R. Zeller

Magnetic properties of 3d transition metal monolayers on metal substrates

We report results of systematic calculations for magnetic properties of 3d transition metal monolayers on Pd(001) and Ag(001). We find large similarities to interactions of magnetic 3d impurities in the bulk. Therefore the overlayer results are supplemented with results for 3d dimers in Cu, Ag, and Pd. Differences between the two classes of systems are utilized to reveal the interaction within the overlayers and between overlayers and substrates. In virtually all cases we find both ferromagnetic and antiferromagnetic solutions, showing large magnetic moments and similar densities of states. From the trend of the calculations we conclude that V, Cr, and Mn overlayers favor the antiferromagnetic c(2×2) structure, while Ti, Fe, Co, and Ni prefer the ferromagnetic one.

Role of vacancies in metal–insulator transitions of crystalline phase-change materials

The study of metal-insulator transitions (MITs) in crystalline solids is a subject of paramount importance, both from the fundamental point of view and for its relevance to the transport properties of materials. Recently, a MIT governed by disorder was observed in crystalline phase-change materials. Here we report on calculations employing density functional theory, which identify the microscopic mechanism that localizes the wavefunctions and is driving this transition. We show that, in the insulating phase, the electronic states responsible for charge transport are localized inside regions having large vacancy concentrations. The transition to the metallic state is driven by the dissolution of these vacancy clusters and the formation of ordered vacancy layers. These results provide important insights on controlling the wavefunction localization, which should help to develop conceptually new devices based on multiple resistance states.

Conceptual improvements of the KKR method

We review some recent conceptual improvements of the Korringa–Kohn–Rostoker (KKR) Green function method for electronic structure calculations. After an introduction into the KKR–Green function method we present an extension of this method into an accurate full-potential scheme, which allows calculation of forces and lattice relaxations. The additional numerical effort compared to the atomic sphere approximation scales only linear with the number of atoms. In addition, we discuss the recently developed screened KKR method which represents a reformulation of the multiple scattering theory with exponentially decreasing structure constants. This method, which has the same accuracy as the standard KKR method, exhibits strong advantages for two-dimensional systems like multilayers or surfaces, since the numerical effort scales linearly with the number of layers. The strength of both methods is illustrated in typical applications.

Broken-bond rule for the surface energies of noble metals

Using two different full-potential ab initio techniques we introduce a simple rule based on the number of broken first-neighbour bonds to determine the surface energies of the three noble metals Cu, Ag and Au. When one bond is broken, the rearrangement of the electronic charge for these metals does not practically lead to a change of the remaining bonds. Thus the energy needed to break a bond is independent of the surface orientation, so that the surface energy is in good approximation proportional to the number of broken nearest-neighbour bonds.

Vacancy-complexes with oversized impurities in Si and Ge

In this paper we examine the electronic and geometrical structure of impurity-vacancy complexes in Si and Ge. Already Watkins suggested that in Si the pairing of Sn with the vacancy produces a complex with the Sn-atom at the bond center and the vacancy split into two half vacancies on the neighboring sites. Within the framework of density-functional theory we use two complementary ab initio methods, the pseudopotential plane wave (PPW) method and the all-electron Kohn-Korringa-Rostoker (KKR) method, to investigate the structure of vacancy complexes with 11 different sp-impurities. For the case of Sn in Si, we confirm the split configuration and obtain good agreement with EPR data of Watkins. In general we find that all impurities of the 5sp and 6sp series in Si and Ge prefer the split-vacancy configuration, with an energy gain of 0.5 to 1 eV compared to the substitutional complex. On the other hand, impurities of the 3sp and 4sp series form a (slightly distorted) substitutional complex. Al impurities show an exception from this rule, forming a split complex in Si and a strongly distorted substitutional complex in Ge. We find a strong correlation of these data with the size of the isolated impurities, being defined via the lattice relaxations of the nearest neighbors.

Ab-initio calculations of the electronic structure of impurities and alloys of ferromagnetic transition metals

Abstract We review recent theoretical work on the electronic structure and the magnetic properties of ferromagnetic transition-metal alloys. All calculations are based on density-functional theory in the local-spin-density approximation. We report about calculations for dilute alloys using the KKR-Greens function method and for concentrated disordered alloys using the charge-self-consistent KKR-CPA method.

Local moments of 3d, 4d, and 5d atoms at Cu and Ag (001) surfaces

Abstract We present ag-initio calculations for the electronic structure of 3d, 4d and 5d transition-metal impurities at the (001) surface of Cu and Ag and determine the surface enhancement of the local moments. For 3d impurities we find a sizable enhancement of the local moments, being most important for V and Cr. Large local moments are obtained for 4d and 5d impurities which are in general non-magnetic in the bulk. Some of the adatoms (Nb, Mo, Tc, W, Re) on Ag (001) have “giant” magnetic moments between 3 and 4 μ B .

Fully relativistic calculation of the hyperfine fields of 5d‐impurity atoms in ferromagnetic Fe

The spin‐polarized, fully relativistic version of the Korringa–Kohn–Rostoker Green’s‐function method of band‐structure calculation has been used to calculate the electronic structure of 5d‐impurity atoms dissolved in Fe. As a new feature of such investigations, the spin as well as the orbital contributions to the magnetic moments and hyperfine fields were accessible to an evaluation. For the hyperfine fields the orbital‐dipolar contributions stemming from non‐s‐electrons of the core and the conduction band were found to be quite large and non‐negligible. In addition, we found a drastic relativistic enhancement of the Fermi‐contact field due to the s‐electrons. For this reason a completely relativistic treatment of the hyperfine interaction of 5d‐transition metals seems to be indispensable to achieve satisfying agreement with experimental data.

Hyperfine fields of impurities in ferromagnets

Abstract Due to the development of Greens function methods the calculation of hyperfine fields of impurities in ferromagnets has become feasible in the last years. We present the result of recent calculations for sp- and d-impurities and their nearest neighbours in Fe. The calculations are based on density functional theory; the potentials of both the impurity and the neighboring host atoms are determined self-consistently. For not too heavy impurities ( Z ≤ 50) the calculations well produce the observed trend of hyperfine fields. We discuss the strong correlation between the local magnetic moment of magnetic impurities and the hyperfine field, observed experimentally, in the light of the calculations. For sp-impurities the so-called systematic behavior of the hyperfine field with increasing Z is explained in terms of the bonding properties between impurity s and host d orbitals.

First-principles calculations for vacancy formation energies in Cu and Al; non-local effect beyond the LSDA and lattice distortion

Abstract We show ab initio calculations for vacancy formation energies in Cu and Al. The calculations are based on density-functional theory and the full-potential Korringa–Kohn–Rostoker Greens function method for impurities. The non-local effect beyond the local-spin-density approximation (LSDA) for density-functional theory is taken into account within the generalized-gradient approximation (GGA) of Perdew and Wang. The lattice relaxation around a vacancy is also investigated using calculated Hellmann–Feynman forces exerted on atoms in the vicinity of a vacancy. We show that the GGA calculations reproduce very well the experimental values of vacancy formation energies and bulk properties of Cu and Al, as they correct the deficiency of LSDA results (underestimation of equilibrium lattice parameters, overestimation of bulk moduli, and vacancy formation energies). It is also shown that the GGA calculations reduce the LSDA results for the lattice relaxation energy for a vacancy in Cu.