Ferroelectric switching of band alignments in LSMO/PZT/Co multiferroic tunnel junctions: an ab initio study.

Band alignments in ferroelectric tunnel junctions (FTJs) are expected to play a critical role in determining the charge transport across the tunneling barrier. In general, however, the interface band discontinuities and their polarization dependence are not well known in these systems. Using a first-principles density-functional-theory approach, we explore the ferroelectric (FE) polarization dependence of the band alignments in [Formula: see text] (LSMO/PZT/Co) multiferroic tunnel junctions, for which recent experiments indicated an ON/OFF conductivity behavior upon switching the PZT FE polarization. Our results on the pseudomorphic defect-free LSMO/PZT/Co FTJs evidence a major FE switching effect on the band discontinuities at both interfaces. Based on the changes in the band alignments, we provide a possible explanation for the observed trends in the resistive switching.

Spin-polarization properties and electronic structure of the ordered c(2 × 2) MnCu/Cu(1 1 0) surface alloy.

The spin-ordering and electronic properties of the c(2 × 2) MnCu/Cu(1 1 0) surface alloy are investigated by means of ab initio density-functional calculations. We first address the magnetic ground state and the robustness of the spin-polarization properties. The lowest-energy state is found to be ferromagnetic with a very low Curie temperature, showing that the paramagnetic phase should be established in this system at room temperature. The calculated trends obtained for the various spin structures considered indicate that the local Mn-spin moment and resulting reduced work function as well as Mn-outward buckling should persist in the paramagnetic phase. We then address the electronic surface-band structure of the paramagnetic phase close to the Fermi energy, in connection with the interpretation of recent angle-resolved-photoemission-spectroscopy experiments at room temperature. Our calculations account for an intriguing new surface-band feature observed experimentally near the [Formula: see text] point upon alloy formation, and provide a microscopic interpretation for this feature and for the alloy-induced changes in the Cu(1 1 0) Shockley surface state.

Self-organization in Pd/W(110): interplay between surface structure and stress.

It has recently been shown that submonolayer Pd on W(110) forms highly ordered linear mesoscopic stripes at high temperatures. The stripes display an internal Pd superstructure with a nano-scale periodicity along the direction perpendicular to the periodicity of the stripes. The same type of superstructure is also observed in a wide range of temperatures below the stripe formation temperature. We present a combined experimental and theoretical study of this superstructure of Pd on W(110) and investigate its influence on the appearance of the linear mesoscopic stripes. By means of low-energy electron diffraction and low-energy electron microscopy we show that it has a far more peculiar dependence on temperature and coverage than expected from a regular surface reconstruction. Using density-functional theory, we model the Pd superstructures as periodic vacancy-line type configurations and investigate their energetics and elastic properties. From our calculated surface stresses and anisotropies for the vacancy-line configurations, and based on the continuum elasticity theory, we demonstrate quantitatively that the vacancy-line type of structure is a prerequisite for the formation of the linear mesoscopic stripes. Moreover, we show that the physics driving the formation of the internal superstructure is very similar to the one at play in forming the mesoscopic stripes themselves.

Surface stress of Ni adlayers on W(110): the critical role of the surface atomic structure.

Puzzling trends in surface stress have been reported experimentally for Ni/W(110) as a function of Ni coverage. In order to explain this behavior, we have performed a density-functional-theory study of the surface stress and atomic structure of the pseudomorphic and several different possible 1 × 7 configurations for this system. For the 1 × 7 phase, we predict a different, more regular atomic structure than previously proposed based on surface x-ray diffraction. At the same time, we reproduce the unexpected experimental change of surface stress between the pseudomorphic and 1 × 7 configuration along the crystallographic surface direction which does not undergo density changes. We show that the observed behavior in the surface stress is dominated by the effect of a change in Ni adsorption/coordination sites on the W(110) surface.

The electron density decay length effect on surface reactivity.

The correlation between the thickness-dependent oxidation rate of ultrathin Al films on W(110) and the quantum-well states (QWS) resulting from electron confinement in the Al film has been explored by combined x-ray photoemission electron microscopy (XPEEM), low energy electron microscopy (LEEM), and first-principles calculations. Hybridization with substrate electronic states is observed to alter the film electronic structure, strongly modifying the electron density decay length in vacuum. The decay length, rather than the density of states at the Fermi energy, is found to dominate the observed reactivity trends.

Magnetism at the V/Gd interface.

Recent experimental investigations into the magnetic properties of V/Gd bilayers have shown that vanadium, which is nonmagnetic in the bulk, can acquire a magnetic moment in such systems. We have performed ab initio pseudopotential calculations to examine the magnetic behavior of V(110)/Gd(0001) bilayers for V layers with thicknesses up to 4 monolayers (ML). We considered both abrupt and atomic intermixed V/Gd interfaces. In both cases, the magnetic moment of the V layer is found to align antiparallel to the moment of the Gd layer, consistent with the experimental observation. However, the magnitude of the V moment at the abrupt interface is considerably smaller than the moments reported experimentally. In the presence of atomic intermixing, instead, substantially larger V moments are found, closer to the experimentally reported moments. On the basis of the calculated atomic and spin resolved density of states, we discuss the possible mechanism responsible for the observed Gd-V antiferromagnetic coupling.

Effects of chemical order and atomic relaxation on the electronic and magnetic properties of La(2/3)Sr(1/3)MnO(3).

We investigate the effects of Sr/La cation ordering/disordering and atomic relaxation on the electronic and magnetic properties of La(2/3)Sr(1/3)MnO(3) (LSMO) by means of ab initio pseudopotential calculations. We consider a cation-ordered layered structure and a more homogeneously Sr-bulk-doped structure. Cation disordering and atomic relaxation both tend to push the LSMO system towards half-metallicity, increasing the minority-spin and spin-flip gaps. Lattice relaxation has a significant effect on the electronic density of states (DOS) of the layered LSMO and drastically reduces the initial differences found comparing the electronic properties of the perovskite structures with different dopant configurations. The trend with structural relaxation is understood in terms of an effective screening, due to the displacement of the O anions and La cations, of the local electric field produced by the ordered Sr dopants.

Structural relaxation due to electronic correlations in the paramagnetic insulator KCuF3.

A computational scheme for the investigation of complex materials with strongly interacting electrons is formulated which is able to treat atomic displacements, and hence structural relaxation, caused by electronic correlations. It combines ab initio band structure and dynamical mean-field theory and is implemented in terms of plane-wave pseudopotentials. The equilibrium Jahn-Teller distortion and antiferro-orbital order found for paramagnetic KCuF3 agree well with experiment.

Surface reactivity and quantum-size effects on the electronic density decay length of ultrathin metal films.

The origin of the correlation between surface reactivity and quantum-size effects, observed in recent experiments on the oxidation of ultrathin magnesium films, is addressed by means of ab initio calculations and model predictions. We show that the decay length in vacuum of the electronic local density of states at the Fermi energy exhibits systematic oscillations with film thickness, with local maxima induced when a quantum-well state at crosses the Fermi energy. The predicted changes in the decay length are expected to have a major impact on the electron transfer rate by tunneling, which has been proposed to control the initial sticking of in the oxidation process.

Work functions at facet edges.

We perform ab initio pseudopotential calculations for metal crystals with finite facets of different crystallographic orientation to investigate the work function profile near crystal edges. We examine local edge effects, and address the problem of the coexistence of different face-dependent local work functions at crystal edges. By modeling the electronic dipoles at the metal surface, we show how nonvanishing surface charges spontaneously appear on metals with inequivalent facets. Our studies of Al crystal nanowires with (100) and (111) facets are extended to derive the dependence of the work function on the crystal morphology in the macroscopic limit.