arXiv subjects

Wouter Jolie

Publications and source records attributed to Wouter Jolie.

At least 19 recordsLinked to original sources

arXiv2026

Emergence of a non-bulk hexagonal Fe$_2$S$_2$ single layer via phase transformation

Two-dimensional materials can stabilize crystal structures that are absent from their bulk counterparts, offering opportunities for materials design. Here, we report the synthesis of a previously unknown hexagonal Fe$_2$S$_2$ single layer with $\beta$-CuI structure, a buckled layer of two vertically stacked FeS honeycomb lattices, realized by thermally induced transformation of single layer mackinawite grown on graphene/Ir(111). In situ scanning tunneling microscopy and low-energy electron diffraction reveal a transition from a tetragonal to a hexagonal lattice accompanied by distinct morphological and electronic signatures. The hexagonal Fe$_2$S$_2$ forms reproducibly upon annealing and represents a new structural motif within the Fe-S material family. First-principles calculations identify the $\beta$-CuI structure as most consistent with experiment. The calculations suggest that on-site Coulomb interactions and magnetic order are relevant to understanding the stability of the new 2D Fe-S compound. The preferred nucleation of single-layer mackinawite, despite being energetically disfavored, is speculated to result from its low edge energy, analogous to the 3D case. Our results establish Fe$_2$S$_2$ as a platform for exploring structural polymorphism in two dimensions and demonstrate that reduced dimensionality can stabilize crystal structures not accessible in bulk materials.

cond-mat.mtrl-sci↗

arXiv2026

Atomic-Scale Detection of N\'eel Vector Switching in the Single-Layer A-type Antiferromagnet Cr2S3-2D

The detection of N\'eel vector switching in a single-layer A-type antiferromagnet marks an important step toward functional two-dimensional spintronics. Here, Cr$_2$S$_3$-2D, grown on graphene on Ir(110), is established as a first single-layer A-type antiferromagnet. Spin-polarized scanning tunneling microscopy reveals hysteresis loops with a large switching field and a pronounced dependence on island size. X-ray magnetic circular dichroism at the Cr L$_{2,3}$ edges exhibits a tiny signal with a linear magnetic field dependence, consistent with a nearly compensated antiferromagnetic ground state and a N\'eel temperature of about 160 K. Quantitative analysis of the island-size dependence of the switching field, together with first principles calculations, indicates a slight imbalance between the magnetic moments of the two Cr planes of Cr$_2$S$_3$-2D when supported on a substrate. This imbalance results in a net magnetization for the A-type antiferromagnet, which enables the 180$^\circ$ rotation of the N\'eel vector. Moreover, Cr$_2$S$_3$-2D retains its magnetic properties after several days of exposure to air.

cond-mat.mes-hall↗

arXiv2026

Atomic-Scale Quantum Control of Single Spin Defects in a Two-Dimensional Semiconductor

Individual spin defects in solids are promising building blocks for quantum technologies, but their deterministic creation, individual addressability, and operation near surfaces remain major challenges. Two-dimensional materials provide an attractive alternative, as their single-layer thickness enables direct atomic-scale access to defect states. Here, we demonstrate single-spin control of solid-state defects in a two-dimensional semiconductor by a combination of scanning tunneling microscopy and electron spin resonance. We create and manipulate individual sulfur vacancies and carbon substitution defects in monolayer molybdenum disulfide and characterize their spin dynamics, including coherent control, at the single-defect level. Using atomic manipulation, we further engineer and probe spin-spin interactions between defect pairs. Our results demonstrate deterministic creation, addressability, coherent manipulation, and controlled coupling of individual spin defects within a single experimental platform. This establishes atomically engineered spin defects in two-dimensional semiconductors as a versatile class of controllable solid-state quantum systems and opens a route towards tailored quantum sensing experiments.

cond-mat.mes-hall↗

arXiv2025

Substrate Role in Polaron Formation on Single-layer Transition Metal Dihalides

Single-layer transition metal dihalides grown on conducting substrates were shown to host stable polarons. Here, we investigate polarons in insulating single-layer MnBr$_2$ grown by molecular beam epitaxy on three different substrates, namely graphene on Ir(110), graphene on Ir(111), and Au(111). The number densities and species of polarons observed vary strongly as a function of the substrate. For MnBr$_2$ grown on Ir(110) the largest number of polaron species is observed, namely four, of which three show clear similarities with the species observed for CoCl$_2$ on graphite. Polarons in single-layer MnBr$_2$ are observed up to 300K. They can be created, converted, and moved by the STM tip when a tunneling current flows at a proper bias voltage. For graphene on Ir(110) as a substrate, mobile polarons in MnBr$_2$ are guided through the periodic potential imposed from the super-moir\'e resulting from the interaction of MnBr$_2$ with graphene and Ir(110). Our findings indicate that modeling of polarons in such single-layer insulators in contact with a conducting substrate requires to take the substrate explicitly into account.

cond-mat.mtrl-sci↗

arXiv2025

MnBr$_2$ on the graphene on Ir(110) substrate: growth, structure, and super-moir\`e

Single-layer MnBr$_2$ is grown on graphene (Gr) supported by Ir(110) and investigated using low-energy electron diffraction, scanning tunneling microscopy, and spectroscopy. The structure and epitaxial relationship with the substrate are systematically characterized. The structure and epitaxial relationship with the substrate are systematically characterized. The growth morphology strongly depends on the growth temperature, evolving from fractal to dendritic and eventually to compact dendritic skeletal islands, reflecting changes in the underlying surface diffusion processes. The pronounced variation in the apparent height with tunneling conditions for the magnetic insulator is explained based on the measured electronic density of states. MnBr$_2$ on Gr/Ir(110) constitutes a three-lattice system, giving rise to a super-moir\'e pattern -- a moir\'e of moir\'es. The super-moir\'e of MnBr$_2$/Gr/Ir(110) is unique, as it involves a virtual moir\'e of MnBr$_2$ with the Ir(110) surface lattice -- two lattices not in contact with each other. Using a careful Fourier analysis, the known properties of Gr/Ir(110), and the results of ab initio calculations, the origin of the virtual moir\'e is uncovered and related to the inhomogeneous binding of Gr to Ir(110). Comparative experiments with MnBr$_2$ on Gr/Ir(111) show similar growth and structure, but highlight the unique properties of the MnBr$_2$/Gr/Ir(110) super-moir\'e.

cond-mat.mtrl-sci↗

arXiv2025

Inelastic tunneling into multipolaronic bound states in single-layer MoS$_2$

Polarons are quasiparticles that arise from the interaction of electrons or holes with lattice vibrations. Though polarons are well-studied across multiple disciplines, experimental observations of polarons in two-dimensional crystals are sparse. We use scanning tunneling microscopy and spectroscopy to measure inelastic excitations of polaronic bound states emerging from coupling of non-polar zone-boundary phonons to Bloch electrons in n-doped metallic single-layer MoS$_2$. The latter is kept chemically pristine via contactless chemical doping. Tunneling into the vibrationally coupled polaronic states leads to a series of evenly spaced peaks in the differential conductance on either side of the Fermi level. Combining density functional (perturbation) theory with a recently developed ab initio electron-lattice downfolding technique, we show that the energy spacing stems from the longitudinal-acoustic phonon mode that flattens at the Brillouin zone edge and is responsible for the formation of stable multipolarons in metallic MoS$_2$.

cond-mat.mes-hall↗

arXiv2025

Which chromium-sulfur compounds exist as 2D material?

Two-dimensional (2D) chromium-sulfides are synthesized by molecular beam epitaxy using graphene as a substrate. Structure characterization by employing scanning tunneling microscopy and low energy electron diffraction indicates that there are two 2D phases, Cr$_2$S$_3$-2D and Cr$_{2\frac{2}{3}}$S$_4$-2D, which have not been reported before. Cr$_{2\frac{2}{3}}$S$_4$-2D is related to bulk Cr$_5$S$_6$, but thinner than a bulk unit cell. For Cr$_2$S$_3$-2D, an even thinner material, no bulk counterpart exists. Both 2D materials are found to be structurally stable under ambient conditions and exhibit interesting electronic properties. Extensive first-principles calculations provide further insight into the electronic structure of these systems and indicate that they should be magnetic. Although single layers of CrS$_2$ were predicted to be stable by density functional theory calculations and reported in previous experimental studies, we were unable to synthesize CrS$_2$ under our range of experimental conditions.

cond-mat.mtrl-sci↗

arXiv2024

Engineering two-dimensional materials from single-layer NbS$_2$

Starting from a single layer of NbS$_2$ grown on graphene by molecular beam epitaxy, the single unit cell thick 2D materials Nb$_{5/3}$S$_3$-2D and Nb$_2$S$_3$-2D are created using two different pathways. Either annealing under sulfur-deficient conditions at progressively higher temperatures or deposition of increasing amounts of Nb at elevated temperature result in phase-pure Nb$_{5/3}$S$_3$-2D followed by Nb$_2$S$_3$-2D. The materials are characterized by scanning tunneling microscopy, scanning tunneling spectroscopy and X-ray photoemission spectroscopy. The experimental assessment combined with systematic density functional theory calculations reveals their structure. The 2D materials are covalently bound without any van der Waals gap. Their stacking sequence and structure are at variance with expectations based on corresponding bulk materials highlighting the importance of surface and interface effects in structure formation.

cond-mat.mtrl-sci↗

arXiv2024

Probing the spin polarization of an Anderson impurity

We report spin-polarized scanning tunneling microscopy measurements of an Anderson impurity system in MoS$_{2}$ mirror twin boundaries, where both the quantum confined impurity state and the Kondo resonance resulting from the interaction with the substrate are accessible. Using a spin-polarized tip, we observe magnetic field induced changes in the peak heights of the Anderson impurity states as well as in the magnetic field-split Kondo resonance. Quantitative comparison with numerical renormalization group calculations provides evidence of the notable spin polarization of the spin-resolved impurity spectral function under the influence of a magnetic field. Moreover, we extract the field and temperature dependence of the impurity magnetization from the differential conductance measurements and demonstrate that this exhibits the universality and asymptotic freedom of the $S=1/2$ Kondo effect. This work shows that mirror twin boundaries can be used as a testing ground for theoretical predictions on quantum impurity models.

cond-mat.mes-hall↗

arXiv2024

Tip-induced creation and Jahn-Teller distortions of sulfur vacancies in single-layer MoS$_{2}$

We present an atomically precise technique to create sulfur vacancies and control their atomic configurations in single-layer MoS$_{2}$. 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.

cond-mat.mtrl-sci↗

arXiv2023

Unconventional charge-density-wave gap in monolayer NbS$_2$

Using scanning tunneling microscopy and spectroscopy, for a monolayer of transition metal dichalcogenide H-NbS$_2$ grown by molecular beam epitaxy on graphene, we provide unambiguous evidence for a charge density wave (CDW) with a 3$\times$3 superstructure, which is not present in bulk NbS$_2$. 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.

cond-mat.mes-hall↗

arXiv2023

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, X-ray photoemission spectroscopy and density-functional theory calculations. The controlled synthesis of stoichiometric single-layer 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. Via the comparison of density-functional theory calculations with scanning tunneling microscopy data, the atomic structure of the S-depleted phase, with a stoichiometry of V4S7, is determined. 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 2x2 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.

cond-mat.mtrl-sci↗

arXiv2022

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.

cond-mat.mes-hall↗

arXiv2021

Metal-insulator transition in monolayer MoS$_2$ via contactless chemical doping

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, even when MoS$_2$ is made metallic. Furthermore, with the periodicity of the 1D states dependent on substrate-induced charging and depletion, the boundaries serve as chemical potential sensors functional up to room temperature.

cond-mat.mtrl-sci↗

arXiv2021

Creating tunable and coupled Rashba-type quantum dots atom-by-atom

Artificial lattices created by assembling atoms on a surface with scanning tunneling microscopy present a platform to create matter with tailored electronic, magnetic and topological properties. However, such artificial lattices studies to date have focused exclusively on surfaces with weak spin-orbit coupling. Here, we created artificial and coupled quantum dots by fabricating quantum corrals from iron atoms on the prototypical Rashba surface alloy, BiCu2, using low-temperature scanning tunneling microscopy. We quantified the quantum confinement of such quantum dots with various diameter and related this to the spatially dependent density of states, using scanning tunneling spectroscopy. We found that the density of states shows complex distributions beyond the typical isotropic patterns seen in radially symmetric structures on (111) noble metal surfaces. We related these to the energy-dependent interplay of the confinement potential with the hexagonal warping and multiple intra- and interband scattering vectors, which we simulated with a particle-in-a-box model that considers the Rashba-type band structure of BiCu2. Based on these results, we studied the effect of coupling two quantum dots and exploited the resultant anisotropic coupling derived from the symmetry of the various scattering channels. The large anisotropy and spin-orbit coupling provided by the BiCu2 platform are two key ingredients toward creation of correlated artificial lattices with non-trivial topology.

cond-mat.mes-hall↗

arXiv2020

Band Bending and Valence Band Quantization at Line Defects in MoS$_2$

The variation of the electronic structure normal to 1D defects in quasi-freestanding MoS$_2$, grown by molecular beam epitaxy, is investigated through high resolution scanning tunneling spectroscopy at $5$K. Strong upwards bending of valence and conduction bands towards 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 upwards 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 MoS$_2$ mirror twin boundaries. These calculations elucidate the origin of band bending and charging at these 1D defects in MoS$_2$. The 4|4E mirror twin boundary not only impairs charge transport of electrons and holes due to band bending, but holes are additionally subject to a potential barrier, which is inferred from the independence of the quantized energy landscape on either side of the boundary.

cond-mat.mes-hall↗

arXiv2020

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.

cond-mat.mes-hall↗

arXiv2019

Tomonaga-Luttinger liquid in a box: electrons confined within MoS$_2$ mirror twin boundaries

Two- or three-dimensional metals are usually well described by weakly interacting, fermionic quasiparticles. This concept breaks down in one dimension due to strong Coulomb interactions. There, low-energy electronic excitations are expected to be bosonic collective modes, which fractionalize into independent spin and charge density waves. Experimental research on one-dimensional metals is still hampered by their difficult realization, their limited accessibility to measurements, and by competing or obscuring effects such as Peierls distortions or zero bias anomalies. Here we overcome these difficulties by constructing a well-isolated, one-dimensional metal of finite length present in MoS$_2$ mirror twin boundaries. Using scanning tunneling spectroscopy we measure the single-particle density of the interacting electron system as a function of energy and position in the 1D box. Comparison to theoretical modeling provides unambiguous evidence that we are observing spin-charge separation in real space.

cond-mat.str-el↗