104 research outputs found
Rocking motion induced charging of C60 on h-BN/Ni(111)
One monolayer of C60 on one monolayer of hexagonal boron nitride on nickel is
investigated by photoemission. Between 150 and 250 K the work function
decreases and the binding energy of the highest occupied molecular orbital
(HOMO) increases by approx. 100 meV. In parallel, the occupancy of the, in the
cold state almost empty, lowest unoccupied molecular orbital (LUMO) changes by
0.4 electrons. This charge redistribution is triggered by onset of molecular
rocking motion, i.e. by orientation dependent tunneling between the LUMO of C60
and the substrate. The magnitude of the charge transfer is large and cannot be
explained within a single particle picture. It is proposed to involve
electron-phonon coupling where C60- polaron formation leads to electron
self-trapping.Comment: 15 pages, 4 figure
Fermi surfaces of single layer dielectrics on transition metals
Single sheets of hexagonal boron nitride on transition metals provide a model
system for single layer dielectrics. The progress in the understanding of h-BN
layers on transition metals of the last 10 years are shortly reviewed.
Particular emphasis lies on the boron nitride nanomesh on Rh(111), which is a
corrugated single sheet of h-BN, where the corrugation imposes strong lateral
electric fields. Fermi surface maps of h-BN/Rh(111) and Rh(111) are compared. A
h-BN layer on Rh(111) introduces no new bands at the Fermi energy, which is
expected for an insulator. The lateral electric fields of h-BN nanomesh violate
the conservation law for parallel momentum in photoemission and smear out the
momentum distribution curves on the Fermi surface.Comment: 14 pages, 6 figures, 1 table, 1 equation, Accepted for publication in
the Special Surface Science issue in honor of Gerhard Ertl's Nobel Priz
Excited states at interfaces of a metal-supported ultrathin oxide film
We report layer-resolved measurements of the unoccupied electronic structure of ultrathin MgO films grown on Ag(001). The metal-induced gap states at the metal/oxide interface, the oxide band gap, and a surface core exciton involving an image-potential state of the vacuum are revealed through resonant Auger spectroscopy of the MgKL23L23 Auger transition. Our results demonstrate how to obtain new insights on empty states at interfaces of metal-supported ultrathin oxide films
Robustness of the charge-ordered phases in IrTe against photoexcitation
We present a time-resolved angle-resolved photoelectron spectroscopy study of
IrTe, which undergoes two first-order structural and charge-ordered phase
transitions on cooling below 270 K and below 180 K. The possibility of inducing
a phase transition by photoexcitation with near-infrared femtosecond pulses is
investigated in the charge-ordered phases. We observe changes of the spectral
function occuring within a few hundreds of femtoseconds and persisting up to
several picoseconds, which we interpret as a partial photoinduced phase
transition (PIPT). The necessary time for photoinducing these spectral changes
increases with increasing photoexcitation density and reaches timescales longer
than the rise time of the transient electronic temperature. We conclude that
the PIPT is driven by a transient increase of the lattice temperature following
the energy transfer from the electrons. However, the photoinduced changes of
the spectral function are small, which indicates that the low temperature phase
is particularly robust against photoexcitation. We suggest that the system
might be trapped in an out-of-equilibrium state, for which only a partial
structural transition is achieved.Comment: 8 pages, 5 figures, accepted for publication in Phys. Rev.
Robustness of the charge-ordered phases in against photoexcitation
We present a time-resolved angle-resolved photoelectron spectroscopy study of IrTe2, which undergoes two first-order structural and charge-ordered phase transitions on cooling below 270 K and below 180 K. The possibility of inducing a phase transition by photoexcitation with near-infrared femtosecond pulses is investigated in the charge-ordered phases. We observe changes of the spectral function occurring within a few hundreds of femtoseconds and persisting up to several picoseconds, which we interpret as a partial photoinduced phase transition (PIPT). The necessary time for photoinducing these spectral changes increases with increasing photoexcitation density and reaches time scales longer than the rise time of the transient electronic temperature. We conclude that the PIPT is driven by a transient increase of the lattice temperature following the energy transfer from the electrons. However, the photoinduced changes of the spectral function are small, which indicates that the low- temperature phase is particularly robust against photoexcitation. We suggest that the system might be trapped in an out-of-equilibrium state, for which only a partial structural transition is achieved
Kagome silicene: a novel exotic form of two-dimensional epitaxial silicon
Since the discovery of graphene, intensive efforts have been made in search
of novel two-dimensional (2D) materials. Decreasing the materials
dimensionality to their ultimate thinness is a promising route to unveil new
physical phenomena, and potentially improve the performance of devices. Among
recent 2D materials, analogs of graphene, the group IV elements have attracted
much attention for their unexpected and tunable physical properties. Depending
on the growth conditions and substrates, several structures of silicene,
germanene, and stanene can be formed. Here, we report the synthesis of a Kagome
lattice of silicene on aluminum (111) substrates. We provide evidence of such
an exotic 2D Si allotrope through scanning tunneling microscopy (STM)
observations, high-resolution core-level (CL) and angle-resolved photoelectron
spectroscopy (ARPES) measurements, along with Density Functional Theory
calculations.Comment: 13 pages, 6 figure
Quasicrystals and their approximants in 2D ternary oxides
2D oxide quasicrystals (OQCs) are recently discovered aperiodic, but well-ordered oxide interfaces. In this topical review, an introduction to these new thin-film systems is given. The concept of quasicrystals and their approximants is explained for BaTiO3 - and SrTiO3 - derived OQCs and related periodic structures in these 2D oxides. In situ microscopy unravels the high-temperature formation process of OQCs on Pt(111). The dodecagonal structure is discussed regarding tiling statistics and tiling decoration based on the results of atomically resolved scanning tunneling microscopy and various diffraction techniques. In addition, angle-resolved ultraviolet photoemission spectroscopy and X-ray photoelectron spectroscopy results prove a metallic character of the 2D oxide
Metamagnetic transition and a loss of magnetic hysteresis caused by electron trapping in monolayers of single-molecule magnet Tb@CN
Whereas bulk Tb@CN is a single-molecule magnet with broad hysteresis, its monolayers on different substrates show the prevalence of a non-magnetic ground state near zero magnetic field and a metamagnetic transition with the field increase.
Realization of stable spin states in surface-supported magnetic molecules is crucial for their applications in molecular spintronics, memory storage or quantum information processing. In this work, we studied the surface magnetism of dimetallo-azafullerene Tb@CN, showing a broad magnetic hysteresis in a bulk form. Surprisingly, monolayers of Tb@CN exhibited a completely different behavior, with the prevalence of a ground state with antiferromagnetic coupling at low magnetic field and a metamagnetic transition in the magnetic field of 2.5–4 T. Monolayers of Tb@CN were deposited onto Cu(111) and Au(111) by evaporation in ultra-high vacuum conditions, and their topography and electronic structure were characterized by scanning tunneling microscopy and spectroscopy (STM/STS). X-ray photoelectron spectroscopy (XPS), in combination with DFT studies, revealed that the nitrogen atom of the azafullerene cage tends to avoid metallic surfaces. Magnetic properties of the (sub)monolayers were then studied by X-ray magnetic circular dichroism (XMCD) at the Tb-M absorption edge. While in bulk powder samples Tb@CN behaves as a single-molecule magnet with ferromagnetically coupled magnetic moments and blocking of magnetization at 28 K, its monolayers exhibited a different ground state with antiferromagnetic coupling of Tb magnetic moments. To understand if this unexpected behavior is caused by a strong hybridization of fullerenes with metallic substrates, XMCD measurements were also performed for Tb2@C79N adsorbed on h-BN|Rh(111) and MgO|Ag(100). The co-existence of two forms of Tb@CN was found on these substrates as well, but magnetization curves showed narrow magnetic hysteresis detectable up to 25 K. The non-magnetic state of Tb@CN in monolayers is assigned to anionic Tb@CN− species with doubly-occupied Tb–Tb bonding orbital and antiferromagnetic coupling of the Tb moments. A charge transfer from the substrate or trapping of secondary electrons are discussed as a plausible origin of these species
- …