arXiv subjects

Affan Safeer

Publications and source records attributed to Affan Safeer.

11 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↗

arXiv2023

Growth of aligned and twisted hexagonal boron nitride on Ir(110)

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 single hexagonal boron nitride 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 (1 $\times$ n) reconstruction, with n such that the missing rows are in registry with the h-BN/Ir(110) moir\'{e} pattern. Our moir\'{e} analysis showcases a precise methodology for determining both the moir\'{e} periodicity and the h-BN lattice parameter on an fcc(110) surface.

cond-mat.mtrl-sci↗

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↗