arXiv subjects

Pawel J. Kowalczyk

Publications and source records attributed to Pawel J. Kowalczyk.

6 recordsLinked to original sources

arXiv2026

Electronic Coupling and Charge-Transfer Landscape of Graphene on Ge(001)/Si(001): Multiscale Analysis Assisted by Machine Learning

Understanding and controlling charge transfer at graphene-semiconductor interfaces is essential for the integration of two-dimensional materials into silicon-compatible technologies. Here, we combine ultraviolet photoelectron spectroscopy (UPS), Kelvin probe force microscopy (KPFM), angle-resolved photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS) and density functional theory (DFT) to resolve work-function modulation and electronic coupling in graphene grown on Ge(001)/Si(001). UPS and KPFM reveal a spatially non-uniform work-function landscape correlated with the nanofaceted morphology of the substrate. ARPES and DFT calculations for the pristine interface consistently indicate n-type doping and electron transfer from Ge to graphene. In contrast, modeling of the oxidized interface predicts a reversal to p-type doping, providing a plausible explanation for the different doping polarities reported in the literature. Machine-learning-assisted classification of the STS data resolves distinct local electronic regimes, ranging from nearly free-standing graphene on nanofacet tops to more strongly coupled inter-facet regions and nanoribbon-like regions with distinct local electronic responses. By identifying the mechanisms governing local graphene-substrate interactions and charge transfer, our study provides guidelines for tailoring the synthesis process and minimizing defect formation during delamination. These insights support the production of high-quality graphene layers for electronic devices and for transfer as protective coatings for air-sensitive materials.

cond-mat.mtrl-sci↗

arXiv2026

Overcoming contact resistance at metal-2D semiconductor interfaces: atomically clean MoS2/Au ohmic junctions

The application of two-dimensional (2D) semiconductors, such as monolayer MoS2, is limited by the high contact resistance commonly attributed to interfacial barriers at metal contacts. Furthermore, the dependence of electrical conductivity on MoS2 thickness is still unsettled, as both increasing and decreasing trends with layer number have been reported. By showing the contrast between electrical transport of mono- and multilayer MoS2 exfoliated on Au under ultra-high vacuum (UHV) and ambient conditions, we experimentally prove that, contrary to the prevailing view in the literature, the intrinsic MoS2/Au junction is highly conductive and exhibits ohmic behaviour. Our results indicate that interfacial contamination is responsible for the high contact resistances reported to date and affects the thickness dependence of electrical transport, explaining the discrepancies observed in the literature. We rationalize those findings using electrical transport simulations. Lastly, we show that local force-mediated lamination on lightly contaminated contacts can recover pristine, ohmic contacts, offering a route towards nanoscale patterning.

cond-mat.mes-hall↗

arXiv2025

Persistent spin texture preserved by local symmetry in graphene/WTe$_2$ heterostructure

Crystal symmetries in solids give rise to spin-momentum locking, which determines how an electron's spin orientation depends on its momentum. This relationship, often referred to as spin texture, influences both charge-to-spin conversion and spin relaxation, making it one of the essential characteristics for spin-orbit-driven phenomena. Materials with strong spin-orbit coupling and broken inversion symmetry can host persistent spin textures (PSTs) - unidirectional spin configurations in momentum space, supporting efficient charge-to-spin conversion and extended spin lifetimes. Monolayer WTe$_2$, a topological material crystallizing in a rectangular lattice, is a notable example; its symmetry enforces a canted PST enabling quantum spin Hall effect with the nontrivial spin orientation. Here, we use first-principles calculations to explore how these properties are modified when WTe$_2$ is interfaced with graphene. We find that the PST is preserved by the local symmetry present in different regions of the heterostructure, while the system develops extended electron and hole pockets, resulting in semimetallic behavior. Although the band gap closes and eliminates the quantum spin Hall phase, spin Hall effects remain robust in both conventional and unconventional geometries. The computed spin Hall conductivities are comparable to those of other two-dimensional materials, and the survival of the PST suggests the possibility of long-range spin transport even in the absence of topological edge states. In addition, the graphene layer serves as an oxidation barrier, helping protect the intrinsic properties of WTe$_2$ and supporting the potential of this heterostructure for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2021

Observation of Unpinned Two-Dimensional Dirac States in Antimony Single Layers with Phosphorene Structure

The discovery of graphene has stimulated enormous interest in two-dimensional (2D) electron gas with linear band structure. 2D Dirac materials possess many intriguing physical properties such as high carrier mobility and zero-energy Landau level thanks to the relativistic dispersion and chiral spin/pseudospin texture. 2D Dirac states discovered so far are exclusively pinned at high-symmetry points of the Brillouin zone, for example, surface Dirac states at $\overlineΓ$ in topological insulators Bi$_2$Se(Te)$_3$ and Dirac cones at $K$ and $K'$ in graphene. In this work, we report the realization of 2D Dirac states at generic $k$-points in antimony atomic layers with phosphorene structure ($i.e.$ $α$-antimonene). The unpinned nature enables versatile ways to control the locations of the Dirac points in momentum space. In addition, dispersions around the unpinned Dirac points exhibit intrinsically anisotropic behaviors due to the reduced symmetry of generic momentum points. These properties make the $α$-antimonene films a promising platform for exploring interesting physics in unpinned 2D Dirac fermions that are distinct from the conventional Dirac states in graphene.

cond-mat.mes-hall↗

arXiv2019

Realisation of Symmetry Enforced Two-Dimensional Dirac Fermions in Nonsymmorphic $α$-Bismuthene

Two-dimensional (2D) Dirac-like electron gases have attracted tremendous research interest ever since the discovery of free-standing graphene. The linear energy dispersion and non-trivial Berry phase play the pivotal role in the remarkable electronic, optical, mechanical and chemical properties of 2D Dirac materials. The known 2D Dirac materials are gapless only within certain approximations, for example, in the absence of SOC. Here we report a route to establishing robust Dirac cones in 2D materials with nonsymmorphic crystal lattice. The nonsymmorphic symmetry enforces Dirac-like band dispersions around certain high-symmetry momenta in the presence of SOC. Through $μ$-ARPES measurements we observe Dirac-like band dispersions in $α$-bismuthene. The nonsymmorphic lattice symmetry is confirmed by $μ$-LEED and STM. Our first-principles simulations and theoretical topological analysis demonstrate the correspondence between nonsymmorphic symmetry and Dirac states. This mechanism can be straightforwardly generalized to other nonsymmorphic materials. The results open the door for the search of symmetry enforced Dirac fermions in the vast uncharted world of nonsymmorphic 2D materials.

cond-mat.mes-hall↗

arXiv2017

Engineering Multiple Topological Phases in Nanoscale Van der Waals Heterostructures: Realisation of $α$-Antimonene

Van der Waals heterostructures have recently been identified as providing many opportunities to create new two-dimensional materials, and in particular to produce materials with topologically interesting states. Here we show that it is possible to create such heterostructures with multiple topological phases in a single nanoscale island. We discuss their growth within the framework of diffusion-limited aggregation, the formation of moiré patterns due to the differing crystallographies of the materials comprising the heterostructure, and the potential to engineer both the electronic structure as well as local variations of topological order. In particular we show that it is possible to build islands which include both the hexagonal $β$- and rectangular $α$-forms of antimonene, on top of the topological insulator $α$-bismuthene. This is the first experimental realisation of $α$-antimonene, and we show that it is a topologically non-trivial material in the quantum spin Hall class.

cond-mat.mtrl-sci↗