157 research outputs found
Metallic charge density waves and surface Mott insulators for adlayer structures on semiconductors: extended Hubbard modeling
Motivated by the recent experimental evidence of commensurate surface CDW in
Pb/Ge(111) and Sn/Ge(111) -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 dangling bond orbitals of
the adsorbate. We model the problem 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 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 the model including the
possibility of commensurate 3x3 phases, using mainly the Hartree-Fock
approximation. For U larger than the bandwidth we find magnetic insulators,
possibly corresponding to the situation in SiC and in K/Si. For smaller U, the
inter-site repulsion V can stabilize metallic CDW phases, reminiscent of the
3x3 structures of Sn/Ge, and possibly of Pb/Ge.Comment: 10 pages, 3 figures, presented at ECOSS-17 sept 199
First Principles Calculations of Charge and Spin Density Waves of sqr3-Adsorbates on Semiconductors
We present ab-initio electronic structure results on the surface of sqr3
adsorbates. In particular, we address the issue of metal-insulator
instabilities, charge-density-waves (CDWs) or spin-density-waves (SDWs), driven
by partly filled surface states and their 2D Fermi surface, and/or by the onset
of magnetic instabilities. The focus is both on the newly discovered
commensurate CDW transitions in the Pb/Ge(111) and Sn/Ge(111) structures, and
on the puzzling semiconducting behavior of the Pb/Ge(111), K/Si(111):B and
SiC(0001) surfaces. In all cases, the main factor driving the instability
appears to be an extremely narrow surface state band. We have carried out so
far preliminary calculations for the Si/Si(111) surface, chosen as our model
system, within the gradient corrected local density (LDA+GC) and local spin
density (LSD+GC) approximations, with the aim of understanding the possible
interplay between 2D Fermi surface and electron correlations in the surface +
adsorbate system. Our spin- unrestricted results show that the sqr3
paramagnetic surface is unstable towards a commensurate SDW with periodicity
3x3 and magnetization 1/3.Comment: 9 pages, 4 Postscript figures, to be published in Surf. Sc
Deuterium adsorption on (and desorption from) SiC(0001)-(3×3), (√3×√3)R30°, (6√3×6√3)R30° and quasi-free standing graphene obtained by hydrogen intercalation
International audienceWe present a comparative high-resolution electron energy-loss spectroscopy study on the interaction of atomic hydrogen and deuterium with various reconstructions of SiC(0 0 0 1). We first show that on both the (3 × 3) and reconstructions, deuterium atoms only bind to silicon atoms, thereby confirming the silicon-rich appellation of these reconstructions. Deuterium passivation of the (3 × 3) is only reversible when exposed to atomic deuterium at a surface temperature of 700 K since tri- and dideuterides, necessary precursors for silicon etching, are not stable. On the other hand, we show that the deuteration of the is always reversible because precursors to silicon etching are scarce on the surface. Then, we demonstrate that hydrogen (deuterium) adsorption at 300 K on both the (buffer-layer) and the quasi-free-standing graphene occurs on carbon atoms justifying their carbon-rich appellation. Comparison of the deuterium binding in the intercalation layer of quasi-free-standing graphene with the deuterated surface provides some indication on the bonding structure at the substrate intercalation layer. Finally, by measuring C-H (C-D) vibrational frequencies and hydrogen (deuterium) desorption temperatures we suggest that partial sp2-to-sp3 rehybridization occurs for the carbon atoms of the buffer-layer because of the corrugation related to covalent bonding to the SiC substrate. In contrast, on quasi-free-standing graphene hydrogen (deuterium) atoms adsorb similarly to what is observed on graphite, i.e. without preferential sticking related to the underlying SiC substrate
The mechanism for the 3 x 3 distortion of Sn/ge (111)
We show that two distinct ground states, one nonmagnetic,
metallic, and distorted, the other magnetic, semimetallic (or insulating) and
undistorted, compete in -phase adsorbates on semiconductor (111)
surfaces. In Sn/Ge(111), LSDA/GGA calculations indicate, in agreement with
experiment, that the distorted metallic ground state prevails. The reason for
stability of this state is analysed, and is traced to a sort of bond density
wave, specifically a modulation of the antibonding state filling between the
adatom and a Ge-Ge bond directly underneath
Electronic structure of tetra(4-aminophenyl)porphyrin studied by photoemission, UV–Vis spectroscopy and density functional theory
The valence and conduction bands of a thin film of tetra(4-aminophenyl)porphyrin (TAPP) are investi-gated by direct and inverse photoemission as well as by comparison to density functional theory (DFT)calculations. By projecting the electronic eigenfunctions onto the molecular framework it was possibleto interpret the origin of each spectroscopic feature. Although the majority of the photoemission spec-trum is attributed to the unsubstituted tetraphenylporphyrin (TPP) parent molecule, several featuresare clearly due to the amino substitution. Substitution also has important consequences for the energypositions of the frontier orbitals and therefore on the low-energy electronic excitations. The measuredelectronic transport energy gap (Eg= 1.85 eV) between the highest occupied molecular orbital (HOMO)and lowest unoccupied (LUMO) in TAPP is found to be significantly reduced with respect to TPP. More-over, an increased energy separation between the two highest occupied states (HOMO and HOMO−1) isfound both experimentally and by DFT calculations. Such evidence is attributed to an increased HOMOorbital destabilization due to an enhanced electron-donor character of the phenyl substituents uponamino functionalization. Finally, the above findings together with further time-dependent DFT calcula-tions are used to interpret the effect of the amino groups on the UV–Vis absorption spectrum, namely anoverall red-shift of the spectrum and remarkable intensity changes within the Q band.© 2017 Elsevier B.V
Magnetic Coupling and Single-Ion Anisotropy in Surface-Supported Mn-based Metal-Organic Networks
The electronic and magnetic properties of Mn coordinated to
1,2,4,5-tetracyanobenzene (TCNB) in the Mn-TCNB 2D metal-ligand networks have
been investigated by combining scanning tunneling microscopy and X-ray magnetic
circular dichroism (XMCD) performed at low temperature (3 K). When formed on
Au(111) and Ag(111) substrates the Mn-TCNB networks display similar geometric
structures. Magnetization curves reveal ferromagnetic (FM) coupling of the Mn
sites with similar single-ion anisotropy energies, but different coupling
constants. Low-temperature XMCD spectra show that the local environment of the
Mn centers differs appreciably for the two substrates. Multiplet structure
calculations were used to derive the corresponding ligand field parameters
confirming an in-plane uniaxial anisotropy. The observed interatomic coupling
is discussed in terms of superexchange as well as substrate-mediated magnetic
interactions.Comment: J. Phys. Chem. C 201
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
High-temperature desorption of C60 covalently bound to 6H-SiC(0001)-(3x3)
The desorption or fragmentation temperature of C-60 bound to Si-rich-(3 x 3) and (root 3 x root 3) R30 degrees. reconstructions of 6H-SiC(0001) is investigated using inverse photoemission spectroscopy (IPES) and LEED experiments. On SiC-(3 x 3), C-60 film is found desorbed after annealing at a high temperature of 1140 K, supporting covalent bonding. Meanwhile, the Si tetramers of the (3 x 3) nanostructured substrate are recovered, as can be inferred from the full reappearance of the Mott-Hubbard surface state in the IPE spectra. SiC-(3 x 3) behaves in a singular way among the other semiconducting substrates, which covalently bind to C-60. This remarkable feature is attributed to the low density of Si dangling bonds and to the highly corrugated character of this reconstruction
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