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Biomedical subjects

S Blügel

Publications and source records attributed to S Blügel.

At least 19 recordsLinked to original sources
PubMed2006

Probing quasiparticle states bound by disparate periodic potentials.

Thin films of Ag(111) with two-dimensional crystallinity of large lateral coherence grow on Ge(111), free of in-plane registry with the underlying substrate. Ag s-p electrons forming two-dimensional quantum well states scatter coherently at the buried interface potential, resulting in an unexpected set of new quasiparticle states, as observed by angle-resolved photoemission. These new features originate from interactions among Ag quantum well bands, gaining a momentum equivalent to a reciprocal vector of the substrate lattice.

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PubMed2006

Thermal collapse of spin polarization in half-metallic ferromagnets.

We propose two novel approaches to study the temperature dependence of the magnetization and the spin polarization at the Fermi level in magnetic compounds, and apply them to half-metallic ferromagnets. We reveal a new mechanism, where the hybridization of states forming the half-metallic gap depends on thermal spin fluctuations and the polarization can drop abruptly at temperatures much lower than the Curie point. We verify this for NiMnSb by ab initio calculations. The thermal properties are studied by mapping ab initio results to an extended Heisenberg model which includes longitudinal fluctuations and is solved by a Monte Carlo method.

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PubMed2006

Half-metallic ferromagnetism induced by dynamic electron correlations in VAs.

The electronic structure of the VAs compound in the zinc-blende structure is investigated using a combined density-functional and dynamical mean-field theory approach. Contrary to predictions of a ferromagnetic semiconducting ground state obtained by density-functional calculations, dynamical correlations induce a closing of the gap and produce a half-metallic ferromagnetic state. These results emphasize the importance of dynamic correlations in materials suitable for spintronics.

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PubMed2006

Observation of a complex nanoscale magnetic structure in a hexagonal Fe monolayer.

We have observed a novel magnetic structure in the pseudomorphic Fe monolayer on Ir(111). Using spin-polarized scanning tunneling microscopy we find a nanometer-sized two-dimensional magnetic unit cell. A collinear magnetic structure is proposed consisting of 15 Fe atoms per unit cell with 7 magnetic moments pointing in one and 8 moments in the opposite direction. First-principles calculations verify that such an unusual magnetic state is indeed lower in energy than all solutions of the classical Heisenberg model. We demonstrate that the complex magnetic structure is induced by the strong Fe-Ir hybridization.

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PubMed2006

Giant magnetocrystalline anisotropies of 4d transition-metal monowires.

The magnetocrystalline anisotropy energy (MAE) for ferromagnetic and antiferromagnetic freestanding monowires of 4d transition metals is investigated on the basis of first-principles calculations. Across the 4d series, the easy axis of the magnetization oscillates between two directions: perpendicular and along the wire axis. The largest values of the MAE occur at the end of the series. Giant values of 30-60 meV/atom can be obtained upon stretching Ru or Rh wires. Ru and Pd chains change the magnetization direction upon wire stretching, opening new perspectives in controlling the spin-dependent ballistic conductance in these structures.

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PubMed2006

Atomic-scale sharpening of silicon tips in noncontact atomic force microscopy.

The atomic-scale stability of clean silicon tips used in noncontact atomic force microscopy (NC-AFM) is simulated by ab initio calculations based on density functional theory. The tip structures are modeled by silicon clusters with and termination. For the often assumed Si(111)-type tip we observe the sharpening of the initially blunt tip via short-range chemical forces during the first approach and retraction cycle. The structural changes corresponding to this intrinsic process are irreversible and lead to stable NC-AFM imaging conditions. In opposition to the picture used in literature, the Si(001)-type tip does not exhibit the so-called "two-dangling bond" feature as a bulklike termination suggests.

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PubMed2005

Revealing antiferromagnetic order of the Fe monolayer on W(001): spin-polarized scanning tunneling microscopy and first-principles calculations.

We prove that the magnetic ground state of a single monolayer Fe on W(001) is c(2x2) antiferromagnetic, i.e., a checkerboard arrangement of antiparallel magnetic moments. Real space images of this magnetic structure have been obtained with spin-polarized scanning tunneling microscopy. An out-of-plane easy magnetization axis is concluded from measurements in an external magnetic field. The magnetic ground state and anisotropy axis are explained based on first-principles calculations.

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PubMed2004

Role of spin in quasiparticle interference.

Quasiparticle interference patterns measured by scanning tunneling microscopy can be used to study the local electronic structure of metal surfaces and high-temperature superconductors. Here, we show that even in nonmagnetic systems the spin of the quasiparticles can have a profound effect on the interference patterns. On Bi(110), where the surface state bands are not spin degenerate, the patterns are not related to the dispersion of the electronic states in a simple way. In fact, the features which are expected for the spin-independent situation are absent and the observed interference patterns can be interpreted only by taking spin-conserving scattering events into account.

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PubMed2004

Strong spin-orbit splitting on bi surfaces.

Using first-principles calculations and angle-resolved photoemission, we show that the spin-orbit interaction leads to a strong splitting of the surface-state bands on low-index surfaces of Bi. The dispersion of the states and the corresponding Fermi surfaces are profoundly modified in the whole surface Brillouin zone. We discuss the implications of these findings with respect to a proposed surface charge density wave on Bi(111) as well as to the surface screening, surface spin-density waves, electron (hole) dynamics in surface states, and to possible applications to the spintronics.

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PubMed2004

Spin-polarized electron scattering at single oxygen adsorbates on a magnetic surface.

Scanning tunneling spectroscopy (STS) on the system of isolated oxygen atoms adsorbed on the double layer of Fe on W(110) shows highly anisotropic spatial oscillations in the local density of states in the vicinity of the adsorbates. We explain this in terms of a single-particle model as electron waves being scattered by the potential induced by the presence of the oxygen atoms. Analysis of the wavelength of the standing electron waves and comparison with ab initio spin-resolved electronic structure calculations reveal that minority-spin bands of d-like symmetry are involved in the scattering process. By applying spin-polarized STS, we observe this standing wave pattern on one particular type of magnetic domain of Fe on W(110) only, thereby proving that the standing electron waves are highly spin polarized.

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PubMed2002

Element specific surface reconstructions of islands during surfactant-mediated growth on Si (111).

The early stages of surfactant- (As, Sb) mediated homoepitaxial growth on Si (111) are examined by scanning tunneling microscopy and extensive ab initio calculations of Si(n) clusters (n</=12). The results reveal the different microscopic behavior of the two surfactants: On As-covered Si (111), one exclusively finds two-dimensional islands with double-layer height which show the (1 x 1) terrace structure. On Sb-covered Si (111), the islands show two different reconstructions: at the rim of the islands a (1 x 1) structure appears, while in the center the (square root 3 x square root 3) terrace structure is observed.

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PubMed2002

Magnetization-direction-dependent local electronic structure probed by scanning tunneling spectroscopy.

Scanning tunneling spectroscopy (STS) of thin Fe films on W(110) shows that the electronic structure of domains and domain walls is different. This experimental result is explained on the basis of first-principles calculations. A detailed analysis reveals that the spin-orbit induced mixing between minority d(xy+xz) and minority d(z(2)) spin states depends on the magnetization direction and changes the local density of states in the vacuum detectable by STS. As a consequence nanometer-scale magnetic structure information is obtained even by using nonmagnetic probe tips.

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PubMed2002

Novel sb induced reconstruction of the (113) surface of ge.

Sb induces on Ge(113) a c(2 x 2) reconstruction in which Sb breaks one Ge-Ge bond and occupies an interstitial site, in contrast to Sb adsorption on other Si or Ge surfaces. Sb saturates the three dangling bonds per unit cell of the (113) surface inducing a large strain which is released by occupation of the interstitial site. Two neighboring Sb at interstitial sites form a dimer. The structure has been determined by x-ray diffraction, applying direct methods, and ab initio density-functional-theory calculations. The adsorption geometry and the high binding energy lead one to expect that Sb cannot be used as a surfactant for the growth of Si/Ge layers on the (113) surface.

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PubMed2002

Surfactant mediated heteroepitaxy versus homoepitaxy: kinetics for group-IV adatoms on As-passivated Si(111) and Ge(111).

Using ab initio calculations we have determined the paths and activation energies for diffusion of group-IV atoms (Si, Ge, and Sn) on top of the As layer on As-passivated Si(111), and for exchange with an As atom. The kinetics of Si, Ge, and Sn adatoms is substantially different: Si adatoms are readily incorporated under the As layer. Ge adatoms diffuse far on top of the As layer and can reach existing steps. We show for the first time that the ratio between diffusion and exchange barriers depends strongly on the strain of the growing Ge film. Sn atoms remain on top of the As layer.

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PubMed2001

Resolving complex atomic-scale spin structures by spin-polarized scanning tunneling microscopy.

The spin-polarized scanning tunneling microscope (SP-STM) operated in the constant current mode is proposed as a powerful tool to investigate complex atomic-scale magnetic structures of otherwise chemically equivalent atoms. The potential of this approach is demonstrated by successfully resolving the magnetic structure of Cr/Ag(111), which is predicted on the basis of ab initio vector spin-density calculations to be a coplanar noncollinear periodic 120 degrees Néel structure. Different operating modes of the SP-STM are discussed on the basis of the model of Tersoff and Hamann.

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PubMed2001

Three-dimensional spin structure on a two-dimensional lattice: Mn/Cu(111).

Based on first-principles vector spin-density total-energy calculations of the magnetic and electronic structure of Cr and Mn transition-metal monolayers on the triangular lattice of a (111) oriented Cu surface, we propose for Mn a three-dimensional noncollinear spin structure on a two-dimensional triangular lattice as magnetic ground state. This new spin structure is a multiple spin-density wave of three row-wise antiferromagnetic spin states and comes about due to magnetic interactions beyond the nearest neighbors and due to higher order spin interactions (i.e., four spin). The magnetic ground state of Cr is a coplanar noncollinear periodic 120 degrees Néel structure.

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