arXiv subjects

Marcin Mucha-Kruczynski

Publications and source records attributed to Marcin Mucha-Kruczynski.

15 recordsLinked to original sources

arXiv2026

Dimensional and doping stability of Peierls charge density waves in arrays of coupled one-dimensional chains

The Peierls instability, the spontaneous dimerization of a one-dimensional metallic chain at half filling, is a paradigmatic mechanism for charge-density-wave (CDW) formation. Here we test its robustness under finite doping and interchain hybridization in finite-thickness arrays of identical chains. We find that the stacking geometry plays a decisive role in stabilizing CDW order away from half filling. In particular, parallel-coupled chains exhibit a bistable regime where the normal and dimerized states coexist as local minima of the total energy, while skew-coupled chains display reentrant CDW order upon doping. Our results demonstrate that even minimal models of coupled atomic chains host rich phase diagrams controlled by doping, lattice rigidity, and interchain coupling geometry.

cond-mat.mes-hall↗

arXiv2026

Rhombohedral graphite junctions as a platform for continuous tuning between topologically trivial and non-trivial electronic phases

Manipulating the topological properties of quantum states can provide a way to protect them against disorder. However, typically, changing the topology of electronic states in a crystalline material is challenging because their nature is underpinned by chemical composition and lattice symmetry that are difficult to modify. We propose junctions between rhombohedral graphite crystals as a platform that enables smooth transition between topologically trivial and non-trivial regimes distinguished by the absence or presence of topological junction states. By invoking an analogy with the Su-Schrieffer-Heeger model, the appearance of topological states is related to the symmetry of the atomic stacking at the interface between the crystals. The possibility to explore both the topological and non-topological phases is provided by sliding the crystals with respect to each other.

cond-mat.mes-hall↗

arXiv2026

Electron localization, charge redistribution, and emergence of topological states at graphite junctions

Low-energy electronic behavior in graphite crystals is highly dependent on the relative stacking arrangement of the constituent layers. Topologically non-trivial electronic states can arise due to interrupted rhombohedral (ABC) stacking, localized at the edges of the stacking region, but not in the case of Bernal (AB) stacking. Here, we study the electronic properties of junctions between half-crystals of graphite of either Bernal or rhombohedral stacking, using a charge self-consistent tight-binding method and embedding potentials to account for the influence of layers far from the junction. We find junction-localized electronic states to be a ubiquitous feature, and all systems but one involving a rhombohedral half-crystal support a flat-band expected to exhibit electronic instabilities and strongly-correlated states. Nascent flat-band states associated with finite rhombohedral stacking sequences extend the physics into pure Bernal systems.

cond-mat.mes-hall↗

arXiv2024

One-dimensional Lieb superlattices: from the discrete to the continuum limit

The Lieb lattice is one of the simplest lattices that exhibits both linear Dirac-like and flat topological electronic bands. We propose to further tailor its electronic properties through periodic 1D electrostatic superlattices (SLs), which, in the long wavelength limit, were predicted to give rise to novel transport signatures, such as the omnidirectional super-Klein tunnelling (SKT). By numerically modelling the electronic structure at tight-binding level, we uncover the evolution of the Lieb SL band structure from the discrete all the way to the continuum regime and build a comprehensive picture of the Lieb lattice under 1D potentials. This approach allows us to also take into consideration the discrete lattice symmetry-breaking that occurs at the well/barrier interfaces created by the 1D SL, whose consequences cannot be explored using the previous low energy and long wavelength approaches. We find novel features in the band structure, among which are intersections of quadratic and flat bands, tilted Dirac cones, or series of additional anisotropic Dirac cones at energies where the SKT is predicted. Such features are relevant to experimental realizations of electronic transport in Lieb 1D SL realized in artificial lattices or in real material systems like 2D covalent organic/metal-organic frameworks and inorganic 2D solids.

cond-mat.mes-hall↗

arXiv2022

2T-POT Hawkes model for left- and right-tail conditional quantile forecasts of financial log-returns: out-of-sample comparison of conditional EVT models

Conditional extreme value theory (EVT) methods promise enhanced forecasting of the extreme tail events that often dominate systemic risk. We present an improved two-tailed peaks-over-threshold (2T-POT) Hawkes model that is adapted for conditional quantile forecasting in both the left and right tails of a univariate time series. This is applied to the daily log-returns of six large cap indices. We also take the unique step of fitting the model at multiple exceedance thresholds (from the 1.25% to 25.00% mirrored quantiles). Quantitatively similar asymmetries in Hawkes parameters are found across all six indices, adding further empirical support to a temporal leverage effect in financial price time series in which the impact of losses is not only larger but also more immediate. Out-of-sample backtests find that our 2T-POT Hawkes model is more reliably accurate than the GARCH-EVT model when forecasting (mirrored) value-at-risk and expected shortfall at the 5% coverage level and below. This suggests that asymmetric Hawkes-type arrival dynamics are a better approximation of the true data generating process for extreme daily log-returns than GARCH-type conditional volatility; our 2T-POT Hawkes model therefore presents a better performing alternative for financial risk modelling.

q-fin.ST↗

arXiv2022

Using in-plane anisotropy to engineer Janus monolayers of rhenium dichalcogenides

The new class of Janus two-dimensional (2D) transition-metal dichalcogenides with two different interfaces are currently gaining increasing attention due to their distinct properties different from the typical 2D materials. Here, we show that in-plane anisotropy of a 2D atomic crystal, like ReS$_{2}$ or ReSe$_{2}$, allows formation of a large number of inequivalent Janus monolayers. We use first-principles calculations to investigate the structural stability of 29 distinct ReX$_{2-x}$Y$_{x}$ ($\mathrm{X,Y \in \{S,Se\}}$) structures, which can be obtained by selective exchange of exposed chalcogens in a ReX$_{2}$ monolayer. We also examine the electronic properties and work function of the most stable Janus monolayers and show that the large number of inequivalent structures provides a way to engineer spin-orbit splitting of the electronic bands. We find that the breaking of inversion symmetry leads to sizable spin splittings and spontaneous diople moments than are larger than those in other Janus dichalcogenides. Moreover, our caluclations suggest that the work function of the Janus monolayers can be tuned by varying the content of the substituting chalcogen. Our work demonstrates that in-plane anisotropy provides additional flexibility in sub-layer engineering of 2D atomic crystals.

cond-mat.mtrl-sci↗

arXiv2022

Moiré superlattice effects and band structure evolution in near-30-degree twisted bilayer graphene

In stacks of two-dimensional crystals, mismatch of their lattice constants and misalignment of crystallographic axes lead to formation of moiré patterns. We show that moiré superlattice effects persist in twisted bilayer graphene (tBLG) with large twists and short moiré periods. Using angle-resolved photoemission, we observe dramatic changes in valence band topology across large regions of the Brillouin zone, including the vicinity of the saddle point at $M$ and across 3 eV from the Dirac points. In this energy range, we resolve several moiré minibands and detect signatures of secondary Dirac points in the reconstructed dispersions. For twists $θ>21.8^{\circ}$, the low-energy minigaps are not due to cone anti-crossing as is the case at smaller twist angles but rather due to moiré scattering of electrons in one graphene layer on the potential of the other which generates intervalley coupling. Our work demonstrates robustness of mechanisms which enable engineering of electronic dispersions of stacks of two-dimensional crystals by tuning the interface twist angles. It also shows that large-angle tBLG hosts electronic minigaps and van Hove singularities of different origin which, given recent progress in extreme doping of graphene, could be explored experimentally.

cond-mat.mes-hall↗

arXiv2021

Interplay of crystal thickness and in-plane anisotropy and evolution of quasi-one dimensional electronic character in ReSe$_{2}$

We study the valence band structure of ReSe$_{2}$ crystals with varying thickness down to a single layer using nanoscale angle-resolved photoemission spectroscopy and density functional theory. The width of the top valence band in the direction perpendicular to the rhenium chains decreases with decreasing number of layers, from 280 meV for the bulk to 61 meV for monolayer. This demonstrates increase of in-plane anisotropy induced by changes in the interlayer coupling and suggests progressively more one-dimensional character of electronic states in few-layer rhenium dichalcogenides.

cond-mat.mtrl-sci↗

arXiv2021

Asymmetric excitation of left- and right-tail extreme events probed using a Hawkes model: application to financial returns

We construct a two-tailed peak-over-threshold Hawkes model that captures asymmetric self- and cross-excitation in and between left- and right-tail extreme values within a time series. We demonstrate its applicability by investigating extreme gains and losses within the daily log-returns of the S&P 500 equity index. We find that the arrivals of extreme losses and gains are described by a common conditional intensity to which losses contribute twice as much as gains. However, the contribution of the former decays almost five times more quickly than that of the latter. We attribute these asymmetries to the different reactions of market traders to extreme upward and downward movements of asset prices: an example of negativity bias, wherein trauma is more salient than euphoria.

q-fin.ST↗

arXiv2018

Electronic band structure of rhenium dichalcogenides

The band structures of bulk transition metal dichalcogenides ReS2 and ReSe2 are presented, showing the complicated nature of the interband transitions in these materials, with several close-lying band gaps. Three-dimensional plots of constant energy surfaces in the Brillouin zone at energies near the band extrema are used to show that the valence band maximum and conduction band minimum may not be located at special high symmetry points. We find that both materials are indirect gap materials and that one must be careful to consider the whole Brillouin zone volume in addressing this question.

cond-mat.mes-hall↗

arXiv2017

Superconductivity-induced features in electronic Raman spectrum of monolayer graphene

Using the continuum model, we investigate theoretically contribution of the low-energy electronic excitations to the Raman spectrum of superconducting monolayer graphene. We consider superconducting phases characterised by an isotropic order parameter in a single valley and find a Raman peak at a shift set by the size of the superconducting gap. The height of this peak is proportional to the square root of the gap and the third power of the Fermi level, and we estimate its quantum efficiency as $I\sim10^{-14}$.

cond-mat.mes-hall↗

arXiv2017

Electronic bandstructure and van der Waals coupling of ReSe2 revealed by high-resolution angle-resolved photoemission spectroscopy

ReSe2 and ReS2 are unusual compounds amongst the layered transition metal dichalcogenides as a result of their low symmetry, with a characteristic in-plane anisotropy due to in-plane rhenium chains. They preserve inversion symmetry independent of the number of layers and, in contrast to more well-known transition metal dichalcogenides, bulk and few-monolayer Re-TMD compounds have been proposed to behave as electronically and vibrational decoupled layers. Here, we probe for the first time the electronic band structure of bulk ReSe2 by direct nanoscale angle-resolved photoemission spectroscopy. We find a highly anisotropic in- and out-of-plane electronic structure, with the valence band maxima located away from any particular high-symmetry direction. The effective mass doubles its value perpendicular to the Re chains and the interlayer van der Waals coupling generates significant electronic dispersion normal to the layers. Our density functional theory calculations, including spin-orbit effects, are in excellent agreement with these experimental findings.

cond-mat.mes-hall↗

arXiv2014

Anomalous sequence of quantum Hall liquids revealing tunable Lifshitz transition in bilayer graphene

Fermi surface topology plays an important role in determining the electronic properties of metals. In bulk metals, the Fermi energy is not easily tunable at the energy scale needed for reaching conditions for the Lifshitz transition - a singular point in the band structure where the connectivity of the Fermi surface changes. Bilayer graphene is a unique system where both Fermi energy and the low-energy electron dispersion can be tuned using the interplay between trigonal warping and a band gap opened by a transverse electric field. Here, we drive the Lifshitz transition to experimentally controllable carrier densities by applying large transverse electric fields through a h-BN-encapsulated bilayer graphene structure, and detect it by measuring the degeneracies of Landau levels. These degeneracies are revealed by filling factor -3 and -6 quantum Hall effect states of holes at low magnetic fields reflecting the existence of three maxima on the top of the valence band dispersion. At high magnetic fields all integer quantum Hall states are observed, indicating that deeper in the valence band the constant energy contours are singly-connected. The fact that we observe ferromagnetic quantum Hall states at odd-integer filling factors testifies to the high quality of our sample, and this enables us to identify several phase transitions between correlated quantum Hall states at intermediate magnetic fields, in agreement with the calculated evolution of the Landau level spectrum.

cond-mat.mes-hall↗

arXiv2014

Strain-induced modifications of transport in gated graphene nanoribbons

We investigate the effects of homogeneous and inhomogeneous deformations and edge disorder on the conductance of gated graphene nanoribbons. Under increasing homogeneous strain the conductance of such devices initially decreases before it acquires a resonance structure, and finally becomes completely suppressed at larger strain. Edge disorder induces mode mixing in the contact regions, which can restore the conductance to its ballistic value. The valley-antisymmetric pseudo-magnetic field induced by inhomogeneous deformations leads to the formation of additional resonance states, which either originate from the coupling into Fabry-Perot states that extend through the system, or from the formation of states that are localized near the contacts, where the pseudo-magnetic field is largest. In particular, the n=0 pseudo-Landau level manifests itself via two groups of conductance resonances close to the charge neutrality point.

cond-mat.mes-hall↗

arXiv2011

Strained bilayer graphene: Band structure topology and Landau level spectrum

We show that topology of the low-energy band structure in bilayer graphene critically depends on mechanical deformations of the crystal which may easily develop in suspended graphene flakes. We describe the Lifshitz transition that takes place in strained bilayers upon splitting the parabollic bands at intermediate energies into several Dirac cones at the energy scale of few meV. Then, we show how this affects the electron Landau level spectra and the quantum Hall effect.

cond-mat.mes-hall↗