arXiv subjects

Dario Bercioux

Publications and source records attributed to Dario Bercioux.

At least 19 recordsLinked to original sources

arXiv2026

Understanding Graphene-Perovskite Interactions: From Flake Chemistry to Crystallisation and Solar Cell Performance

Graphene-derivatives are widely employed materials to improve bulk and interface properties of metal-halide perovskite devices. Yet the implications of their flake chemistry and interactions with the perovskite precursors remain unclear. Here, we show that pristine graphene flakes (GF) and more conventional graphene oxide flakes (GOF) are not interchangeable. Density functional theory calculations reveal that GOF interacts more strongly with the perovskite lattice but induces larger structural distortions, stronger interfacial polarisation, and localised gap states. In contrast, GF forms comparatively non-disruptive contacts, a response retained across a wide compositional range. Machine-learning atomistic simulations further show that GF contacts both Pb- and I-containing regions of solvated perovskite nanocrystals, with a strong solvent dependency. Solution spectroscopic characterization indicates that GF additives serve as scaffold for preorganised Pb/I-containing precursors, favouring film crystallisation. In this sense, GF enhances solar cell performance across perovskite compositions, but particularly those facing a more challenging crystallisation. In mixed Sn-Pb perovskite solar cells, GF raises the champion power-conversion efficiency from 21.5\% to 23.7\% with improved storage stability. These results establish pristine GF as a chemically defined additive and connect its atomic-scale interactions with precursor organisation, crystallisation, device performance, and stability.

cond-mat.mtrl-sci↗

arXiv2026

Tuning topological phases and exceptional points in a non-Hermitian Rice Mele model beyond nearest neighbors

Exceptional points are degeneracies characteristic of non-Hermitian operators, where eigenvalues and eigenvectors coalesce, rendering the Hamiltonian defective. We investigate the exceptional-point structure and topological properties of a generalized non-Hermitian Rice-Mele model with balanced gain and loss, as well as next-nearest-neighbor hopping. The system hosts only second-order exceptional points under both periodic and open boundary conditions. Under periodic boundary conditions, the exceptional points in parameter space lie on lines and ellipses that are independent of the next-nearest-neighbor hopping, since the latter enters the bulk Hamiltonian only as an identity contribution. Under open boundary conditions, this independence is broken: the next-nearest-neighbor hopping not only shifts the energy of existing exceptional points but also generates new ones, with a specific condition signaling a topological gap closing observed only in the open-boundary spectrum. At special parameter points, multiple simultaneous second-order exceptional points yield degenerate configurations whose degeneracy grows with system size. Exceptional point locations are identified numerically via the condition number of the eigenvector matrix and confirmed by Jordan decomposition. The topological phase diagram, computed via a winding number framework for non-Hermitian systems without symmetry protection, reveals sectors with zero, one, and two edge states; the bulk-boundary correspondence is confirmed, and the non-Hermitian skin effect is absent.

quant-ph↗

arXiv2026

Effects of Electron Form Factor on Quasiparticle Interference in Twisted Bilayer Graphene

The overlap matrix of electronic energy eigenstates, sometimes referred to as the form factor, determines the quantum geometric tensor of electrons in solids. Here, we show that the variation in the overlap of two eigenstates with opposite momenta can be directly observed via quasiparticle interference (QPI) imaging. We study the QPI in twisted bilayer graphene using a real-space tight-binding model combined with the kernel polynomial method. The resulting QPI patterns, which are largely independent of whether the two graphene layers are commensurate or incommensurate, reveal all intralayer and interlayer interference processes. While the intralayer interference signals resemble those of monolayer graphene, the interlayer interference - which vanishes at large twist angles - displays a chiral structure that reverses between the two layers and between the valence and conduction bands. Furthermore, the QPI patterns explicitly demonstrate the approximate translational symmetries and valley charge conservation in twisted bilayer graphene, validating the topological obstruction to constructing the Wannier orbitals of states at the Dirac cones. Using a continuum model of twisted bilayer graphene, we show that all characteristics of the observed QPI patterns can be explained by the form factor of eigenstates projected onto a single layer. Our results provide fundamental insights into the electronic spectrum and wave functions of twisted bilayer graphene, and establish QPI as an experimental probe for the form factor of back-scattering states.

cond-mat.mes-hall↗

arXiv2026

Topological Lasing from Thouless Pumping in Bilayer Photonic Crystal

Topological lasing leverages concepts from topological physics to achieve single-mode light amplification within topological bandgaps, offering robustness against fabrication imperfections. Recent advances in microelectromechanical systems (MEMSs) and phase-change materials (PCMs) at the subwavelength scale promise new avenues for dynamically reconfigurable topological lasers, enabling robust and tunable nanoscale light sources. Here, we numerically demonstrate a dynamically reconfigurable lasing action at telecom wavelengths in a bilayer photonic crystal through the mechanisms of Thouless pumping. By designing two competing periodic potentials -- one slowly translating photonic grating atop another stationary one -- we observe a transition between a topological pumping regime and conventional mode oscillation. A carefully engineered heterojunction between these phases supports a robust lasing mode that can be dynamically tuned via MEMSs or reversible PCMs. Our work establishes bilayer photonic crystals as a programmable platform for achieving topological light sources, showcasing a potential pathway for merging topological photonics with reconfigurable photonic devices.

physics.optics↗

arXiv2025

Colloquium: Synthetic quantum matter in non-standard geometries

Quantum simulation is making a significant impact on scientific research. The prevailing tendency of the field is to build quantum simulators that get closer to real-world systems of interest, in particular electronic materials. However, progress in the microscopic design also provides an opportunity for an orthogonal research direction: building quantum many-body systems beyond real-world limitations. This colloquium takes this perspective: Concentrating on synthetic quantum matter in non-standard lattice geometries, such as fractal lattices or quasicrystals, higher-dimensional or curved spaces, it aims at providing a fresh introduction to the field of quantum simulation aligned with recent trends across various quantum simulation platforms, including atomic, photonic, and electronic devices. We also shine light on the novel phenomena which arise from these geometries: Condensed matter physicists may appreciate the variety of different localization properties as well as novel topological phases which are offered by such exotic quantum simulators. But also in the search of quantum models for gravity and cosmology, quantum simulators of curved spaces can provide a useful experimental tool.

quant-ph↗

arXiv2025

Tunable hyperbolic Landau-level polaritons in charge-neutral graphene nanoribbon metasurfaces

Magnetized charge-neutral graphene supports collective hybrid electronic excitations - polaritons - which have quantum origin. In contrast to polaritons in doped graphene, which arise from intraband electronic transitions, those in charge-neutral graphene originate from interband transitions between Landau levels, enabled by the applied magnetic field. Control of such quantum polaritons and shaping their wavefronts remains totally unexplored. Here we design an artificial two-dimensional quantum material formed by charge-neutral graphene nanoribbons exposed to an external magnetic field. In such metasurface, quantum polaritons acquire a hyperbolic dispersion. We find that the topology of the isofrequency curves of quantum hyperbolic magnetoexciton polaritons excited in this quantum material can change, so that the shape of isofrequency curves transforms from a closed to open one by tuning the external magnetic field strength. At the topological transition, we observe canalization phenomena, consisting of the propagation of all the polaritonic plane waves in the continuum along the same direction when excited by a point source. From a general perspective, our fundamental findings introduce a novel type of actively-tunable quantum polaritons with hyperbolic dispersion and can be further generalized to other types of quantum materials and polaritons in them. In practice, quantum hyperbolic polaritons can be used for applications related to quantum sensing and computing.

cond-mat.mes-hall↗

arXiv2025

Impurity-induced counter skin-effect and linear modes in non-Hermitian systems

Non-reciprocal lattice systems are among the simplest non-Hermitian systems, exhibiting several key features absent in their Hermitian counterparts. In this study, we investigate the Hatano-Nelson model with impurity and unveil how the impurity influences the intrinsic non-Hermitian skin effect of the system. We present an exact analytical solution to the problem under open and periodic boundary conditions, irrespective of the impurity's position and strength. Numerical simulations thoroughly validate this exact solution. Our analysis reveals a distinctive phenomenon where a specific impurity strength, determined by the non-reciprocal hopping parameters, induces a unique skin state at the impurity site. This impurity state exhibits a skin effect that counterbalances the boundary-induced skin effect, a phenomenon we term the \emph{impurity-induced counter skin-effect}. These findings offer insights into the dynamics of non-Hermitian systems with impurities, elucidating the complex interplay between impurities and the system's non-reciprocal nature. We propose a possible implementation of this system for a non-Hermitian discrete-time quantum walk, and we demonstrate that an impurity-induced counter skin-effect also exists in multi-band models.

quant-ph↗

arXiv2025

Berry Monopole Scattering in the Synthetic Momentum Space of a Bilayer Photonic Crystal Slab

Berry monopoles-quantized sources of Berry curvature-are fundamental to topological phases, yet their scattering remains unexplored. Here, we report for the first time the adiabatic scattering of Berry monopoles in a bilayer photonic crystal slab combining one genuine and one synthetic momentum. Two monopoles approach, collide, and scatter within this hybrid parameter space. The process is described by an effective coupled-mode model and confirmed by full-wave simulations. We further propose an experimental scheme using chiral edge states, opening a route to probe monopole interactions in synthetic photonic systems.

physics.optics↗

arXiv2024

Implementation and characterization of the dice lattice in the electron quantum simulator

Materials featuring touching points, localized states, and flat bands are of great interest in condensed matter and artificial systems due to their implications in topology, quantum geometry, superconductivity, and interactions. In this theoretical study, we propose the experimental realization of the dice lattice with adjustable parameters by arranging carbon monoxide molecules on a two-dimensional electron system at a (111) copper surface. First, we develop a theoretical framework to obtain the spectral properties within a nearly free electron approximation and then compare them with tight-binding calculations. Our investigation reveals that the high mobility of Shockley state electrons enables an accurate theoretical description of the artificial lattice using a next-nearest-neighbor tight-binding model, resulting in the emergence of a touching point, a quasi-flat band, and localized lattice site behavior in the local density of states. Additionally, we present theoretical results for a long-wavelength low-energy model that accounts for next-nearest-neighbor hopping terms. Furthermore, we theoretically examine the model's behavior under an external magnetic field by employing Peierl's substitution, a commonly used technique in theoretical physics to incorporate magnetic fields into lattice models. Our theoretical findings suggest that, owing to the exceptional electron mobility, the highly degenerate eigenenergy associated with the Aharonov-Bohm caging mechanism may not manifest in the proposed experiment.

cond-mat.mes-hall↗

arXiv2023

Chiral spin channels in curved graphene $pn$ junctions

We show that the chiral modes in circular graphene $pn$ junctions provide an advantage for spin manipulation via spin-orbit coupling compared to semiconductor platforms. We derive the effective Hamiltonian for the spin dynamics of the junction's zero modes and calculate their quantum phases. We find a sweet spot in parameter space where the spin is fully in-plane and radially polarized for a given junction polarity. This represents a shortcut to singular spin configurations that would otherwise require spin-orbit coupling strengths beyond experimental reach.

cond-mat.mes-hall↗

arXiv2023

Spin-texture topology in curved circuits driven by spin-orbit interactions

Interferometry is a powerful technique used to extract valuable information about the wave function of a system. In this work, we study the response of spin carriers to the effective field textures developed in curved one-dimensional interferometric circuits subject to the joint action of Rashba and Dresselhaus spin-orbit interactions. By using a quantum network technique, we establish that the interplay between these two non-Abelian fields and the circuit's geometry modify the geometrical characteristics of the spinors, particularly on square circuits, leading to the localisation of the electronic wave function and the suppression of the quantum conductance. We propose a topological interpretation by classifying the corresponding spin textures in terms of winding numbers.

cond-mat.mes-hall↗

arXiv2023

Fermi arc reconstruction in synthetic photonic lattice

The chiral surface states of Weyl semimetals have an open Fermi surface called Fermi arc. At the interface between two Weyl semimetals, these Fermi arcs are predicted to hybridize and alter their connectivity. In this letter, we numerically study a one-dimensional (1D) dielectric trilayer grating where the relative displacements between adjacent layers play the role of two synthetic momenta. The lattice emulates 3D crystals without time-reversal symmetry, including Weyl semimetal, nodal line semimetal, and Chern insulator. Besides showing the phase transition between Weyl semimetal and Chern insulator at telecom wavelength, this system allows us to observe the Fermi arc reconstruction between two Weyl semimetals, confirming the theoretical predictions.

cond-mat.mes-hall↗

arXiv2021

Higher-order topology in plasmonic kagome lattices

We study the topological properties of a kagome plasmonic metasurface, modelled with a coupled dipole method which naturally includes retarded long range interactions. We demonstrate the system supports an obstructed atomic limit phase through the calculation of Wilson loops. Then we characterise the hierarchy of topological boundary modes hosted by the subwavelength array of plasmonic nanoparticles: both one-dimensional edge modes as well as zero-dimensional corner modes. We determine the properties of these modes which robustly confine light at subwavelength scales, calculate the local density of photonic states at edge and corner modes frequencies, and demonstrate the selective excitation of delocalised corner modes in a topological cavity, through non-zero orbital angular momentum beam excitation.

cond-mat.mes-hall↗

arXiv2020

On the robustness of topological corner modes in photonic crystals

We analyze the robustness of corner modes in topological photonic crystals, taking a $C_6$-symmetric breathing honeycomb photonic crystal as an example. First, we employ topological quantum chemistry and Wilson loop calculations to demonstrate that the topological properties of the bulk crystal stem from an obstructed atomic limit phase. We then characterize the topological corner modes emerging within the gapped edge modes employing a semi-analytical model, determining the appropriate real space topological invariants. For the first time, we provide a detailed account of the effect of long-range interactions on the topological modes in photonic crystals, and we quantify their robustness to perturbations. We conclude that, while photonic long-range interactions inevitably break chiral symmetry, the corner modes are protected by lattice symmetries.

cond-mat.mes-hall↗

arXiv2020

Volkov-Pankratov states in topological graphene nanoribbons

In topological systems, a modulation in the gap onset near interfaces can lead to the appearance of massive edge states, as were first described by Volkov and Pankratov. In this work, we study graphene nanoribbons in the presence of intrinsic spin-orbit coupling smoothly modulated near the system edges. We show that this space modulation leads to the appearance of Volkov-Pankratov states, in addition to the topologically protected ones. We obtain this result by means of two complementary methods, one based on the effective low-energy Dirac equation description and the other on a fully numerical tight-binding approach, finding excellent agreement between the two. We then show how transport measurements might reveal the presence of Volkov-Pankratov states, and discuss possible graphene-like structures in which such states might be observed.

cond-mat.mes-hall↗

arXiv2019

Tutorial: Computing topological invariants in two-dimensional photonic crystals

The field of topological photonics emerged as one of the most promising areas for applications in transformative technologies: possible applications are in topological lasers or quantum optics interfaces. Nevertheless, efficient and simple methods for diagnosing the topology of optical systems remain elusive for an important part of the community. In this tutorial, we provide a summary of numerical methods to calculate topological invariants emerging from the propagation of light in photonic crystals. We first describe the fundamental properties of wave propagation in lattices with a space-dependent periodic electric permittivity. Next, we provide an introduction to topological invariants; proposing an optimal strategy to calculate them through the numerical evaluation of Maxwell's equation in a discretized reciprocal space. Finally, we will complement the tutorial with a few practical examples of photonic crystal systems showing different topological properties, such as photonic valley-Chern insulators, photonic crystals presenting an "obstructed atomic limit", photonic systems supporting fragile topology and finally photonic Chern insulators, where we also periodically modulated the magnetic permeability.

cond-mat.mes-hall↗

arXiv2019

Living on the edge: Topology, electrostatics and disorder

We address the co-existence of massless and massive topological edge states at the interface between two materials with different topological phases. We modify the well known Bernevig-Hughes-Zhang model to introduce a smooth function describing the band inversion and the band bending due to electrostatic effects between the bulk of the quantum well and the vacuum. Within this minimal model we identify distinct parameter sets that can lead to the co-existence of the two types of edge states, and that determine their number and characteristics. We propose several experimental setups that could demonstrate their presence in two-dimensional topological systems, as well as ways to regulate or tune the contribution of the massive edge states to the conductance of associated electronic devices. Our results suggest that such states may also be present in novel two-dimensional Van der Waals topological materials.

cond-mat.mes-hall↗

arXiv2019

Engineering Fragile Topology in Photonic Crystals: Topological Quantum Chemistry of Light

In recent years, there have been rapid advances in the parallel fields of electronic and photonic topological crystals. Topological photonic crystals in particular show promise for coherent transport of light and quantum information at macroscopic scales. In this work, we apply for the first time the recently developed theory of "Topological quantum chemistry" to the study of band structures in photonic crystals. This method allows us to design and diagnose topological photonic band structures using only group theory and linear algebra. As an example, we focus on a family of crystals formed by elliptical rods in a triangular lattice. We show that the symmetry of Bloch states in the Brillouin zone can determine the position of the localized photonic wave packets describing groups of bands. By modifying the crystal structure and inverting bands, we show how the centers of these wave packets can be moved between different positions in the unit cell. Finally, we show that for shapes of dielectric rods, there exist isolated topological bands which do not admit a well-localized description, representing the first physical instance of "fragile" topology in a truly noninteracting system. Our work demonstrates how photonic crystals are the natural platform for the future experimental investigation of fragile topological bands.

cond-mat.mes-hall↗