arXiv subjects

Péter Nemes-Incze

Publications and source records attributed to Péter Nemes-Incze.

At least 19 recordsLinked to original sources

arXiv2026

One-way electron mirrors from single-layer stacking shifts in rhombohedral graphite

One-way mirrors or asymmetric scatterers are key components of photonic systems, protecting sources from back-reflections and routing light along a chosen path. Acoustic systems can also be engineered to show asymmetric reflection. However, a condensed matter platform acting as a one-way mirror without breaking linearity, time-reversal symmetry or energy conservation has been lacking. One route to achieving this is mode selection, where a probe coupled to only some modes sees a boundary as a mirror from one side and as transparent from the other. Here we show that single-layer stacking shifts in six-layer rhombohedral graphite provide this platform, acting as one-way mirrors for electrons. In scanning tunneling microscopy, 10 nm scale interference fringes appear only on the stacking fault side of a wall, tracking its Dirac band, the rhombohedral side remaining transparent. Layer-resolved tight-binding and wave-packet dynamics calculations reproduce this and trace the asymmetry to the mismatched layer polarization across the wall, with the tip supplying the selective coupling. Because stacking solitons glide under external fields, this mirror forms a movable element of electron optics defined by the crystal structure alone.

cond-mat.mes-hall↗

arXiv2026

Ultrastrong Au-Te bonding drives disorder in monolayer ZrTe$_5$ on gold

Monolayer ZrTe$_5$ is predicted to host a large-gap quantum spin Hall phase, motivating efforts to isolate single layers of the material. Gold-assisted exfoliation produces clean monolayers of many chalcogen-terminated van der Waals crystals, but the strong Te-Au bond may compete with the bonding network of the ZrTe$_5$ layer itself. Across tens of samples, low-temperature scanning tunneling microscopy on gold-exfoliated flakes shows a disordered monolayer surface in the overwhelming majority of cases, while thicker flakes preserve the characteristic quasi-one-dimensional chain structure. \emph{Ab initio} calculations of the ZrTe$_5$/Au(111) interface reproduce this asymmetry and resolve its mechanism: the Te atoms facing the gold chemisorb, rupturing the weak zigzag Te-Te bonds that cross-link the ZrTe$_3$ chains, turning the discrete bond-length spectrum of the crystal into a continuous one, while the chains persist as distorted units. Charge analysis shows the signature of covalent Te-Au bonding with a modest transfer that hole-dopes the monolayer; in the bilayer, the distortion and the doping stay confined to the layer contacting the gold, and the chemisorption and disorder each preclude the predicted quantum spin Hall phase. The same quasi-covalent Te-Au bond may perturb the contact layer of other gold-exfoliated Te-terminated crystals, particularly those with weakly connected intralayer networks.

cond-mat.mtrl-sci↗

arXiv2026

Mechanical strain induced topological phase changes of monolayer and bilayer ZrTe$_5$

Two-dimensional materials offer exceptional tunability of their properties through strain, electrostatic gating, and related mechanisms. Among these, ZrTe$_5$ stands out as a material in which band topology can be switched via mechanical deformation. In its bulk form, it lies near a topological phase boundary, allowing efficient modulation of its topological state through strain. While extensive studies exist on bulk ZrTe$_5$, the topological phase transitions in monolayer and bilayer forms remain largely unexplored. Here we show that monolayer and bilayer ZrTe$_5$ exhibit distinct strain-induced topological transitions. Mechanical deformation in monolayers can close the topological gap, while bilayers display a richer phase diagram, including topological and trivial insulating phases as well as an intermediate metallic phase. Since thinner van der Waals materials are more amenable to strain tuning, our results identify bilayer ZrTe$_5$ as a candidate for achieving tunable topological phase transitions using moderate (1-2{\%}) strain as a tuning knob.

cond-mat.mtrl-sci↗

arXiv2026

Proximity-Induced Spin Reorientation in Monolayer CrI$_3$ on Hexagonal WTe$_2$

Magnetic anisotropy controls the orientation and thermal stability of two-dimensional magnetic order. Predicting proximity-induced changes in anisotropy requires linking the electronic structure to microscopic magnetic interactions and finite-temperature behavior. Here we study monolayer CrI$_3$ on hexagonal WTe$_2$ using a first-principles-to-finite-temperature workflow centered on relativistic spin-Hamiltonian mapping. We find that WTe$_2$ reorients the CrI$_3$ magnetization from out-of-plane to in-plane and substantially enhances the magnetic ordering scale within the extracted spin models. Analysis of the extracted spin Hamiltonians shows that the reorientation is driven by a substrate-induced change in the balance between symmetric anisotropic exchange and onsite anisotropy. We establish a transferable workflow for proximity-controlled magnetism in two-dimensional van der Waals heterostructures.

cond-mat.mtrl-sci↗

arXiv2026

Enrichment of rhombohedral stacking by mechanical exfoliation of graphite

Rhombohedral (ABC) graphite hosts a surface-localized flat band that supports correlated and topological electronic phases, but its experimental study is limited by the scarcity of ABC stacking in natural graphite, which is dominated by Bernal (AB) stacking. Here we show that the routine mechanical exfoliation step itself enriches the rhombohedral content of graphite flakes, and that a simple blade-assisted exfoliation step, which introduces additional shear, amplifies the effect further. Using large-area Raman 2D-band skewness mapping we measure ABC content at area fractions of 3\% in the pristine source crystal, 16\% after conventional exfoliation, and 26\% after blade-assisted exfoliation for thick flakes. In thin flakes ($<20$ layers) the per-flake area fraction reaches 75\% in the upper tail of the distribution. Tracking individual flakes before and after blade-assisted exfoliation shows that wrinkles seed AB-ABC domain walls, and uniaxial strain can move these walls. Blade-assisted mechanical exfoliation therefore removes one of the bottlenecks to the preparation of ABC-rich graphite samples for studies of correlated and topological phases in rhombohedral graphite.

cond-mat.mtrl-sci↗

arXiv2026

The role of stacking and strain in mean-field magnetic moments of multilayer graphene

Rhombohedral or ABC stacked multilayer graphene hosts a correlated magnetic ground state at charge neutrality, making it one of the simplest systems to investigate strong electronic correlations. We investigate this ground state in multilayer graphene structures using the Hubbard model in a distance dependent Slater-Koster tight binding framework. We show that by using a universal Hubbard-$U$ term, we can accurately capture the spin polarization predicted by hybrid density functional theory calculations for both hexagonal (ABA) and rhombohedral (ABC) stackings. Using this $U$ value, we calculate the magnetic moments of 3-8 layers of ABC and ABA graphene multilayers. We demonstrate that the structure and magnitude of these magnetic moments are robust when heterostructures are built from varying numbers of ABC and ABA multilayers. By applying different types of mechanical distortions, we study the behaviour of the magnetism in graphene systems under uniaxial strain and pressure. Our results establish a computationally efficient framework to investigate correlation-driven magnetism across arbitrary stacking configurations of graphite polytypes.

cond-mat.str-el↗

arXiv2026

Identification of graphite with perfect rhombohedral stacking by electronic Raman scattering

Rhombohedral graphite (RG) shows strong correlations in its topological flat band and is pivotal for exploring emergent, correlated electronic phenomena. One key advantage is the enhancement of electronic interactions with the increase in the number of rhombohedrally stacked graphene layers. Increasing thickness also leads to an exponential increase in the number of stacking faults, necessitating a precise method to identify flawless rhombohedral stacking. Overcoming this challenge is difficult because the established technique for stacking sequence identification, based on the Raman 2D peak, fails in thick RG samples. We demonstrate that the strong layer dependence of the band structure can be harnessed to identify RG without stacking faults, or alternatively, to detect their presence. For thicknesses ranging from 3 to 12 layers, we show that each perfect RG structure presents distinctive peak positions in electronic Raman scattering (ERS). This measurement can be carried out using a conventional confocal Raman spectrometer at room temperature, using visible excitation wavelengths. Consequently, this overcomes the identification challenge by providing a simple and fast optical measurement technique, thereby helping to establish RG as a platform for studying strong correlations in one of the simplest crystals possible.

cond-mat.mes-hall↗

arXiv2025

Revealing the impact of ambient molecular contamination on scanning tunneling microscopy and spectroscopy of layered materials

Hydrocarbon contamination is an ever-present factor to consider in surface science measurements. In the case of van der Waals material surfaces, the structure of this contamination has become known in recent years as a self-assembled layer of normal-alkanes, resulting from a few days' exposure to ambient air. Knowledge of its composition and structure enables systematic investigation of its influence on surface properties. Here, we investigate the effect of this contamination on scanning tunneling microscopy (STM) and spectroscopy measurements by comparing clean and ambient alkane-contaminated surfaces of graphite. Our results reveal that the ambient alkane layer suppresses the well-known phonon-induced gap near the Fermi energy, resolving a long-standing inconsistency in STM studies, where this feature is often absent. Furthermore, we show that the presence of the contamination layer alters the current-distance ($I(z)$) characteristics, flattening its exponential decay by a factor of 1.5 to 5 compared to the clean surface. This change arises from extra conductance channels through the alkane layer alongside the tunnel junction, as the tip penetrates the contaminant overlayer. Finally, based on the $I(z)$ characteristics, we provide a practical guide to detect the presence of surface contamination in STM measurements.

cond-mat.mes-hall↗

arXiv2024

Signature of pressure-induced topological phase transition in ZrTe$_5$

The layered van der Waals material ZrTe$_5$ is known as a candidate topological insulator (TI), however its topological phase and the relation with other properties such as an apparent Dirac semimetallic state is still a subject of debate. We employ a semiclassical multicarrier transport (MCT) model to analyze the magnetotransport of ZrTe$_5$ nanodevices at hydrostatic pressures up to 2 GPa. The temperature dependence of the MCT results between 10 and 300 K is assessed in the context of thermal activation, and we obtain the positions of conduction and valence band edges in the vicinity of the chemical potential. We find evidence of the closing and re-opening of the band gap with increasing pressure, which is consistent with a phase transition from weak to strong TI. This matches expectations from ab initio band structure calculations, as well as previous observations that CVT-grown ZrTe$_5$ is a weak TI in ambient conditions.

cond-mat.mes-hall↗

arXiv2023

Revealing the band structure of ZrTe$_5$ using Multicarrier Transport

The layered material ZrTe$_5$ appears to exhibit several exotic behaviors which resulted in significant interest recently, although the exact properties are still highly debated. Among these we find a Dirac/Weyl semimetallic behavior, nontrivial spin textures revealed by low temperature transport, and a potential weak or strong topological phase. The anomalous behavior of resistivity has been recently elucidated as originating from band shifting in the electronic structure. Our work examines magnetotransport behavior in ZrTe$_5$ samples in the context of multicarrier transport. The results, in conjunction with ab-initio band structure calculations, indicate that many of the transport features of ZrTe$_5$ across the majority of the temperature range can be adequately explained by the semiclassical multicarrier transport model originating from a complex Fermi surface.

cond-mat.mes-hall↗

arXiv2022

The composition and structure of the ubiquitous hydrocarbon contamination on van der Waals materials

The behavior of single layer van der Waals (vdW) materials is profoundly influenced by the immediate atomic environment at their surface, a prime example being the myriad of emergent properties in artificial heterostructures. Equally significant are adsorbates deposited onto their surface from ambient. While vdW interfaces are well understood, our knowledge regarding atmospheric contamination is severely limited. Here we show that the common ambient contamination on the surface of: graphene, graphite, hBN and MoS2 is composed of a self-organized molecular layer, which forms during a few days of ambient exposure. Using low-temperature STM measurements we image the atomic structure of this adlayer and in combination with infrared spectroscopy identify the contaminant molecules as normal alkanes with lengths of 20-26 carbon atoms. Through its ability to self-organize, the alkane layer displaces the manifold other airborne contaminant species, capping the surface of vdW materials and possibly dominating their interaction with the environment.

cond-mat.mes-hall↗

arXiv2022

Observation of competing, correlated ground states in the flat band of rhombohedral graphite

In crystalline solids the interactions of charge and spin can result in a variety of emergent quantum ground states, especially in partially filled, topological flat bands such as Landau levels or in 'magic-angle' bilayer graphene. Much less explored is rhombohedral graphite (RG), perhaps the simplest and structurally most perfect condensed matter system to host a flat band protected by symmetry. By scanning tunneling microscopy we map the flat band charge density of 8, 10 and 17 layers and identify a domain structure emerging from a competition between a sublattice antiferromagnetic insulator and a gapless correlated paramagnet. Our density-matrix renormalization group calculations explain the observed features and demonstrate that the correlations are fundamentally different from graphene based magnetism identified until now, forming the ground state of a quantum magnet. Our work establishes RG as a new platform to study many-body interactions beyond the mean-field approach, where quantum fluctuations and entanglement dominate.

cond-mat.mes-hall↗

arXiv2022

Revealing the topological phase diagram of ZrTe$_5$ using the complex strain fields of microbubbles

Topological materials host robust properties, unaffected by microscopic perturbations, owing to the global topological properties of the bulk electron system. Materials in which the topological invariant can be changed by easily tuning external parameters are especially sought after. Zirconium pentatelluride (ZrTe$_5$) is one of a few experimentally available materials that reside close to the boundary of a topological phase transition, allowing the switching of its invariant by mechanical strain. Here, we unambiguously identify a topological insulator - metal transition as a function of strain, by a combination of ab initio calculations and direct measurements of the local charge density. Our model quantitatively describes the response to complex strain patterns found in bubbles of few layer ZrTe$_5$ without fitting parameters, reproducing the mechanical deformation dependent closing of the band gap observed using scanning tunneling microscopy. We calculate the topological phase diagram of ZrTe$_5$ and identify the phase at equilibrium, enabling the design of device architectures which exploit the unique topological switching characteristics of the system.

cond-mat.mtrl-sci↗

arXiv2020

Robust quantum point contact operation of narrow graphene constrictions patterned by AFM cleavage lithography

Detecting conductance quantization in graphene nanostructures turned out more challenging than expected. The observation of well-defined conductance plateaus through graphene nanoconstrictions so far has only been accessible in the highest quality suspended or h-BN encapsulated devices. However, reaching low conductance quanta in zero magnetic field, is a delicate task even with such ultra-high mobility devices. Here, we demonstrate a simple AFM-based nanopatterning technique for defining graphene constrictions with high precision (down to 10 nm width) and reduced edge-roughness (+/- 1 nm). The patterning process is based on the in-plane mechanical cleavage of graphene by the AFM tip, along its high symmetry crystallographic directions. As-defined, narrow graphene constrictions with improved edge quality enable an unprecedentedly robust QPC operation, allowing the observation of conductance quantization even on standard $SiO_2/Si$ substrates, down to low conductance quanta. Conductance plateaus, were observed at $ne^2/h$, evenly spaced by $2e^2/h$ (corresponding to n = 3, 5, 7, 9, 11) in the absence of an external magnetic field, while spaced by $e^2/h$ (n = 1, 2, 3, 4, 5, 6) in 8T magnetic field.

cond-mat.mes-hall↗

arXiv2020

Signature of large-gap quantum spin Hall state in the layered mineral jacutingaite

Quantum spin Hall (QSH) insulators are materials that feature an insulating bulk and host edge states protected by time-reversal symmetry. The helical locking of spin and momentum in these states suppresses backscattering of charge carriers, promising applications from low-power electronics to quantum computing. A major challenge for applications is the identification of large gap QSH materials, which would enable room temperature dissipationless transport in their edge states. Here we show that the layered mineral jacutingaite (Pt$_2$HgSe$_3$) is a candidate QSH material, realizing the long sought after the Kane-Mele insulator. Using scanning tunneling microscopy, we measure a band gap of 110 meV, above room temperature, and identify the hallmark edge states. By calculating the $\mathbb{Z}_2$ invariant, we confirm the topological nature of the gap. Being a layered mineral, it is stable in air and can be thinned down to a few atomic layers by mechanical exfoliation. Furthermore, we demonstrate that it can be integrated into heterostructures with other two-dimensional materials. This adds a topological insulator to the 2D quantum material library, greatly expanding the possibilities for tuning 2D electron systems using stacks of layered materials.

cond-mat.mes-hall↗

arXiv2019

Large intravalley scattering due to pseudo-magnetic fields in crumpled graphene

The pseudo-magnetic field generated by mechanical strain in graphene can have dramatic consequences on the behavior of electrons and holes. Here we show that pseudo-magnetic field fluctuations present in crumpled graphene can induce significant intravalley scattering of charge carriers. We detect this by measuring the confocal Raman spectra of crumpled areas, where we observe an increase of the D'/D peak intensity ratio by up to a factor of 300. We reproduce our observations by numerical calculation of the double resonant Raman spectra and interpret the results as experimental evidence of the phase shift suffered by Dirac charge carriers in the presence of a pseudo-magnetic field. This lifts the restriction on complete intravalley backscattering of Dirac fermions.

cond-mat.mes-hall↗

arXiv2017

Preparing local strain patterns in graphene by atomic force microscope based indentation

Patterning graphene into various mesoscopic devices such as nanoribbons, quantum dots, etc. by lithographic techniques has enabled the guiding and manipulation of graphene's Dirac-type charge carriers. Graphene, with well-defined strain patterns, holds promise of similarly rich physics while avoiding the problems created by the hard to control edge configuration of lithographically prepared devices. To engineer the properties of graphene via mechanical deformation, versatile new techniques are needed to pattern strain profiles in a controlled manner. Here we present a process by which strain can be created in substrate supported graphene layers. Our atomic force microscope-based technique opens up new possibilities in tailoring the properties of graphene using mechanical strain.

cond-mat.mes-hall↗

arXiv2014

Controlling the nanoscale rippling of graphene with SiO2 nanoparticles

The electronic properties of graphene can be significantly influenced by mechanical strain. One practical approach to induce strain in graphene is to transfer this atomically thin membrane onto pre-patterned substrates with specific corrugation. The possibility to use nanoparticles to impart extrinsic rippling to graphene has not been fully explored yet. Here we study the structure and elastic properties of graphene grown by chemical vapour deposition and transferred onto a continuous layer of SiO2 nanoparticles with diameters of around 25 nm, prepared on Si substrate by Langmuir-Blodgett technique. We show that the corrugation of the transferred graphene and thus the membrane strain can be modified by annealing at moderate temperatures. The membrane parts bridging the nanoparticles are suspended and can be reversibly lifted by the attractive forces between an atomic force microscope tip and graphene. This allows the dynamic control of the local morphology of graphene nanomembranes.

cond-mat.mes-hall↗