Jeison Fischer

When a single electron is confined to an impurity state in a metal, a many-body resonance emerges at the Fermi energy if the electron bath screens the impurity’s magnetic moment. This is the Kondo effect, originally introduced to explain the abnormal resistivity behaviour in bulk magnetic alloys, and it has been realized in many quantum systems over the past decades, ranging from heavy-fermion lattices down to adsorbed single atoms. Here we describe a Kondo system that allows us to experimentally resolve the spectral function consisting of impurity levels and a Kondo resonance in a large Kondo temperature range, as well as their spatial modulation. Our approach is based on a discrete half-filled quantum confined state within a MoS2 grain boundary, which—in conjunction with numerical renormalization group calculations—enables us to test the predictive power of the Anderson model that is the basis of the …

Das Anderson‐Modell (Anderson Impurity Model) wird in der Festkörperphysik zur Beschreibung von korrelierten, also stark wechselwirkenden Elektronen genutzt. Es besteht aus lokalen Energieniveaus, die aufgrund der Wechselwirkung mit delokalisierten Leitungselektronen eine nach Yun Kondo benannte Kondo‐Resonanz hervorrufen. Nun ist es erstmals gelungen, diese Energieniveaus zusammen mit der Kondo‐Resonanz präzise zu vermessen, wodurch eine quantitative Übereinstimmung zwischen Theorie und Experiment demonstriert wurde.

We present an atomically precise technique to create sulfur vacancies and control their atomic configurations in single-layer MoS. It involves adsorbed Fe atoms and the tip of a scanning tunneling microscope, which enables single sulfur removal from the top sulfur layer at the initial position of Fe. Using scanning tunneling spectroscopy, we show that the STM tip can also induce two Jahn-Teller distorted states with reduced orbital symmetry in the sulfur vacancies. Density functional theory calculations rationalize our experimental results. Additionally, we provide evidence for molecule-like hybrid orbitals in artificially created sulfur vacancy dimers, which illustrates the potential of our technique for the development of extended defect lattices and tailored electronic band structures.

Using scanning tunneling microscopy and spectroscopy, for a monolayer of transition metal dichalcogenide H-NbS2 grown by molecular beam epitaxy on graphene, we provide unambiguous evidence for a charge density wave (CDW) with a 3 × 3 superstructure, which is not present in bulk NbS2. Local spectroscopy displays a pronounced gap on 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 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 2D materials and …

The occupied electron energy bands of monolayer MoS 2 are composed from out-of-plane d orbitals at the Brillouin zone (BZ) center and from in-plane d orbitals at the BZ corner. If a dopant would interact in an orbital selective manner with the MoS 2 bands, it could provide a tuning knob to modulate the MoS 2 energy bands according to the electron wave vector. Here we directly show by angle-resolved photoemission spectroscopy (ARPES) that Fe doping of epitaxial MoS 2 is orbital selective. That is, Fe doping causes a larger energy upshift of the valence band at the BZ center compared to the BZ corner. The optical properties of Fe-doped MoS 2 are investigated by ultrahigh vacuum photoluminescence and reveal a loss of photoluminescence upon Fe doping. The sample morphology is investigated by scanning tunneling microscopy and shows a two-dimensional core-shell structure with Fe chemisorbed along …

The growth of monolayer hexagonal boron nitride (h-BN) on Ir (110) through low-pressure chemical vapor deposition is investigated using low energy electron diffraction and scanning tunneling microscopy. We find that the growth of aligned h-BN on Ir (110) requires a growth temperature of 1500 K, whereas lower growth temperatures result in coexistence of aligned h-BN with twisted h-BN. The presence of the h-BN overlayer suppresses the formation of the nano-faceted ridge pattern known from clean Ir (110). Instead, we observe the formation of a

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.

Much effort has been made to modify the properties of transition metal dichalcogenide layers via their environment as a route to new functionalization. However, it remains a challenge to induce large electronic changes without chemically altering the layer or compromising its two-dimensionality. Here, a non-invasive technique is used to shift the chemical potential of monolayer MoS 2 through p-and n-type doping of graphene (Gr), which remains a well-decoupled 2D substrate. With the intercalation of oxygen (O) under Gr, a nearly rigid Fermi level shift of 0.45 eV in MoS 2 is demonstrated, whereas the intercalation of europium (Eu) induces a metal–insulator transition in MoS 2, accompanied by a giant band gap reduction of 0.67 eV. Additionally, the effect of the substrate charge on 1D states within MoS 2 mirror-twin boundaries (MTBs) is explored. It is found that the 1D nature of the MTB states is not compromised …

A single-crystal sheet of graphene is synthesized on the low-symmetry substrate Ir (110) by thermal decomposition of C 2 H 4 at 1500 K. Using scanning tunneling microscopy, low-energy electron diffraction, angle-resolved photoemission spectroscopy, and ab initio density functional theory, the structure and electronic properties of the adsorbed graphene sheet and its moiré with the substrate are uncovered. The adsorbed graphene layer forms a wave pattern of nanometer wavelength with a corresponding modulation of its electronic properties. This wave pattern is demonstrated in density functional theory calculations to enable the templated adsorption of naphthalene molecules, and in experiment to uniaxially align sandwich-molecular wires composed of Eu and cyclooctatetraene molecules.

The variation of the electronic structure normal to 1D defects in quasi-freestanding MoS2, grown by molecular beam epitaxy, is investigated through high resolution scanning tunneling spectroscopy at 5 K. Strong upward bending of valence and conduction bands toward the line defects is found for the 4|4E mirror twin boundary and island edges but not for the 4|4P mirror twin boundary. Quantized energy levels in the valence band are observed wherever upward band bending takes place. Focusing on the common 4|4E mirror twin boundary, density functional theory calculations give an estimate of its charging, which agrees well with electrostatic modeling. We show that the line charge can also be assessed from the filling of the boundary-localized electronic band, whereby we provide a measurement of the theoretically predicted quantized polarization charge at MoS2 mirror twin boundaries. These calculations …