607 research outputs found
Co atoms on BiSe revealing a coverage dependent spin reorientation transition
We investigate Co nanostructures on BiSe by means of scanning
tunneling microscopy and spectroscopy [STM/STS], X-ray absorption spectroscopy
[XAS], X-ray magnetic dichroism [XMCD] and calculations using the density
functional theory [DFT]. In the single adatom regime we find two different
adsorption sites by STM. Our calculations reveal these to be the fcc and hcp
hollow sites of the substrate. STS shows a pronounced peak for only one species
of the Co adatoms indicating different electronic properties of both types.
These are explained on the basis of our DFT calculations by different
hybridizations with the substrate. Using XMCD we find a coverage dependent spin
reorientation transition from easy-plane toward out-of-plane. We suggest
clustering to be the predominant cause for this observation.Comment: 10 pages, 4 figure
Magnetic properties of substitutional Mn in (110) GaAs surface and subsurface layers
Motivated by recent STM experiments, we present a theoretical study of the
electronic and magnetic properties of the Mn-induced acceptor level obtained by
substituting a single Ga atom in the (110) surface layer of GaAs or in one of
the atoms layers below the surface. We employ a kinetic-exchange tight-binding
model in which the relaxation of the (110) surface is taken into account. The
acceptor wave function is strongly anisotropic in space and its detailed
features depend on the depth of the sublayer in which the Mn atom is located.
The local-density-of-states (LDOS) on the (110) surface associated with the
acceptor level is more sensitive to the direction of the Mn magnetic moment
when the Mn atom is located further below the surface. We show that the total
magnetic anisotropy energy of the system is due almost entirely to the
dependence of the acceptor level energy on Mn spin orientation, and that this
quantity is strongly dependent on the depth of the Mn atom.Comment: 14 pages, 13 figure
Magnetic Scanning Tunneling Microscopy with a Two-Terminal Non-Magnetic Tip: Quantitative Results
We report numerical simulation result of a recently proposed \{P. Bruno,
Phys. Rev. Lett {\bf 79}, 4593, (1997)\} approach to perform magnetic scanning
tunneling microscopy with a two terminal non-magnetic tip. It is based upon the
spin asymmetry effect of the tunneling current between a ferromagnetic surface
and a two-terminal non-magnetic tip. The spin asymmetry effect is due to the
spin-orbit scattering in the tip. The effect can be viewed as a Mott scattering
of tunneling electrons within the tip. To obtain quantitative results we
perform numerical simulation within the single band tight binding model, using
recursive Green function method and Landauer-B\"uttiker formula for
conductance. A new model has been developed to take into account the spin-orbit
scattering off the impurities within the single-band tight-binding model. We
show that the spin-asymmetry effect is most prominent when the device is in
quasi-ballistic regime and the typical value of spin asymmetry is about 5%.Comment: 5 pages, Late
Direct comparison between potential landscape and local density of states in a disordered two-dimensional electron system
The local density of states (LDOS) of the adsorbate induced two-dimensional
electron system (2DES) on n-InAs(110) is studied by low-temperature scanning
tunneling spectroscopy. The LDOS exhibits irregular structures with fluctuation
lengths decreasing with increasing energy. Fourier transformation reveals that
the k-values of the unperturbed 2DES dominate the LDOS, but additional lower
k-values contribute significantly. To clarify the origin of the additional
k-space intensity, we measure the potential landscape of the same 2DES area
with the help of the tip induced quantum dot. This allows to calculate the
expected LDOS from the single particle Schroedinger equation and to directly
compare it with the measured one. Reasonable correspondance between calculated
and measured LDOS is found.Comment: 7 pages, 4 figures, submitted to PR
Correction of systematic errors in scanning tunnelling spectra on semiconductor surfaces: the energy gap of Si(111)-7x7 at 0.3 K
The investigation of the electronic properties of semiconductor surfaces
using scanning tunnelling spectroscopy (STS) is often hindered by
non-equilibrium transport of the injected charge carriers. We propose a
correction method for the resulting systematic errors in STS data, which is
demonstrated for the well known Si(111)-(7x7) surface. The surface has an odd
number of electrons per surface unit cell and is metallic above 20 K. We
observe an energy gap in the ground state of this surface by STS at 0.3 K.
After correction, the measured width of the gap is (70 +- 15) meV which is
compatible with previous less precise estimates. No sharp peak of the density
of states at the Fermi level is observed, in contrast to proposed models for
the Si(111)-(7x7) surface.Comment: 10 pages, 4 figure
Spin-sensitive shape asymmetry of adatoms on noncollinear magnetic substrates
The spin-resolved density of states of Co atoms on a noncollinear magnetic support displays a distinct shape contrast, which is superimposed on the regular height contrast in spin-polarized scanning tunneling microscopy. The apparent atom height follows the well-known cosine dependence on the angle formed by the tip and adatom local magnetization directions, whereas the shape contrast exhibits a sine dependence. We explain this effect in terms of a noncollinear spin density induced by the substrate, which in our case is the spin spiral of the Mn monolayer on W(110). The two independent contrast channels, apparent height and shape, are identified with the Co magnetization projections onto two orthogonal axes. As a result, all components of the overall atom magnetic moment vector can be determined with a single spin-sensitive tip in the absence of an external magnetic field. This result should be general for any atom deposited on noncollinear magnetic layers
Intra- and Interband Electron Scattering in the Complex Hybrid Topological Insulator Bismuth Bilayer on BiSe
The band structure, intra- and interband scattering processes of the
electrons at the surface of a bismuth-bilayer on BiSe have been
experimentally investigated by low-temperature Fourier-transform scanning
tunneling spectroscopy. The observed complex quasiparticle interference
patterns are compared to a simulation based on the spin-dependent joint density
of states approach using the surface-localized spectral function calculated
from first principles as the only input. Thereby, the origin of the
quasiparticle interferences can be traced back to intraband scattering in the
bismuth bilayer valence band and BiSe conduction band, and to interband
scattering between the two-dimensional topological state and the
bismuth-bilayer valence band. The investigation reveals that the bilayer band
gap, which is predicted to host one-dimensional topological states at the edges
of the bilayer, is pushed several hundred milli-electronvolts above the Fermi
level. This result is rationalized by an electron transfer from the bilayer to
BiSe which also leads to a two-dimensional electron state in the
BiSe conduction band with a strong Rashba spin-splitting, coexisting
with the topological state and bilayer valence band.Comment: 11 pages, 5 figure
Jahn-Teller stabilization of a "polar" metal oxide surface: Fe3O4(001)
Using ab initio thermodynamics we compile a phase diagram for the surface of
Fe3O4(001) as a function of temperature and oxygen pressures. A hitherto
ignored polar termination with octahedral iron and oxygen forming a wave-like
structure along the [110]-direction is identified as the lowest energy
configuration over a broad range of oxygen gas-phase conditions. This novel
geometry is confirmed in a x-ray diffraction analysis. The stabilization of the
Fe3O4(001)-surface goes together with dramatic changes in the electronic and
magnetic properties, e.g., a halfmetal-to-metal transition.Comment: 4 pages, 4 figure
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