arXiv subjects

Konrád Kandrai

Publications and source records attributed to Konrád Kandrai.

8 recordsLinked to original sources

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

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

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↗

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

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↗