perdew, Burke y Ernzerhof para la energía de intercambio y correlación, así como la aproximación de densidad local (local density approximation, LDA) para el cálculo de la densidad de estados y la estructura de bandas de la perovskita doble Sr2HoNbO6. Para los cálculos se consideró el grupo Fmm (#225), experimentalmente obtenido a partir de mediciones de difracción de rayos X y del método de refinamiento de Rietveld. El parámetro de red experimental fue de 8.018 Å, el cual concuerda en un 99,2 % con las predicciones teóricas efectuadas a partir de la minimización de la energía mediante la ecuación de estado de Murnaghan. A partir de mediciones de susceptibilidad magnética en función de la temperatura y del ajuste con la ley de Curie, se obtuvo el valor del momento magnético efectivo 10,01 μB. Este valor es muy cercano del esperado teóricamente a partir de las reglas de Hund (10,60 μB). La brecha de energía determinada entre las bandas de valencia y de conducción fue de 3,3 eV, lo que revela el carácter aislante de la perovskita compleja Sr2HoNbO6 para la configuración de espín hacia arriba, en tanto que se observó el carácter semiconductor para la polarización de espín hacia abajo, con una brecha de energía de 0,77 eV. Las propiedades termodinámicas se calcularon a partir de la ecuación de estado usando el modelo cuasi-armónico de Debye. Un comportamiento del calor específico, con CV≈CP, se encontró a temperaturas inferiores a T = 500 K, con valores del límite de Dulong-Petit que doblaban los que se han reportado para materiales del tipo de la perovskita. In this work we used the Wien2k code, within the framework of the Kohn-Sham Density Functional Theory (DFT), applying the Full-Potential Linearized Augmented Plane Wave method (FP-LAPW) and adopting the Generalized Gradient approximation (GGA) for the exchange-correlation energy due to Perdew, Burke, and Ernzerhof, as well as the Local Density approximation (LDA) for the calculation of the Density of States and band structure of the Sr2HoNbO6 double perovskite. For calculations, we considered the Fmm (#225) space group, which was experimentally obtained from X-ray diffraction measurements and Rietveld refinement. The experimental lattice parameter was 8.018 Å, which is 99.2% in agreement with the theoretical prediction from the minimization of energy through the Murnaghan state equation. From the measurements of magnetic susceptibility as a function of temperature and the adjustment with the Curie law, we obtained a value for the effective magnetic moment of 10.01 μB, which is close to the theoretical expected from Hund’s Rule (10.60 μB). An energy gap of 3.3 eV between the valence band and the conduction band revealed the insulator character of the Sr2HoNbO6 complex perovskite for the spin up configuration, but a semiconductor feature was observed for the spin down polarization, with an energy gap of 0.77 eV. The thermodynamic properties were calculated from the state equation by using the Debye quasiharmonic model. A specific heat behavior of CV≈CP was found at temperatures below T = 500 K, with Dulong-Petit limit values doubling those reported for perovskite materials.

van der Waals interactions govern the physics of a plethora of molecular structures. It is well known that the leading term in the distance-based London expansion of the van der Waals energy for atomic and molecular dimers decays as $1/R^6$, where $R$ is the dimer distance. Using perturbation theory, we find the leading term in the distance-based expansion of the intracule pair density at the interatomic distance. Our results unveil a universal $1/R^3$ decay, which is less prone to numerical errors than the $1/R^6$ dependency, and it is confirmed numerically in H$_2$ and He$_2$ molecules. This \textit{signature} of van der Waals interactions can be directly used in the construction of approximate pair density and energy functionals including vdW corrections., 4 pages, 3 figures

We study the effect that a statically screened Coulomb potential represented by a Debye-Hückel-Yukawa potential has in the electronic structure of the simplest molecule H2+ within the Born-Oppenheimer approximation. The method of solution is based on a two-center partial-wave expansion expressed in confocal elliptic coordinates using B-spline polynomials. General algorithms for the computation of energies, wave functions, and dipole and nonadiabatic radial matrix elements are given in detail. As it occurs in atoms, screening in simple molecules shifts the energies of bound states upwards so that, as screening increases, every bound state eventually crosses the upper ionization threshold at a critical screening value. The loss of long-range Coulomb interactions has its effect in the structure of wave functions, and consequently in the dipole and nonadiabatic matrix elements at intermediate and long internuclear distances, which determine the dynamics in external electromagnetic fields and collisional processes. Other issues related to a practical solution of the arbitrary sign problem, as well as the assignment of angular and radial nodes to the variational eigenfunctions, and the appearance of molecular shape resonances and Borromean states in H2+ as screening increases, are also addressed in this work, Work supported by the Advanced Grant of the European Research Council XCHEM 290853, the European COST Action XLIC CM1204, and the MINECO Project No. FIS2013-42002-R. A.F.O.-L. acknowledges financial support from EACEA through an Erasmus Mundus scholarship. J.L.S.-V. acknowledges financial support from Vicerrectoria de investigacion at Universidad de Antioquia (Contract No. E01538 and Estrategia de Sostenibilidad 2016-2017) and from Departamento Administrativo de Ciencia, Tecnologia e Innovacion (COLCIENCIAS, Colombia) under Contract No.111565842968

En esta Tesis Doctoral se han estudiado teóricamente las propiedades electrónicas y ópticas de electrones y huecos confinados en puntos cuánticos semiconductores mediante el uso del método k·p dentro de las aproximaciones de masa efectiva y de función envolvente. Para tal fin, se han desarrollado modelos computacionales capaces de describir adecuadamente la estructura electrónica de las bandas de conducción y valencia de sistemas sometidos a varios fenómenos de interés. En concreto, se ha prestado especial atención a la respuesta de estas nanoestructuras frente a interacciones con el entorno (tensiones de deformación y piezoelectricidad) y a la aplicación de campos eléctricos y magnéticos externos. Adicionalmente, se ha estudiado la relajación de espín inducida por el acoplamiento espín-órbita teniendo en cuenta todas las posibles fuentes de mezcla de espín mediante modelos tridimensionales. Por último, se ha explorado también la aparición de estados de borde en nanoestructuras formadas por MoS2 monocapa., In this PhD Thesis we theoretically investigate the optical and electronic properties of semiconductor nanostructures by using the k·p method within the effective mass and the envelope function approximations. To this end, computational models are built to properly describe the conduction and valence bands of nanoscopic systems subject to various relevant phenomena. Particularly, we focus on quantum dots of different shape, dimensions, and composition to explore their behavior under external magnetic fields and interactions with the environment such as strain and piezoelectricity. In addition, the spin-orbit-induced relaxation of the spin degree of freedom confined in quantum dots is also studied taking into account all relevant sources of spin mixing in fully three-dimensional models. Finally, we also study the emergence of edge states in nanoribbons and quantum dots of monolayer MoS2, which is a novel two-dimensional material. The obtained results reveal several interesting features which may be useful for future applications.

Rational design of improved transition metal based materials mostly relies on their electronic structure descriptors, typically estimated by density functional theory and so unduly affected by self-interaction or static correlation errors. Here we show for all 30 transition metals that original or width-corrected d-band centers, and Hilbert transform highest peak descriptors are unaffected by self-interaction, while poor treatment of static correlation by hybrid functionals leads to an unbalanced description. Thus, descriptors have a general validity unbiased by a specific computational method.

An Euler-Lagrange-type equation is solved to different orders for a plane metallic system perturbed by the external operator Q^ used previously in a sum-rule calculation. This approach managed to reproduce to within a few percent the multipole surface-plasmon energies. The jellium model is used for the positive ionic background and local-density functionals for the Hamiltonian. From the zeroth-order Euler-Lagrange equation a self-consistent ground-state density is obtained and used in the first-order equation to obtain the induced density. From the induced density, several aspects of the system response are considered and special attention is focused on the static polarizability.

The non-coherent rate equation approach to the electrical transport in a serial quantum dot system is presented. The charge density in each quantum dot is obtained using the transfer Hamiltonian formalism for the current expressions. The interactions between the quantum dots and between the quantum dots and the electrodes are introduced by transition rates and capacitive couplings. Within this framework analytical expressions for the current and the charge in each quantum dot are presented. The effects of the local potential are computed within the self-consistent field regime. Despite the simplicity of the model, well-known effects are satisfactorily explained and reproduced. We also show how this approach can be extended into a more general case.

A general mapping between the energy of pertinent magnetic solutions and the diagonal terms of the spin Hamiltonian in a local representation provides the first general framework to extract accurate values for the many body terms of extended spin Hamiltonians from periodic first-principle calculations. Estimates of these terms for La2CuO4, the paradigm of high-Tc superconductor parent compounds, and for the SrCu2O3 ladder compound are reported. For La2CuO4, present results support experimental evidence by Toader et al. [Phys. Rev. Lett. 94, 197202 (2005)10.1103/PhysRevLett.94.197202]. For SrCu2O3 even larger four-body spin amplitudes are found together with Jl/Jr = 1 and non-negligible ferromagnetic interladder exchange.

Empleando un esquema de cálculo tight-binding que usa una base de orbitales sp3s*d5, se estudian propiedades de la estructura electrónica de un grupo de materiales semiconductores III-V los cuales son de interés para la tecnología de dispositivos electrónicos y optoelectrónicos. En específico, se analiza la influencia sobre estas propiedades de una tensión aplicada según la dirección cristalográfica [111], haciendo uso de una formulación presentada en la primera parte del trabajo [Mora-Ramos 2009]. Especial atención se presta a la inclusión del efecto de deformación interna de la red cristalina. Para cada material de los estudiados presentamos las dependencias de las brechas energéticas asociadas a los puntos Γ, X y L de la zona de Brillouin como funciones de la tensión uniaxial en AlP, InP, AlSb, GaSb, InSb. Asimismo, reportamos expresiones de ajuste para los valores de las principales brechas energéticas en esos cinco materiales. Se detecta una fuerte dependencia no lineal de estas magnitudes, así como de las masas efectivas de conducción, con la tensión.

We study the scaling behavior of the characteristic length of precursor magnetic nanostructures above the Curie temperature with the correlation length of quenched-in disorder. We found that the modulation length of the nanostructures $\ensuremath{\xi}$ follows the scaling law $\ensuremath{\xi}\ensuremath{\sim}\sqrt{\overline{D}}$, where $\overline{D}$ is the average size of the magnetized regions in the material. The scaling behavior of the average size of these regions, $\overline{D}$, with the correlation length of the disorder, $\ensuremath{\sigma}$, depends on the properties of the disorder. For Gaussian disorder, we find that $\overline{D}$ scales with the disorder correlation length as $\overline{D}\ensuremath{\sim}{\ensuremath{\sigma}}^{a∕2}$, where $a$ is the exponent of the leading term of the pair correlation function of the disorder in the limit $r\ensuremath{\rightarrow}0$, $\ensuremath{\Gamma}(r)\ensuremath{\approx}\ensuremath{\epsilon}[1\ensuremath{-}(1∕a){(r∕\ensuremath{\sigma})}^{a}]$. These results are quite general and applicable to other systems, e.g., ferroelectric precursors, independent of the nature of the long-range dipolar forces.

6 pages.-- PACS numbers: 73.21.-b, 73.22.-f, 71.15.Mb.-- Final full-text version of the paper available at: http://dx.doi.org/10.1103/PhysRevB.76.245302., Within density functional theory, we address the effect of an in-plane electric field [script E] on the ground state and the density dipole response of a many-electron quantum ring, which is also submitted to a perpendicular magnetic field B. Addition energies and density dipole spectra are discussed as a function of [script E]. For the two-electron case, an exact numerical calculation is performed, obtaining the spin-phase diagram in the [script E]-B plane and showing that transitions between singlet and triplet spin states can be induced by varying the fields. We also find that, in spite of the deformation that [script E] causes in the electronic density, the spin of the ground state of a given electron number ring is very robust, changing little as the strength of the electric field is reasonably increased., This work has been performed under Grants No. FIS2005-01414 and No. FIS2005-02796 from DGI (Spain), No. 2005SGR00343 from Generalitat de Catalunya, and Grant No. INFN07-30 from the Italian INFN-Spanish DGI agreement.

The computational approach to the Hirshfeld [Theor. Chim. Acta 44, 129 (1977)] atom in a molecule is critically investigated, and several difficulties are highlighted. It is shown that these difficulties are mitigated by an alternative, iterative version, of the Hirshfeld partitioning procedure. The iterative scheme ensures that the Hirshfeld definition represents a mathematically proper information entropy, allows the Hirshfeld approach to be used for charged molecules, eliminates arbitrariness in the choice of the promolecule, and increases the magnitudes of the charges. The resulting Hirshfeld-I charges correlate well with electrostatic potential derived atomic charges

A consistent extension of local spin density approximation (LSDA) to account for mass and dielectric mismatches in nanocrystals is presented. The extension accounting for variable effective mass is exact. Illustrative comparisons with available configuration interaction calculations show that the approach is also very reliable when it comes to account for dielectric mismatches. The modified LSDA is as fast and computationally low demanding as LSDA. Therefore, it is a tool suitable to study large particle systems in inhomogeneous media without much effort.

Within local-spin-density functional theory, we have investigated the `dissociation' of few-electron circular vertical semiconductor double quantum ring artificial molecules at zero magnetic field as a function of inter-ring distance. In a first step, the molecules are constituted by two identical quantum rings. When the rings are quantum mechanically strongly coupled, the electronic states are substantially delocalized, and the addition energy spectra of the artificial molecule resemble those of a single quantum ring in the few-electron limit. When the rings are quantum mechanically weakly coupled, the electronic states in the molecule are substantially localized in one ring or the other, although the rings can be electrostatically coupled. The effect of a slight mismatch introduced in the molecules from nominally identical quantum wells, or from changes in the inner radius of the constituent rings, induces localization by offsetting the energy levels in the quantum rings. This plays a crucial role in the appearance of the addition spectra as a function of coupling strength particularly in the weak coupling limit., 18 pages, 8 figures, submitted to Physical Review B

The energy associated with bosonic and fermionic pairs of topological spin defects in doped antiferromagnetic quantum spin-1/2 square lattice is estimated within a resonating valence bond scenario, as described by a t-t'-J-like model Hamiltonian, plus a t-perpendicular, responsible of a three-dimensional screening of the electrostatic repulsion within the bosonic pairs. For parameters appropriate for monolayered high-T_c superconductors, both fermionic and bosonic pairs show x^2-y^2 symmetry. We find a critical value of doping such that the energy of the bosonic pairs goes below twice the energy of two fermionic pairs at their Fermi level. This finding could be related to the onset of high-T_c superconductivity., Comment: 10 pages, 6 figures. To be published in Phys. Rev. B

Both the intermolecular interaction energies and the geometries for $M\ensuremath{\cdots}$ thiophene, $M\ensuremath{\cdots}$ pyrrole, ${M}^{n+}\ensuremath{\cdots}$ thiophene, and ${M}^{n+}\ensuremath{\cdots}$ pyrrole (with $M=\mathrm{Li}$, Na, K, Ca, and Mg; and ${M}^{n+}={\mathrm{Li}}^{+},\phantom{\rule{0.3em}{0ex}}{\mathrm{Na}}^{+},\phantom{\rule{0.3em}{0ex}}{\mathrm{K}}^{+},\phantom{\rule{0.3em}{0ex}}{\mathrm{Ca}}^{2+}$, and ${\mathrm{Mg}}^{2+}$) have been estimated using four commonly used density functional theory (DFT) methods: B3LYP, B3PW91, PBE, and MPW1PW91. Results have been compared to those provided by HF, MP2, and MP4 conventional ab initio methods. The PBE and MPW1PW91 are the only DFT methods able to provide a reasonable description of the $M\ensuremath{\cdots}\ensuremath{\pi}$ complexes. Regarding ${M}^{n+}\ensuremath{\cdots}\ensuremath{\pi}$ complexes, the four DFT methods have been proven to be adequate in the prediction of these electrostatically stabilized systems, even though they tend to overestimate the interaction energies.

has been addressed with quantum chemicalcalculations on large embedded clusters. We find a charge ordered state for the crystal structure reported byRadaelli et al. [Phys. Rev. B 55, 3015 (1997)] and Zener polaron formation in the crystal structure withequivalent Mn sites proposed by Daoud-Aladineet al. [Phys. Rev. Lett. 89, 097205 (2002)]. Important O toMn charge transfer effects are observed for the Zener polaron.DOI: 10.1103/PhysRevB.70.235104 PACS number(s): 75.47.Lx, 71.15.Ap, 71.27.1aI. INTRODUCTION

We explore the ability of the recently established quasilocal density functional theory for describing the isoscalar giant monopole resonance. Within this theory we use the scaling approach and perform constrained calculations for obtaining the cubic and inverse energy weighted moments (sum rules) of the RPA strength. The meaning of the sum rule approach in this case is discussed. Numerical calculations are carried out using Gogny forces and an excellent agreement is found with HF + RPA results previously reported in literature. The nuclear matter compression modulus predicted in our model lies in the range 210-230 MeV which agrees with earlier findings. The information provided by the sum rule approach in the case of nuclei near the neutron drip line is also discussed., 10 pages, LaTeX

Incompressible (magic) states of vertically coupled quantum dots submitted to strong magnetic fields such that only the lowest Landau level is relevant are studied within an exact diagonalization calculation for N=3, 5 and 6, electrons. We find that the sequences of total angular momentum M for which incompressible states exists depend on the interplay between the inter-dot hopping parameter \Delta_t and the inter-dot distance d. For d of the order of the magnetic length and for all values of \Delta_t, we conclude that, in contrast to previous claims, the incompressible states appear at magic values of M which do not differ from those obtained for a single dot, namely M=N(N-1)/2+jN where j is a positive integer number. For large inter-dot distance and simultaneously small inter-dot hopping parameter, new sequences of magic values of $M$ are observed. These new sequences can be easily understood in terms of a transition regime towards a system of two decoupled single dots. However, important differences in the nature of the incompressible ground states are found with respect to those of a single dot., Comment: 23 pages, 11 figures; discussion improved, references added, journal version

Thomas-Fermi theory is developed to evaluate nuclear matrix elements averaged on the energy shell, on the basis of independent particle Hamiltonians. One- and two-body matrix elements are compared with the quantal results and it is demonstrated that the semiclassical matrix elements, as function of energy, well pass through the average of the scattered quantum values. For the one-body matrix elements it is shown how the Thomas-Fermi approach can be projected on good parity and also on good angular momentum. For the two-body case the pairing matrix elements are considered explicitly., Comment: 15 pages, REVTeX, 6 ps figures; changed content

The role of the bridging ligand on the effective Heisenberg coupling parameters is analyzed in detail. This analysis strongly suggests that the ligand-to-metal charge transfer excitations are responsible for a large part of the final value of the magnetic coupling constant. This permits to suggest a new variant of the Difference Dedicated Configuration Interaction (DDCI) method, presently one of the most accurate and reliable for the evaluation of magnetic effective interactions. This new method treats the bridging ligand orbitals mediating the interaction at the same level than the magnetic orbitals and preserves the high quality of the DDCI results while being much less computationally demanding. The numerical accuracy of the new approach is illustrated on various systems with one or two magnetic electrons per magnetic center. The fact that accurate results can be obtained using a rather reduced configuration interaction space opens the possibility to study more complex systems with many magnetic centers and/or many electrons per center., Comment: 7 pages, 4 figures

The magnetic coupling constant of selected cuprate superconductor parent compounds has been determined by means of embedded cluster model and periodic calculations carried out at the same level of theory. The agreement between both approaches validates the cluster model. This model is subsequently employed in state-of-the-art configuration interaction calculations aimed to obtain accurate values of the magnetic coupling constant and hopping integral for a series of superconducting cuprates. Likewise, a systematic study of the performance of different ab initio explicitly correlated wave function methods and of several density functional approaches is presented. The accurate determination of the parameters of the t-J Hamiltonian has several consequences. First, it suggests that the appearance of high- Tc superconductivity in existing monolayered cuprates occurs with J/t in the 0.20‐0.35 regime. Second, J/t50.20 is predicted to be the threshold for the existence of superconductivity and, third, a simple and accurate relationship between the critical temperatures at optimum doping and these parameters is found. However, this quantitative electronic structure versus Tc relationship is only found when both J and t are obtained at the most accurate level of theory.

Electron transport in a self-consistent potential along a ballistic two-terminal conductor has been investigated. We have derived general formulas which describe the nonlinear current-voltage characteristics, differential conductance, and low-frequency current and voltage noise assuming an arbitrary distribution function and correlation properties of injected electrons. The analytical results have been obtained for a wide range of biases: from equilibrium to high values beyond the linear-response regime. The particular case of a three-dimensional Fermi-Dirac injection has been analyzed. We show that the Coulomb correlations are manifested in the negative excess voltage noise, i.e., the voltage fluctuations under high-field transport conditions can be less than in equilibrium., Comment: 16 pages, 10 figs; minor corrections

We investigate the dissociation of few-electron circular vertical semiconductor double quantum dot artificial molecules at 0 T as a function of interdot distance. Slight mismatch introduced in the fabrication of the artificial molecules from nominally identical constituent quantum wells induces localization by offsetting the energy levels in the quantum dots by up to 2 meV, and this plays a crucial role in the appearance of the addition energy spectra as a function of coupling strength particularly in the weak coupling limit., Accepted for publication in Phys. Rev. Lett

A series of new compounds has been synthesized by insertion of alkali or alkaline earth-metals in chalcopyrite, CuFeS2. Their crystal structures vary from layered arrangements of tetrahedra in a trigonal (LiCuFeS2, NaCuFeS2) or a tetragonal symmetry (KCuFeS2, CsCuFeS2), to three-dimensional cubic framework (Ba6Cu12Fe13S 27). Despite these differences, the synthesized compounds follow a common structural pattern: the sulfur and M atoms (M= Li, Na, K, Cs, or Ba) are ordered in a distorted face-centered-cubic lattice, where all tetrahedral holes surrounded by S atoms are occupied by the transition metal elements. The tetrahedral holes surrounded by both S atoms and at least one M element are empty. The band structures of these compounds are discussed in the framework of an extended Hückel Hamiltonian Una serie de nuevos compuestos han sido sintetizados por inserción de iones de metales alcalinos o alcalinos-térreos en calcopirita. Sus estructuras cristalinas varían desde un ordenamiento bidimensional de tetraedros con simetría trigonal (LiCuFeS2, NaCuFeS2) o con simetría tetragonal (KCuFeS2, CsCuFeS2), hasta un ordenamiento cúbico tridimensional (Ba6Cu 12Fe13S27). A pesar de estas diferencias, los compuestos sintetizados siguen un patrón estructural común: los átomos de azufre y del tipo M (M= Li, Na, K, Cs, or Ba ) están ordenados en una red cúbica de cara centrada distorsionada, donde todos los huecos tetraédricos que están rodeados por átomos de azufre, son ocupados por Cu o Fe . Los huecos tetraédricos que están rodeados por átomos de S y al menos un elemento del tipo M están vacíos. La estructura de bandas de estos compuestos se calcularon por el método de extended Hückel

A self-consistent theory of shot noise in ballistic two-terminal conductors under the action of long-range Coulomb correlations is presented. Analytical formulas for the electron distribution function and its fluctuation along the conductor, which account for the Coulomb correlations, have been derived. Based upon these formulas, the current-noise reduction factor has been obtained for biases ranging from thermal to shot-noise limits as dependent on two parameters: the ratio between the length of the sample and the Debye screening length \lambda=d/L_D and the applied voltage qU/k_BT. The difference with the formulas for a vacuum diode is discussed., Comment: 21 pages, 11 figs, minor changes

We investigate the current injection into a ballistic conductor under the space-charge limited regime, when the distribution function of injected carriers is an arbitrary function of energy F_c(epsilon). The analysis of the coupled kinetic and Poisson equations shows that the injected current fluctuations may be essentially suppressed by Coulomb correlations, and the suppression level is determined by the shape of F_c(epsilon). This is in contrast to the time-averaged quantities: the mean current and the spatial profiles are shown to be insensitive to F_c(epsilon) in the leading-order terms at high biases. The asymptotic high-bias behavior for the energy resolved shot-noise suppression has been found for an arbitrary (non-Poissonian) injection, which may suggest a new field of investigation on the optimization of the injected energy profile to achieve the desired noise-suppression level., extended version 4 -> 8 pages, examples and figure added

Recent improvements in the determination of the running of the fine-structure constant also allow an update of the hadronic vacuum-polarization contribution to the Lamb shift. We find a shift of -3.40(7) kHz to the 1S level of hydrogen. We also comment on the contribution of this effect to the determination by elastic electron scattering of the r.m.s. radii of nuclei., Comment: 7 pages, latex, 1 figure -- Submitted to Phys. Rev. A -- epsfig.sty required

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

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}

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.

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.

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.

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#

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.

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

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.