254 research outputs found
Role of tip size, orientation, and structural relaxations in first-principles studies of magnetic exchange force microscopy and spin-polarized scanning tunneling microscopy
Using first-principles calculations based on density functional theory (DFT),
we investigate the exchange interaction between a magnetic tip and a magnetic
sample which is detected in magnetic exchange force microscopy (MExFM) and also
occurs in spin-polarized scanning tunneling microscopy (SP-STM) experiments. As
a model tip-sample system, we choose Fe tips and one monolayer Fe on W(001)
which exhibits a checkerboard antiferromagnetic structure and has been
previously studied with both SP-STM and MExFM. We calculate the exchange forces
and energies as a function of tip-sample distance using different tip models
ranging from single Fe atoms to Fe pyramids consisting of up to 14 atoms. We
find that modelling the tip by a single Fe atom leads to qualitatively
different tip-sample interactions than using clusters consisting of several
atoms. Increasing the cluster size changes the calculated forces quantitatively
enhancing the detectable exchange forces. Rotating the tip with respect to the
surface unit cell has only a small influence on the tip-sample forces.
Interestingly, the exchange forces on the tip atoms in the nearest and
next-nearest layers from the apex atom are non-negligible and can be opposite
to that on the apex atom for a small tip. In addition, the apex atom interacts
not only with the surface atoms underneath but also with nearest-neighbors in
the surface. We find that structural relaxations of tip and sample due to their
interaction depend sensitively on the magnetic alignment of the two systems. As
a result the onset of significant exchange forces is shifted towards larger
tip-sample separations which facilitates their measurement in MExFM. At small
tip-sample separations, structural relaxations of tip apex and surface atoms
can either enhance or reduce the magnetic contrast measured in SP-STMComment: 14 pages, 13 figure
One-dimensional Si chains embedded in Pt(111)and protected by a hexagonal boron-nitride monolayer
Using scanning tunneling microscopy, we show that Si deposition on Pt(111) at
300K leads to a network of one-dimensional Si chains. On the bare Pt(111)
surface, the chains, embedded into the Pt surface, are orientated along the
-direction. They disappear within a few hours in ultrahigh vacuum due to
the presence of residual gas. Exposing the chains to different gases
deliberately reveals that CO is largely responsible for the disappearance of
the chains. The chains can be stabilized by a monolayer of hexagonal boron
nitride, which is deposited prior to the Si deposition. The resulting Si chains
are rotated by 30{\deg} with respect to the chains on the bare Pt(111) surface
and survive even an exposure to air for 10 minutes.Comment: 8 pages, 4 Figure
The impact of ias/ifrs on the romanian accounting rules
The accounting standardization process is in progress at international regional level, more and more countries have reached the same conclusion of enforcing high quality accounting standards like IAS/IFRS. At international level, on one hand it is thought to implement IASB's international standards and on the other hand, to converge American standards with IASB standards. There are various reasons for Romania adopting the IASB reference system, but most of them are subordinated to the central aim, respectively EU accession. There are also some secondary reasons required by the IAS/IFRS transition, which in our country is less present than in more economic developed countries. In our country accountancy is subordinated to the taxation system, financing still comes prevalent from banks and very few Romanian companies are listed on foreign capital markets. According to this, starting with 2006 the International Financial Reporting Standards (IFRS), as presented and published by the International Accounting Standards Board, shall be applied in Romania by the following categories of companies: trade companies applying OMF no. 94/2001, loan institutions, assurance and reassurance companies, institutions supervised by the National Commission for Movable Assets, independent public companies and other state owned companies, companies to be consolidated by a company applying IFRS standards, companies, which at the end of the previous year fulfilled two of the following three criteria: turnover exceeding EUR 7.3 Million, total assets over EUR 3.65 Million, average number of employees over 50, as well as other companies subject to the Finance Ministry’s approval.romanian accounting rules, IAS/IFRS, romanian accounting normalization body, capital market
Hybridisation at the organic-metal interface: a surface-scientific analogue of H\"uckel's rule?
We demonstrate that cyclooctatetraene (COT) can be stabilised in different
conformations when adsorbed on different noble-metal surfaces due to varying
molecule-substrate interaction. While at first glance the behaviour seems to be
in accordance with H\"uckel's rule, a theoretical analysis reveals no
significant charge transfer. The driving mechanism for the conformational
change is hybridisation at the organic-metal interface and does not necessitate
any charge transfer.Comment: Accepted for publication in Chemical Communications. Main article: 6
pages, 2 figures; Supplementary Information: 4 pages, 3 figures, 1 table. All
in one fil
Chemical versus van der Waals Interaction: The Role of the Heteroatom in the Flat Absorption of Aromatic Molecules C6H6, C5NH5, and C4N2H4 on the Cu(110) Surface
We perform first-principles calculations aimed at investigating the role of a heteroatom such as N in the chemical and long-range van der Waals (vdW) interactions for a flat adsorption of several-conjugated molecules on the Cu(110) surface. Our study reveals that the alignment of the molecular orbitals at the adsorbate-substrate interface depends on the number of heteroatoms. As a direct consequence, the molecule-surface vdW interactions involve not only pi-like orbitals which are perpendicular to the molecular plane but also sigma-like orbitals delocalized in the molecular plane
Spin- and energy-dependent tunneling through a single molecule with intramolecular spatial resolution
We investigate the spin- and energy dependent tunneling through a single
organic molecule (CoPc) adsorbed on a ferromagnetic Fe thin film, spatially
resolved by low-temperature spin-polarized scanning tunneling microscopy.
Interestingly, the metal ion as well as the organic ligand show a significant
spin-dependence of tunneling current flow. State-of-the-art ab initio
calculations including also van-der-Waals interactions reveal a strong
hybridization of molecular orbitals and surface 3d states. The molecule is
anionic due to a transfer of one electron, resulting in a non-magnetic (S= 0)
state. Nevertheless, tunneling through the molecule exhibits a pronounced
spin-dependence due to spin-split molecule-surface hybrid states.Comment: Version of Submission, 18-03-201
Role of the van der Waals interactions on the bonding mechanism of pyridine on Cu(110) and Ag(110) surfaces: A first-principles study
We performed density-functional calculations aimed to investigate the adsorption mechanism of a single pyridine (C5H5N) molecule on Cu(110) and Ag(110) surfaces. Our ab initio simulations show that, in the ground state, the pyridine molecule adsorbs with its molecular plane perpendicular to these substrates and is oriented along the [001] direction. In this case, the bonding mechanism involves a sigma bond through the lone-pair electrons of the nitrogen atom. When the heterocyclic ring is parallel to the surface, the bonding takes place via pi-like molecular orbitals. However, depending on the position of the N atom on the surface, the planar adsorption configuration can relax to a perpendicular geometry. The role of the long-range van der Waals interactions on the adsorption geometries and energies was analyzed in the framework of the semiempirical method proposed by Grimme [J. Comput. Chem. 27, 1787 (2006)]. We demonstrate that these dispersion effects are very important for geometry and electronic structure of flat adsorption configurations
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