3,318 research outputs found
Ge-substitutional defects and the r3xr3 <--> 3x3 transition in alpha--SnGe(111)
The structure and energetics of Ge substitutional defects on the
alpha-Sn/Ge(111) surface are analyzed using Density Functional Theory (DFT)
molecular dynamics (MD) simulations. An isolated Ge defect induces a very local
distortion of the 3x3 reconstruction, confined to a significant downwards
displacement (-0.31 A) at the defect site and a modest upward displacement
(0.05 A) of the three Sn nearest neighbours with partially occupied dangling
bonds. Dynamical fluctuations between the two degenerate ground states yield
the six-fold symmetry observed around a defect in the experiments at room
temperature. Defect-defect interactions are controlled by the energetics of the
deformation of the 3x3 structure: They are negligible for defects on the
honeycomb lattice and quite large for a third defect on the hexagonal lattice,
explaining the low temperature defect ordering.Comment: 4 pages, Revtex, 7 Encapsulated Postscript figures, uses epsf.sty.
Submitted to Phys. Rev. Let
Disproportionation Phenomena on Free and Strained Sn/Ge(111) and Sn/Si(111) Surfaces
Distortions of the Sn/Ge(111) and Sn/Si(111) surfaces
are shown to reflect a disproportionation of an integer pseudocharge, ,
related to the surface band occupancy. A novel understanding of the
-1U (``1 up, 2 down'') and 2U (``2 up, 1 down'') distortions of
Sn/Ge(111) is obtained by a theoretical study of the phase diagram under
strain. Positive strain keeps the unstrained value Q=3 but removes distorsions.
Negative strain attracts pseudocharge from the valence band causing first a
-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a
-3U (``all up'') state with Q=6. The possibility of a
fluctuating phase in unstrained Sn/Si(111) is discussed.Comment: Revtex, 5 pages, 3 figure
Phase transitions in two dimensions - the case of Sn adsorbed on Ge(111) surfaces
Accurate atomic coordinates of the room-temperature (root3xroot3)R30degree
and low-temperature (3x3) phases of 1/3 ML Sn on Ge(111) have been established
by grazing-incidence x-ray diffraction with synchrotron radiation. The Sn atoms
are located solely at T4-sites in the (root3xroot3)R30degree structure. In the
low temperature phase one of the three Sn atoms per (3x3) unit cell is
displaced outwards by 0.26 +/- 0.04 A relative to the other two. This
displacement is accompanied by an increase in the first to second double-layer
spacing in the Ge substrate.Comment: RevTeX, 5 pages including 2 figure
Charge density waves and surface Mott insulators for adlayer structures on semiconductors: extended Hubbard modeling
Motivated by the recent experimental evidence of commensurate surface charge
density waves (CDW) in Pb/Ge(111) and Sn/Ge(111) sqrt{3}-adlayer structures, as
well as by the insulating states found on K/Si(111):B and SiC(0001), we have
investigated the role of electron-electron interactions, and also of
electron-phonon coupling, on the narrow surface state band originating from the
outer dangling bond orbitals of the surface. We model the sqrt{3} dangling bond
lattice by an extended two-dimensional Hubbard model at half-filling on a
triangular lattice. We include an on-site Hubbard repulsion U and a
nearest-neighbor Coulomb interaction V, plus a long-ranged Coulomb tail. The
electron-phonon interaction is treated in the deformation potential
approximation. We have explored the phase diagram of this model including the
possibility of commensurate 3x3 phases, using mainly the Hartree-Fock
approximation. For U larger than the bandwidth we find a non-collinear
antiferromagnetic SDW insulator, possibly corresponding to the situation on the
SiC and K/Si surfaces. For U comparable or smaller, a rich phase diagram
arises, with several phases involving combinations of charge and
spin-density-waves (SDW), with or without a net magnetization. We find that
insulating, or partly metallic 3x3 CDW phases can be stabilized by two
different physical mechanisms. One is the inter-site repulsion V, that together
with electron-phonon coupling can lower the energy of a charge modulation. The
other is a novel magnetically-induced Fermi surface nesting, stabilizing a net
cell magnetization of 1/3, plus a collinear SDW, plus an associated weak CDW.
Comparison with available experimental evidence, and also with first-principle
calculations is made.Comment: 11 pages, 9 figure
Determination of the (3x3)-Sn/Ge(111) structure by photoelectron diffraction
At a coverage of about 1/3 monolayer, Sn deposited on Ge(111) below 550 forms
a metastable (sqrt3 x sqrt3)R30 phase. This phase continuously and reversibly
transforms into a (3x3) one, upon cooling below 200 K. The photoemission
spectra of the Sn 4d electrons from the (3x3)-Sn/Ge(111) surface present two
components which are attributed to inequivalent Sn atoms in T4 bonding sites.
This structure has been explored by photoelectron diffraction experiments
performed at the ALOISA beamline of the Elettra storage ring in Trieste
(Italy). The modulation of the intensities of the two Sn components, caused by
the backscattering of the underneath Ge atoms, has been measured as a function
of the emission angle at fixed kinetic energies and viceversa. The bond angle
between Sn and its nearest neighbour atoms in the first Ge layer (Sn-Ge1) has
been measured by taking polar scans along the main symmetry directions and it
was found almost equivalent for the two components. The corresponding bond
lengths are also quite similar, as obtained by studying the dependence on the
photoelectron kinetic energy, while keeping the photon polarization and the
collection direction parallel to the Sn-Ge1 bond orientation (bond emission). A
clear difference between the two bonding sites is observed when studying the
energy dependence at normal emission, where the sensitivity to the Sn height
above the Ge atom in the second layer is enhanced. This vertical distance is
found to be 0.3 Angstroms larger for one Sn atom out of the three contained in
the lattice unit cell. The (3x3)-Sn/Ge(111) is thus characterized by a
structure where the Sn atom and its three nearest neighbour Ge atoms form a
rather rigid unit that presents a strong vertical distortion with respect to
the underneath atom of the second Ge layer.Comment: 10 pages with 9 figures, added reference
Surface soft phonon and the root3 x root3 <--> 3 x 3 phase transition in Sn/Ge(111) and Sn/Si(111)
Density Functional Theory (DFT) calculations show that the reversible
Sn/Ge(111) phase transition
can be described in terms of a surface soft phonon. The isovalent Sn/Si(111)
case does not display this transition since the phase
is the stable structure at low temperature, although it presents a partial
softening of the surface phonon. The rather flat energy surfaces for
the atomic motion associated with this phonon mode in both cases explain the
experimental similarities found at room temperature between these systems. The
driving force underlying the
phase transition is shown to be associated with the electronic energy gain due
to the Sn dangling bond rehybridization.Comment: 4 pages, Revtex, 4 Encapsulated Postscript figures, uses epsf.sty.
Final version published in Phys. Rev. Let
Influence of pharmacogenetic variability on the pharmacokinetics and toxicity of the aurora kinase inhibitor danusertib
Objectives Danusertib is a serine/threonine kinase inhibitor of multiple kinases, including aurora-A, B, and C. This explorative study aims to identify possible relationships between single nucleotide polymorphisms in genes coding for drug metabolizing enzymes and transporter proteins and clearance of danusertib, to clarify the interpatient variability in exposure. In addition, this study explores the relationship between target receptor polymorphisms and toxicity of danusertib. Methods For associations with clearance, 48 cancer patients treated in a phase I study were analyzed for ABCB1, ABCG2 and FMO3 polymorphisms. Association analyses between neutropenia and drug target receptors, including KDR, RET, FLT3, FLT4, AURKB and AURKA, were performed in 30 patients treated at recommended phase II dose-levels in three danusertib phase I or phase II trials. Results No relationships between danusertib clearance and drug metabolizing enzymes and transporter protein polymorphisms were found. Only, for the one patient with FMO3 18281AA polymorphism, a significantly higher clearance was noticed, compared to patients carrying at least 1 wild type allele. No effect of target receptor genotypes or haplotypes on neutropenia was observed. Conclusions As we did not find any major correlations between pharmacogenetic variability in the studied enzymes and transporters and pharmacokinetics nor toxicity, it is unlikely that danusertib is highly susceptible for pharmacogenetic variation. Therefore, no dosing alterations of danusertib are expected in the future, based on the polymorphisms studied. However, the relationship between FMO3 polymorphisms and clearance of danusertib warrants further research, as we could study only a small group of patients
Theory of the "honeycomb chain-channel" reconstruction of Si(111)3x1
First-principles electronic-structure methods are used to study a structural
model for Ag/Si(111)3x1 recently proposed on the basis of transmission electron
diffraction data. The fully relaxed geometry for this model is far more
energetically favorable than any previously proposed, partly due to the unusual
formation of a Si double bond in the surface layer. The calculated electronic
properties of this model are in complete agreement with data from
angle-resolved photoemission and scanning tunneling microscopy.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett (the ugly postscript
error on page 4 has now been repaired
Surface Phase Transitions Induced by Electron Mediated Adatom-Adatom Interaction
We propose that the indirect adatom-adatom interaction mediated by the
conduction electrons of a metallic surface is responsible for the
structural phase transitions
observed in Sn/Ge (111) and Pb/Ge (111). When the indirect interaction
overwhelms the local stress field imposed by the substrate registry, the system
suffers a phonon instability, resulting in a structural phase transition in the
adlayer. Our theory is capable of explaining all the salient features of the
transitions observed in
Sn/Ge (111) and Pb/Ge (111), and is in principle applicable to a wide class of
systems whose surfaces are metallic before the transition.Comment: 4 pages, 5 figure
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