Hannes Raebiger

Charge self-regulation upon changing the oxidation state of transition metals in insulators

Transition-metal atoms embedded in an ionic or semiconducting crystal can exist in various oxidation states that have distinct signatures in X-ray photoemission spectroscopy and ‘ionic radii’ which vary with the oxidation state of the atom. These oxidation states are often tacitly associated with a physical ionization of the transition-metal atoms—that is, a literal transfer of charge to or from the atoms. Physical models have been founded on this charge-transfer paradigm, but first-principles quantum mechanical calculations show only negligible changes in the local transition-metal charge as the oxidation state is altered. Here we explain this peculiar tendency of transition-metal atoms to maintain a constant local charge under external perturbations in terms of an inherent, homeostasis-like negative feedback. We show that signatures of oxidation states and multivalence—such as X-ray photoemission core-level shifts, ionic radii and variations in local magnetization—that have often been interpreted as literal charge transfer are instead a consequence of the negative-feedback charge regulation.

Electronic and magnetic properties of substitutional Mn clusters in (Ga,Mn)As

Donostia International Physics Centre (DIPC), POB 1072, 20018 San Sebastian, Spain(Dated: February 2, 2008)The magnetization and hole distribution of Mn clusters in (Ga,Mn)As are investigated by all-electron total energy calculations using the projector augmented wave method within the density-functional formalism. It is found that the energetically most favorable clusters consist of Mn atomssurrounding one center As atom. As the Mn cluster grows the hole band at the Fermi level splitsincreasingly and the hole distribution gets increasingly localized at the center As atom. The holedistribution at large distances from the cluster does not depend significantly on the cluster size.As a consequence, the spin-flip energy differences of distant clusters are essentially independent ofthe cluster size. The Curie temperature T

Intrinsic hole localization mechanism in magnetic semiconductors

The interplay between clustering and exchange coupling in magnetic semiconductors for the prototype (Ga1−x,Mnx)As is investigated considering manganese concentrations x of 1/16 and 1/32, which are in the interesting experimental range. For , we study all possible arrangements of two Mn atoms on the Ga sublattice within a large supercell and find the clustering of Mn atoms at nearest-neighbour Ga sites energetically preferred. As shown by analysis of spin density and projected density of states, this minimum-energy configuration localizes further one hole and reduces the effective charge carrier concentration. Also the exchange coupling constant increases to a value corresponding to lower Mn concentrations with decreasing inter-Mn distance.

Spontaneous magnetization of aluminum nanowires deposited on the NaCl(100) surface

We investigate electronic structures of Al quantum wires, both unsupported and supported on the (100) NaCl surface, using the density-functional theory. We confirm that unsupported nanowires, constrained to be linear, show magnetization when elongated beyond the equilibrium length. Allowing ions to relax, the wires deform to zigzag structures with lower magnetization but no dimerization occurs. When an Al wire is deposited on the NaCI surface, a zigzag geometry emerges again. The magnetization changes moderately from that for the corresponding unsupported wire. We analyze the findings using electron band structures and simple model wires.

High Curie temperatures in "Ga,Mn…N from Mn clustering

The effect of microscopic Mn cluster distribution on the Curie temperature (TC) of (Ga,Mn)N is studied using density-functional calculations together with the mean field approximation. We find that the calculated TC depends crucially on the microscopic cluster distribution, which can explain the abnormally large variations in experimental TC values from a few K to well above room temperature. The partially dimerized Mn2-Mn1 distribution is found to give the highest TC>500K, and in general, the presence of the Mn2 dimer has a tendency to enhance TC. The lowest TC values close to zero are obtained for the Mn4-Mn1 and Mn4-Mn3 distributions.

Charge storage in oxygen deficient phases of TiO2: defect Physics without defects

Defects in semiconductors can exhibit multiple charge states, which can be used for charge storage applications. Here we consider such charge storage in a series of oxygen deficient phases of TiO2, known as Magnéli phases. These Magnéli phases (TinO2n−1) present well-defined crystalline structures, i.e., their deviation from stoichiometry is accommodated by changes in space group as opposed to point defects. We show that these phases exhibit intermediate bands with an electronic quadruple donor transitions akin to interstitial Ti defect levels in rutile TiO2. Thus, the Magnéli phases behave as if they contained a very large pseudo-defect density: ½ per formula unit TinO2n−1. Depending on the Fermi Energy the whole material will become charged. These crystals are natural charge storage materials with a storage capacity that rivals the best known supercapacitors.

Diffusion and clustering of substitutional Mn in (Ga,Mn)As

The Ga vacancy mediated microstructure evolution of (Ga,Mn)As during growth and postgrowth annealing is studied using a multiscale approach. The migration barriers for the Ga vacancies and substitutional Mn together with their interactions are calculated using first principles, and temporal evolution at temperatures 200–350°C is studied using lattice kinetic Monte Carlo simulations. We show that at the typical growth and annealing temperatures (i) Ga vacancies provide an efficient diffusion transport for Mn and (ii) in 10–20h the diffusion of Mn promotes the formation of clusters. Clustering reduces the Curie temperature, and explains its decrease during long-term annealing.

A multiscale study of ferromagnetism in clustered (Ga,Mn)N

Magnetic interactions in (Ga,Mn)N are studied on the microscopic intercluster and intra-cluster scales using first-principles calculations. Ferromagnetic transition temperatures are calculated using Monte Carlo methods. Randomness in Mn substitution is found to reduce Curie temperatures by about 10–20% fro mt hose of the corresponding regular (Ga,Mn)N lattice due to clustering. Nevertheless, high Curie temperatures reaching above room temperature are obtained even for completely random Mn distribution in (Ga,Mn)N for the Mn concentration of x 13.5%. Increasing clustering is always found to decrease the Curie temperature—especially when tetramer clusters are formed. The active search for diluted magnetic semiconductors (DMSs) with high Curie temperatures (TCs) exceeding room temperature started with the discovery of ferromagnetism in (In,Mn)As [1]. The calculations by Dietl et al predicted that (Ga,Mn)N should have the highest TC (� 400 K) among the prospective III–V DMS materials suggested fo rs pintronics applications [2]. However, the measured TC values for (Ga,Mn)N range from 10 to 940 K [3–6] and also paramagnetic behaviour is reported at lower Mn concentrations (typically x < 0.02) [7]. The reasons for the wide range of the observed TC values and especially for the high values are poorly understood. One of the suggestions for this anomalous behaviour of TC is that the increase is due to clustering (or precipitation) of Mn atoms and the formation of giant magnetic moments at the Mn clusters [6, 8–10]. However, Mn clustering is usually found to decrease calculated TC s[ 11–15 ]a lthough in some very special situations an increase may also be obtained [9–11]. In this paper we consider small clusters and define a Mnn cluster as a collection of n substitutional Mn atoms (n = 2, 3, and 4 for dimers, trimers, and tetramers, respectively) which have a common N neighbour. At high Mn concentrations x ac onsiderable amount of Mn clusters are present even in the case of a completely random distribution of substitutional

Control of defect binding and magnetic interaction energies in dilute magnetic semiconductors by charge state manipulation

Dilute magnetic semiconductors exhibit a unique entanglement of magnetism and semiconductor properties. Their properties are dominated by short-range chemical and magnetic interactions among the magnetic impurities diluted therein. The microscopic structure and defect distribution are of crucial importance; and indeed, it has been shown that clustering, phase separation, and complex formation of the magnetic impurities, possibly involving other intrinsic or extrinsic defects, can dramatically alter the magnetic properties of a given sample. Detailed knowledge of the underlying short-range chemical and magnetic interactions, in turn, can be used to “design” new materials with target magnetic properties. This paper describes the Fermi-level dependence of these short-range chemical and magnetic interactions, i.e., how these interactions depend on defect charge states.

Term rules for simple metal clusters.

Hund’s term rules are only valid for isolated atoms, but have no generalization for molecules or clusters of several atoms. We present a benchmark calculation of Al2 and Al3, for which we find the high and low-spin ground states 3Πu and , respectively. We show that the relative stabilities of all the molecular terms of Al2 and Al3 can be described by simple rules pertaining to bonding structures and symmetries, which serve as guiding principles to determine ground state terms of arbitrary multi-atom clusters.