Linear Muffin-tin Orbital Atomic Sphere Research Articles

Positron confinement in embedded lithium nanoclusters

Quantum confinement of positrons in nanoclusters offers the opportunity to obtain detailed information on the electronic structure of nanoclusters by application of positron annihilation spectroscopy techniques. In this work, positron confinement is investigated in lithium nanoclusters embedded in monocrystalline MgO. These nanoclusters were created by means of ion implantation and subsequent annealing. It was found from the results of Doppler broadening positron beam analysis that approximately 92% of the implanted positrons annihilate in lithium nanoclusters rather than in the embedding MgO, while the local fraction of lithium at the implantation depth is only 1.3 at. %. The results of two-dimensional angular correlation of annihilation radiation confirm the presence of crystalline bulk lithium. The confinement of positrons is ascribed to the difference in positron affinity between lithium and MgO. The nanocluster acts as a potential well for positrons, where the depth of the potential well is equal to the difference in the positron affinities of lithium and MgO. These affinities were calculated using the linear muffin-tin orbital atomic sphere approximation method. This yields a positronic potential step at the $\mathrm{MgO}\ensuremath{\Vert}\mathrm{Li}$ interface of 1.8 eV using the generalized gradient approximation and 2.8 eV using the insulator model.

XPS and theoretical studies of the electronic structure of FeX and NiX (X = Al, Si, P)

For the stoichiometric compounds FeX and NiX (X = Si, Al, P) the x-ray photoelectron spectra of valence bands are obtained and calculations of their electronic structure are carried out by the tight-binding linear muffin-tin orbital atomic sphere approximation method (TB-LMTO-ASA). A satisfactory agreement between the x-ray photoelectron spectra and the calculated density-of-states curves is obtained. It is shown that with an increase of the second-component p-electrons, hybridization of d(metal) and p(X) electrons decreases, the probability of d–p resonance increases and the contribution of p(X) states near Ef increases. Copyright © 2002 John Wiley & Sons, Ltd.

Magnetism of Co clusters embedded in Cu( [formula omitted]) surfaces: an ab initio study

We report calculations of electronic structure and magnetic properties of Co clusters (up to 50 atoms) embedded in Cu( 0 0 1 ) surfaces, performed using the first-principles linear muffin tin orbital-atomic sphere approximation method, implemented directly in real space. Co agglomerates of different sizes and shapes are considered in order to investigate the influence of the local environment around the Co sites to the magnetism in this class of systems. We find that the magnitude of the Co moments is mainly governed by two factors: the position of the site relative to the Cu( 0 0 1 ) surface layer and the number of Cu neighbors. The results show moment enhancement for sites located above the surface and/or placed substitutionally in the surface layer, due to their reduced coordination numbers. For sites with the same coordination number, the moment tends to decrease as the number of Cu neighbors increases. As a consequence, in Co agglomerates, the magnetic moment decreases considerably as one goes from more central sites to those close to the grain boundary at the Co–Cu interface.

Interlayer exchange coupling and electronic structure of Co/Mn multilayers

The interlayer exchange coupling between magnetic layers and the electronic structure in Co/Mn multilayers have been studied by using the linear muffin-tin orbital atomic sphere approximation method based on the local spin density approximation. It is found that the interlayer exchange coupling in Co/Mn multilayers shows oscillation with a period of 2 atomic layers because of the antiferromagnetic order of the Mn. The strength of the interlayer exchange coupling is quite large. The interface magnetic coupling between Co and Mn layers is ferromagnetic when the Mn layer is γ-Mn phase; however, it is antiferromagnetic when the Mn layer is α-Mn phase. In addition, the GMR effect of Co/Mn multilayers is discussed.

Screened exact exchange functional calculations of the spin-wave dispersion in transition metals

The screened “exact” nonlocal exchange approach is applied to the calculation of magnetic susceptibility of transition metals, particularly to the calculation of spin-wave dispersion. We discuss the choice of an appropriate approach and aspects of linear muffin-tin orbital-atomic sphere approximation (ASA) calculations, in particular calculation of the Green function in the three-center approximation. We calculated transverse spin-wave frequencies in Fe with the local density approximation and with the nonlocal functional using both the “frozen” magnon method and calculation of bound state energies (corresponding to spin-wave excitations).

Influence of spin-dependent scattering at rough interface on the giant magnetoresistance in magnetic multilayers

Spin-dependent scattering in Co/Cu and Co/Ru multilayers with interface roughness is estimated. The electronic band structures of Co/Cu and Co/Ru multilayers with smooth interfaces for parallel and antiparallel spin configurations are calculated self-consistently using linear muffin-tin orbital atomic sphere approximation (LMTO-ASA) band structure calculations based on density functional theory. The spin-dependent scattering caused by interface roughness is calculated from the transition probabilities obtained from self-consistent wave functions and potential differences found from these band structure calculations. Our results indicate that spin-dependent scattering caused by interface roughness plays an important role in the giant magnetoresistance (GMR) effect for magnetic multilayers, as predicted by calculations using a simple model. Furthermore, it is concluded that the scattering depends on differences in the band structure for different spin configurations, as well as for interface roughness. The GMR difference between Co/Cu and Co/Ru can be explained qualitatively by difference in spin-dependent scattering due to interface roughness, which is caused by the band structure with different spin configuration.

Electronic structure of the 1 x 1 YBa2Cu3O7/PrBa2Cu3O7 superlattice: A local-spin-density approximation with on-site Coulomb interaction.

We performed self-consistent linear muffin-tin orbital atomic sphere approximation calculations of the electronic structure of the 1{times}1 YBa{sub 2}Cu{sub 3}O{sub 7}/PrBa{sub 2}Cu{sub 3}O{sub 7} (YBCO/PBCO) superlattice, based on the local-spin-density approximation with on-site Coulomb correlation. From our results, we can exclude that the experimentally observed depression of the critical temperature is originated intrinsically by a modification of the electronic structure of the CuO{sub 2} planes. We find that the charge transfer between YBCO and PBCO layers is one order lower than the value required to explain the strong depression of both the critical temperature {ital T}{sub {ital c}}(onset) and the zero-resistance transition temperature {ital T}{sup 0}{sub {ital c}}, within the hole-filling model. {copyright} {ital 1996 The American Physical Society.}

Pressure-induced charge transfer in Li and Al alloys.

In pure metals, the s,p,d, . . . bands have different energy shifts as the metals are compressed. Changes that are produced in this way in the amount of s,p,d, . . . type valence charge affect, e.g., the bonding properties of the atoms, which in turn can lead to structural phase transitions at high pressures. In alloys, the band shifts can produce complex changes in the electronic structure. The present work deals with the pressure-induced charge transfer in alloys. In Li-X alloys (X=Be,Na,Mg,Al,Si,P,Ca,Sr) the number of Li valence electrons is found to increase along with increasing pressure. This is attributed to the properties of Li p states. A similar type of charge transfer is also found in Al-X alloys (X=Ti,Zr,Hf,Cu,Ag,Au). The effects of fully occupied and partially occupied d bands can be studied by considering Al alloyed with noble metals and transition metals. The electronic structure of the alloys is calculated with the linear muffin-tin orbital atomic sphere approximation method. To check the magnitude of the possible computational errors involved in our results we also performed pseudopotential plane-wave calculations for Li-Mg alloys. \textcopyright{} 1996 The American Physical Society.

Effect of pressure on the Curie temperature of Fe3Pt.

We have performed self-consistent spin-polarized calculations for ${\mathrm{Fe}}_{3}$Pt (which crystallizes in the ${\mathrm{Cu}}_{3}$Au structure) at various Wigner-Seitz radii using the linear muffin-tin orbital atomic sphere approximation method. The average magnetic moment obtained at equilibrium lattice constant (1.99${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$) is close to the experimental value (2.15${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$). We have calculated the Curie temperature of this compound at various Wigner-Seitz radii, using the model proposed by Mohn and Wohlfarth. The quantities which are obtainable from the band-structure calculations are the only requisite for this model. The variation of the Curie temperature with Wigner-Seitz radii, i.e., with pressure, ${\mathit{T}}_{\mathit{C}}$(P) is then compared with the experimental data.

The electronic structures of the pyrite-type disulphides (MS2, where M = Mn, Fe, Co, Ni, Cu, Zn) and the bulk properties of pyrite from local density approximation (LDA) band structure calculations

A local density approximation (LDA) band structure method, the Linear Muffin-Tin Orbital Atomic Sphere Approximation (LMTO-ASA) method has been used to calculate the electronic structures of the pyrite-type disulphides (MS2, where M = Mn, Fe, Co, Ni, Cu, Zn). The total density of states has been calculated for 10 eV above and below the Fermi Level, along with the separate contributions from metal and sulphur and shows that the metal d band occurs above the sulphur p bands in MnS2, FeS2, CoS2 and NiS2, whereas in CuS2, the d band passes through the sulphur p band and in ZnS2, it lies below the sulphur p band. Substantial hybridization of the metal d states with the sulphur states occurs. FeS2 is calculated to be a semiconductor with a direct band gap of 0.64 eV in good agreement with experiment. The calculated local densities of states have been used in turn to calculate X-ray photoelectron spectra and Bremsstrahlung Isochromat spectra for this series of compounds, and these also show reasonable agreement with experimental data. A particular strength of the LMTO-ASA method is the ability to calculate and predict certain bulk properties of solids of interest in mineral physics. This has enabled the first reasonably accurate calculations of the total energy of the valence electrons of the system for pyrite (FeS2), given as — 345.885 rydbergs per unit cell, and the equilibrium unit cell volume which is within 3.3% of that determined experimentally. A theoretical pressure vs. volume curve for pyrite was also calculated along with values for the bulk modulus. However, our calculations predict a bulk modulus of 6.75 Mbar which is too high by a factor of 4.6 due to the simplifying assumption of a uniform scaling of interatomic distances on compression.

First application of the real space linear muffin-tin orbital atomic sphere approximation method for calculating Fermi contact hyperfine fields

In this work we apply the recently developed first-principles real space linear muffin-tin orbital atomic sphere approximation (RS-LMTO-ASA) scheme to calculate the hyperfine field for 3d impurities (V, Cr, Mn andFe) in Cu. We obtain the Fermi contact term at the impurities andat four shells of host atoms around the impurities. We compare our results with theoretical values obtained using the KKR-Green-function method andwith NMR measurements. The overall agreement is excellent, confirming the reliability of the RS-LMTO-ASA approach as means of obtaining information on hyperfine fields.

The influence of deformation twin boundaries and doping on the electronic structure of CaCuO 2

The influence of deformation twin boundaries and doping on the electronic structure of CuO planes has been examined. Electronic structure calculations have been performed for ideal CaCuO 2,and CaCuO 2 containing a deformation twin boundary. The linear muffin-tin orbital atomic sphere approximation methods in the tight-binding (TB-LMTO-ASA) representation has been used. The twin boundary energy has been calculated in the deformed structure. In addition the influence of electron holes on the magnetic moment of copper and the energy gap is examined using the Hubbard model in the Hartree-Fock approximation.

Band calculations for Mn4N and body-centred cubic Mn, fixing local spin directions

The authors performed band calculations by the linear muffin-tin orbital (LMTO) atomic sphere approximation (ASA) method for Mn4N and BCC Mn using an exchange-correlation potential that depends on each of local spin directions at atomic spheres. They discussed the magnetic structure of Mn4N on the basis of results for two prototypes among several models proposed experimentally for Mn4N. The authors discussed the coexistence of ferromagnetic and antiferromagnetic states in BCC Mn.

One- and two-dimensional oxygen ordering in YBa 2Cu 3O z

It is shown that the (three-dimensional) tetragonal to orthorhombic transition in the 1-2-3 superconducting compound can be mapped onto a two-dimensional Ising model with anisotropic second neighbor effective pair interactions. The cluster variation method (CVM) has been used to calculate a phase diagram based on interaction parameters provided by first principles linear muffin-tin orbital-atomic sphere approximation (LMTO-ASA) calculations performed by P. Sterne at Lawrence Livermore National Laboratory. At high temperature, agreement with experimental phase transition data is excellent. At low temperatures, two-dimensional ordering gives way to quasi one-dimensional states of order as described by the linear chain Ising model for which the thermodynamics are known exactly. Structures produced by Monte Carlo simulation and theoretical diffraction patterns are in striking agreement with recent experimental results.

Thermodynamics of oxygen ordering in YBa 2Cu 3O z

It is shown that the (three-dimensional) tetragonal to orthorhombic transition in the 1-2-3 superconducting compound can be mapped onto a two-dimensional lsing model with anisotropic second neighbor effective pair interactions. The Cluster Variation Method (CVM) has been used to calculate a phase diagram based on interaction parameters provided by first principles linear muffin-tin orbital-atomic sphere approximation (LMTO-ASA) calculations performed by P. Sterne at Lawrence Livermore National Laboratory. At high temperature, agreement with experimental phase transition data is excellent. At low temperatures, two-dimensional ordering gives way to quasi one-dimensional states of order as described by the linear chain Ising model for which the thermodynamics are known exactly. Structures produced by Monte Carlo simulation and theoretical diffraction patterns are in striking agreement with recent experimental results.

Calculation of orbital moments in solids

Two methods for the calculation of orbital moments are discussed. In particular previous calculations for YCo 5 with the recursion method are compared to a new linear muffin-tin orbital atomic sphere approximation (LMTO-ASA) calculation. The magnetic properties of YCo 5 are discussed with emphasis on contributions of the different cobalt atom sites (3 g and 2 c). The effect of magnetocrystalline anisotropy is discussed. The orbital moment contribution is calculated for CaCuO 2, the parent of the new superconducting cuprates.

Real-space method for calculation of the electric field gradient: comparison with k-space results

Results for the electric field gradient (EFG) in eight HCP metals are obtained using the linear muffin-tin orbital (LMTO) atomic sphere approximation (ASA) method and a recently proposed LMTO ASA recursion scheme. In all cases the k-space LMTO ASA results for the EFG agree well with those obtained using the real-space procedure and a first-order LMTO ASA tight-binding Hamiltonian with interactions limited to first neighbours. This is remarkable considering that the EFG is determined by differences between partial occupation numbers, which are of order of a hundredth of an electron and shows that the first-order Hamiltonian is very reliable. A comparison with more precise full-potential linear augmented plane-wave calculations and experimental values shows reasonably good agreement. This opens the way for predictions of the EFG in more complicated and disordered systems.

Electronic structure of Si/disilicide interfaces

Using supercells, the electronic structures of Si(111)/CoSi2 and Si(111)/NiSi2 interfaces are studied by the linear muffin-tin orbital atomic sphere approximation method (LMTO-ASA). Schottky barrier heights (SBH's) are strongly correlated with the interface atomic structures and are determined mainly by interface bonding states and the screening effect of the semiconductor. Metal-induced gap states (MIGS) are metal wave function tails caused by the Schottky barriers.

Spin-polarised band structure for the semimagnetic semiconductor Zn0.5Mn0.5Se

The spin-polarised band structure, density of states (DOS), magnetic moments and exchange interaction coefficients for a layer structure Zn0.5Mn0.5Se in the ferromagnetic phase have been calculated using the self-consistent linear muffin-tin orbital atomic sphere approximation method. Its band structure and DOS distribution are explained through a p-d repulsion model. The band structure in the antiferromagnetic phase is also analysed qualitatively in terms of the p-d repulsion mechanism. The results of the present work show that the bands are strongly polarised. In the case of spin up, the bands are direct, with a band gap of 0.68 eV and a filled Mn 3d valence band whereas, for the spin-down case, the bands are indirect, with a band gap of 1.81 eV and an empty Mn 3d conduction band. These results are in agreement with Hund's rule. The calculated total magnetic moment is 4.81 mu B/cell; the p-d and s-d exchange interaction coefficients are -1.10 eV and 0.04 eV, respectively.

Thermodynamics of oxygen ordering in YBa2Cu

It is shown that the (three-dimensional) tetragonal to orthorhombic transition in the 1-2-3 superconducting compound can be mapped onto a two-dimensional Ising model with anisotropic second neighbor effective pair interactions. The Cluster Variation Method (CVM) has been used to calculate a phase diagram based on interaction parameters provided by first principles linear muffin-tin orbital-atomic sphere approximation (LMTO-ASA) calculations performed by P. Sterne at Lawrence Livermore National Laboratory. At high temperature, agreement with experimental phase transition data is excellent. At low temperatures, two-dimensional ordering gives way to quasi one-dimensional states of order as described by the linear chain Ising model for which the thermodynamics are known exactly. Structures produced by Monte Carlo simulation and theoretical diffraction patterns are in striking agreement with recent experimental results.