40 research outputs found
Understanding, controlling and manipulating the electronic properties of layered materials
This thesis is devoted to the electronic properties of layered materials, with a focus on
the underlying interactions. Scanning tunneling microscopy and spectroscopy are used to
measure the local density of states present on the surface at low temperatures. The layered
materials studied as part of this thesis are bilayer graphene, topological insulators and
transition metal dichalcogenides. These materials have attracted broad interest in recent
years for their exceptional electronic properties. The experimental results are compared
to density functional theory calculations or model Hamiltonians
Quantifying Exchange Forces of a Non-Collinear Magnetic Structure on the Atomic Scale
The large interest in chiral magnetic structures for realization of nanoscale
magnetic storage or logic devices has necessitated methods which can quantify
magnetic interactions at the atomic scale. To overcome the limitations of the
typically used current-based sensing of atomic-scale exchange interactions, a
force-based detection scheme is highly advantageous. Here, we quantify the
atomic-scale exchange force field between a ferromagnetic tip and a cycloidal
spin spiral using our developed combination of current and exchange force
detection. Compared to the surprisingly weak spin polarization, the exchange
force field is more sensitive to atomic-scale variations in the magnetization.
First-principles calculations reveal that the measured atomic-scale variations
in the exchange force originate from different contributions of direct and
indirect (Zener) type exchange mechanisms, depending on the chemical tip
termination. Our work opens the perspective of quantifying different exchange
mechanisms of chiral magnetic structures with atomic-scale precision using 3D
magnetic exchange force field measurements
Unconventional charge-density-wave gap in monolayer NbS
Using scanning tunneling microscopy and spectroscopy, for a monolayer of
transition metal dichalcogenide H-NbS grown by molecular beam epitaxy on
graphene, we provide unambiguous evidence for a charge density wave (CDW) with
a 33 superstructure, which is not present in bulk NbS. Local
spectroscopy displays a pronounced gap of the order of 20 meV at the Fermi
level. Within the gap low energy features are present. The gap structure with
its low energy features is at variance with the expectation for a gap opening
in the electronic band structure due to a CDW. Instead, comparison with \it{ab
initio} calculations indicates that the observed gap structure must be
attributed to combined electron-phonon quasiparticles. The phonons in question
are the elusive amplitude and phase collective modes of the CDW transition. Our
findings advance the understanding of CDW mechanisms in two dimensional
materials and their spectroscopic signatures
Novel 2D vanadium sulphides: synthesis, atomic structure engineering and charge density waves
Two new ultimately thin vanadium rich 2D materials based on VS2 are created
via molecular beam epitaxy and investigated using scanning tunneling microscopy
and X-ray photoemission spectroscopy. The controlled synthesis of
stoichiometric singlelayer VS2 or either of the two vanadium-rich materials is
achieved by varying the sample coverage and the sulphur pressure during
annealing. Through annealing of small stoichiometric single-layer VS2 islands
without S pressure, S-vacancies spontaneously order in 1D arrays, giving rise
to patterned adsorption. We provide an atomic model of the 1D patterned phase,
with a stoichiometry of V4S7. By depositing larger amounts of vanadium and
sulphur, which are subsequently annealed in a S-rich atmosphere,
self-intercalated ultimately thin V5S8-derived layers are obtained, which host
2 x 2 V-layers between sheets of VS2. We provide atomic models for the thinnest
V5S8-derived structures. Finally, we use scanning tunneling spectroscopy to
investigate the charge density wave observed in the 2D V5S8-derived islands
Graphene on weakly interacting metals: Dirac states versus surface states
We investigate the interplay between graphene and different, weakly interacting metal substrates by measuring the local density of states of the surface with scanning tunneling spectroscopy. Energy-resolved Friedel oscillations, confined states, and a prominent signal in point spectra are found after intercalating several monolayers of silver between graphene and Ir(111) and correspond to the shifted surface state of silver. These features outweigh spectroscopic signatures of graphene, which are retrieved when the amount of silver is reduced to one monolayer. Hence, suppressing the surface states of the metal substrate enhances the sensitivity to the Dirac states of quasi-free-standing graphene
Interfacial Carbon Nanoplatelet Formation by Ion Irradiation of Graphene on Iridium(111)
Peer reviewe
Confinement of Dirac electrons in graphene quantum dots
We observe spatial confinement of Dirac states on epitaxial graphene quantum dots with low-temperature scanning tunneling microscopy after using oxygen as an intercalant to suppress the surface state of Ir(111) and to effectively decouple graphene from its metal substrate. We analyze the confined electronic states with a relativistic particle-in-a-box model and find a linear dispersion relation. The oxygen-intercalated graphene is p doped [ED=(0.64±0.07) eV] and has a Fermi velocity close to the one of free-standing graphene [vF=(0.96±0.07)×106 m/s]
Modulated Kondo screening along magnetic mirror twin boundaries in monolayer MoS2 on graphene
A many-body resonance emerges at the Fermi energy when an electron bath
screens the magnetic moment of a half-filled impurity level. This Kondo effect,
originally introduced to explain the abnormal resistivity behavior in bulk
magnetic alloys, has been realized in many quantum systems over the past
decades, such as quantum dots, quantum point contacts, nanowires,
single-molecule transistors, heavy-fermion lattices, down to adsorbed single
atoms. Here we describe a unique Kondo system which allows us to experimentally
resolve the spectral function consisting of impurity levels and Kondo resonance
in a large Kondo temperature range, as well as their spatial modulation. Our
experimental Kondo system, based on a discrete half-filled quantum confined
state within a MoS2 grain boundary, in conjunction with numerical
renormalization group calculations, enables us to test the predictive power of
the Anderson model which is the basis of the microscopic understanding of Kondo
physics
Charge density wave phase of VSe2 revisited
Scanning tunneling microscopy and spectroscopy are used to image the charge density wave at the surface of cleaved VSe2 and to probe its local density of states at 5 K. The main features in the spectrum are linked to the contributions of the p-like and d-like bands of VSe2 found in angle-resolved photoemission spectroscopy and tight-binding calculations. Different from previous tunneling spectroscopy work, we find a narrow partial gap at the Fermi level that we associate with the charge density wave phase. The energy scale of the gap found in the experiment is in good agreement with the charge density wave transition temperature of VSe2, under the assumption of weak electron-phonon coupling, consistent with the Peierls model of Fermi surface nesting. The role of defects is investigated, which reveals that the partial gap in the density of states and hence the charge density wave itself is extremely stable, though the order, phase, and amplitude of the charge density waves on the surface are strongly perturbed by defects