arXiv subjects

Jonas Erhardt

Publications and source records attributed to Jonas Erhardt.

9 recordsLinked to original sources

arXiv2026

Enhanced Screening in Epitaxial Graphene via Nearly Free-Electron Metal Intercalation

Graphene exhibits extraordinarily high carrier mobility, making it a promising platform for next-generation electronics. Scalable growth on SiC, however, suffers from limited dielectric screening at the graphene-substrate interface, degrading electronic performance. In this work, we systematically enhance dielectric screening by intercalating a bilayer of indium at the graphene-SiC interface. Using graphene's plasmaronic signature observed in angle-resolved photoemission spectroscopy as a proxy for interaction strength, we quantitatively demonstrate strong dielectric screening arising from the interplay of both indium layers. Layer-resolved density functional theory shows that the first indium layer acts as a buffer that absorbs substrate interactions, enabling the second layer to form a nearly free-electron system that efficiently screens the graphene layer above. Experiments with only a single intercalated indium layer reveal reduced screening, confirming the essential role of the second layer. Our results establish 2ML indium intercalation as a powerful route for engineering dielectric environments in graphene.

cond-mat.mtrl-sci↗

arXiv2026

Resonantly-enhanced Raman response in graphene-capped bismuthene on SiC

Two-dimensional quantum spin Hall insulators based on atomic monolayers offer a promising route toward dissipationless electronics, yet their practical use is often limited by environmental instability. Encapsulating the system with a graphene capping layer has been shown to be a reliable method to prevent oxidation and degradation. However, the confirmation of a successful encapsulation still relies on ultra-high vacuum techniques, that considerably slow the process. Here, we present an ex situ, rapid, nondestructive and spatially resolved Raman characterization of graphene-capped bismuthene, a honeycomb monolayer of Bi on SiC. A pronounced Raman scattering peak at around 122 cm-1 is identified as the E2g phonon of bismuthene, via a comparison with density functional perturbation theory calculations. We use excitation-energy and polarization-dependent Raman measurements to enable an unambiguous assignment of the spectral features. Tuning the excitation energy close to the excitonic transition in pristine bismuthene, we observe a strong enhancement of the Raman response and the emergence of additional scattering peaks. In this regime, higher-order phonon features, as well as interfacial modes between bismuthene and the SiC substrate, become visible, suggesting the involvement of resonant scattering processes. Our results establish Raman micro-spectroscopy as a versatile tool for probing graphene-protected quantum materials, providing access to lattice dynamics and interlayer coupling.

cond-mat.mtrl-sci↗

arXiv2025

Moir\'e-resonant surface state in ultrathin RuO$_2$

RuO$_2$ has emerged as a prototypical candidate for altermagnetism. In the face of daunting evidence for magnetic order in the bulk, the focus naturally shifted to surfaces and ultrathin films, where Coulomb interactions are dimensionally quenched and electron correlations strongly enhanced. Here, we examine atomically ordered, ultrathin RuO$_2$(110) grown on Ru(0001) using a combination of scanning tunneling microscopy (STM), density functional theory, and density matrix renormalization group methods. We observe a nonmagnetic charge order that is imprinted by the incommensurate moir\'e stacking with the substrate and enhanced by the electronic Fermi surface scattering within the flat-band surface state. We further identify a nonmagnetic, metastable $c(2 \times 2)$ surface reconstruction that arises from surface phonon softening and can be toggled reversibly via STM tip manipulation. Spin-polarized STM measurements, however, reveal no evidence of magnetic order on the RuO$_2$(110) surface. Our findings of a nonmagnetic charge-modulation position ultrathin RuO$_2$(110) as an intriguing platform for exploring moir\'e-assisted electronic orders.

cond-mat.mtrl-sci↗

arXiv2025

Backscattering in Topological Edge States Despite Time-Reversal Symmetry

Spin-momentum-locked edge states of quantum spin Hall insulators (QSHIs) provide a compelling platform for spintronic applications, owing to their intrinsic protection against backscattering from non-magnetic disorder. This protection emerges from time-reversal symmetry, which pairs Kramers partners of helical edge modes with opposite spin and momentum, thereby strictly forbidding elastic single-particle backscattering within the pair. Yet, contrary to the idealized notion of linear edge bands, the non-monotonic dispersions of realistic materials can host multiple Kramers pairs, reintroducing backscattering channels between them without violating time-reversal symmetry. Here, we investigate inter-Kramers pair backscattering in the non-linear edge bands of the QSHI indenene, highlighting a critical aspect of edge-state stability. Using quasiparticle interference in scanning tunneling spectroscopy -- a direct probe of backscattering -- we observe pairwise coupling between energy-degenerate Kramers pairs, while energy regions with only a single Kramers pair remain strictly protected. Supported by theoretical analysis, our findings provide an unprecedented experimental demonstration of edge state backscattering fully consistent with their underlying topological protection. This insight has profound implications for numerous QSHI candidates, emphasizing that the mere presence of gap-traversing edge modes does not inherently guarantee their protection against backscattering.

cond-mat.mes-hall↗

arXiv2025

Graphene intercalation of the large gap quantum spin Hall insulator bismuthene

The quantum spin Hall insulator bismuthene, a two-third monolayer of bismuth on SiC(0001), is distinguished by helical metallic edge states that are protected by a groundbreaking 800 meV topological gap, making it ideal for room temperature applications. This massive gap inversion arises from a unique synergy between flat honeycomb structure, strong spin orbit coupling, and an orbital filtering effect that is mediated by the substrate. However, the rapid oxidation of bismuthene in air has severely hindered the development of applications, so far confining experiments to ultra-high vacuum conditions. Here, we successfully overcome this barrier, intercalating bismuthene between SiC and a protective sheet of graphene. As we demonstrate through scanning tunneling microscopy and photoemission spectroscopy, graphene intercalation preserves the structural and topological integrity of bismuthene, while effectively shielding it from oxidation in air. We identify hydrogen as the critical component that was missing in previous bismuth intercalation attempts. Our findings facilitate ex-situ experiments and pave the way for the development of bismuthene based devices, signaling a significant step forward in the development of next-generation technologies.

cond-mat.mtrl-sci↗

arXiv2023

Stabilizing an atomically thin quantum spin Hall insulator at ambient conditions: Graphene-intercalation of indenene

Atomic monolayers on semiconductor surfaces represent a new class of functional quantum materials at the ultimate two-dimensional limit, ranging from superconductors [1, 2] to Mott insulators [3, 4] and ferroelectrics [5] to quantum spin Hall insulators (QSHI) [6, 7]. A case in point is the recently discovered QSHI indenene [7, 8], a triangular monolayer of indium epitaxially grown on SiC(0001), exhibiting a $\sim$120meV gap and substrate-matched monodomain growth on the technologically relevant $\mu$m scale [9]. Its suitability for room-temperature spintronics is countered, however, by the instability of pristine indenene in air, which destroys the system along with its topological character, nullifying hopes of ex-situ processing and device fabrication. Here we show how indenene intercalation into epitaxial graphene offers effective protection from the oxidizing environment, while it leaves the topological character fully intact. This opens an unprecedented realm of ex-situ experimental opportunities, bringing this monolayer QSHI within realistic reach of actual device fabrication and edge channel transport.

cond-mat.mtrl-sci↗

arXiv2022

Real-space Obstruction in Quantum Spin Hall Insulators

The recently introduced classification of two-dimensional insulators in terms of topological crystalline invariants has been applied so far to "obstructed" atomic insulators characterized by a mismatch between the centers of the electronic Wannier functions and the ionic positions. We extend this notion to quantum spin Hall insulators in which the ground state cannot be described in terms of time-reversal symmetric localized Wannier functions. A system equivalent to graphene in all its relevant electronic and topological properties except for a real-space obstruction is identified and studied via symmetry analysis as well as with density functional theory. The low-energy model comprises a local spin-orbit coupling and a non-local symmetry breaking potential, which turn out to be the essential ingredients for an obstructed quantum spin Hall insulator. An experimental fingerprint of the obstruction is then measured in a large-gap triangular quantum spin Hall material.

cond-mat.mes-hall↗

arXiv2021

Design and realization of topological Dirac fermions on a triangular lattice

Large-gap quantum spin Hall insulators are promising materials for room-temperature applications based on Dirac fermions. Key to engineer the topologically non-trivial band ordering and sizable band gaps is strong spin-orbit interaction. Following Kane and Mele's original suggestion, one approach is to synthesize monolayers of heavy atoms with honeycomb coordination accommodated on templates with hexagonal symmetry. Yet, in the majority of cases, this recipe leads to triangular lattices, typically hosting metals or trivial insulators. Here, we conceive and realize "indenene", a triangular monolayer of indium on SiC exhibiting non-trivial valley physics driven by local spin-orbit coupling, which prevails over inversion-symmetry breaking terms. By means of tunneling microscopy of the 2D bulk we identify the quantum spin Hall phase of this triangular lattice and unveil how a hidden honeycomb connectivity emerges from interference patterns in Bloch $p_x \pm ip_y$-derived wave functions.

cond-mat.mtrl-sci↗

arXiv2019

Tailoring the topological surface state in ultrathin $\alpha$-Sn (111) films

We report on the electronic structure of $\alpha$-Sn films in the very low thickness regime grown on InSb(111)A. High-resolution low photon energies angle-resolved photoemission (ARPES) allows for the direct observation of the linearly dispersing 2D topological surface states (TSSs) that exist between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200meV below the Fermi level in 10-nm-thick $\alpha$-Sn films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The crossover to a quasi-2D electronic structure is accompanied by a full gap opening at the Brillouin zone center, in agreement with our density functional theory calculations. We further identify the thickness regime of $\alpha$-Sn films where the hybridization gap in TSS coexists with the topologically non-trivial electronic structure and one can expect the presence of a 1D helical edge states.

cond-mat.str-el↗