arXiv subjects

Leonor Chico

Publications and source records attributed to Leonor Chico.

At least 19 recordsLinked to original sources

arXiv2026

Symmetry selection rule for the band-edge shift current in two dimensions

The shift current is the intrinsic bulk photovoltaic response of a crystal without an inversion center. In graphene multilayers, calculations report large band-edge shift currents that reverse sign under a gate, a behavior that neither the quantum metric nor the Berry curvature captures. We show that this follows from a symmetry principle. At the absorption edge of a gapped, inversion-broken two-dimensional Dirac-like system, an emergent rotational symmetry forbids the response, and the trigonal warping switches it on linearly in its strength, an angular selection rule that unifies multilayer graphene and the kagome lattice. The released current is governed by a signed, detuning-weighted three-point Bargmann invariant: the optical amplitude stays fixed while the sign alignment of the Bargmann triangles grows with the warping, invisible to any positive-definite figure of merit. In bilayer and trilayer graphene the gate alone reverses the sign, making the band-edge shift current a gate-switchable bulk photovoltaic response.

cond-mat.mes-hall↗

arXiv2025

Sliding-induced topological transitions in bilayer biphenylene

Sliding-induced topological transitions in biphenylene bilayers are investigated, considering various stacking configurations which are analyzed from a symmetry perspective and described in detail,highlighting the intricate patterns of type-II Dirac cone crossings. Topological changes in the Fermi surface are assessed via the Euler characteristic, linking each transition to its corresponding symmetry, which can be experimentally tested by conductance measurements. Moreover, the ability to tune these topological properties by sliding the layers provides a simpler and more effective way to observe such phenomena.

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arXiv2025

Tuning magnetism in graphene nanoribbons via strain and adatoms

We investigate the impact of strain and adsorbed H adatoms on the magnetic properties of zigzag graphene nanoribbons (ZGNRs) using a combination of tight-binding and density functional theory methods for both, ferromagnetic (FM) and antiferromagnetic edge configurations (AFM). Our study reveals that longitudinal strain induces a significant enhancement in the edge magnetic moment, that we attribute to strain-driven modifications in the band structure. In addition, we describe H~adatoms within the tight-binding approach by employing both an unrelaxed vacancy model and the Anderson impurity model. By comparing to density functional theory results, we corroborate that the Anderson impurity model is best suited to describe H adsorption. We then focus on the metallic FM edge configuration of the ZGNRs to better exploit the tuning of its properties. We find that the magnetic configuration of H~adatoms is strongly influenced by the edges, with an AFM coupling between edges and the H~adatom. In fact, the magnetic spatial pattern of the H adatom differs to that found in graphene due to this edge coupling. Importantly, we find robust discrete plateaus of integer magnetic moment as strain is varied in the defected ZGNRs, that we relate to changes in the band structure, namely, a half-metallic character or the opening of a gap. This behavior can be of interest for magnetic applications of carbon-based nanostructures

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arXiv2024

Two-dimensional orbital-obstructed insulators with higher-order band topology

Obstructed atomic phases, with their realizations in systems of diverse dimensionality, have recently arisen as one of the topological states with greatest potential to show higher-order phenomena. In this work we report a special type of obstruction, known as orbital-mediated atomic obstruction, in monolayers of materials with spatial symmetry described by the space group $P$-$3m1$. By means of a minimal tight-binding model and first-principles calculations, we show that this obstructed phase is related to the mismatch of the charge centers coming from the atomic limit with respect to the centers that are obtained from a reciprocal space description. Although we find atomic limits that correspond with occupied atomic sites, orbital-mediated atomic obstruction requires the presence of orbitals that have no support in real space. In order to demonstrate the nontrivial character of the obstruction, we confirm the presence of a filling anomaly for finite geometries that is directly associated with the bulk configuration, and discuss the role of the boundary states and their underlying mechanism. Several material examples are presented to illustrate the ubiquity of these nontrivial responses and, in turn, to discuss the differences related to the particular ground state configuration. In addition, we perform a survey of materials and elaborate a list of candidate systems which will host this obstructed phase in monolayer form.

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arXiv2024

Robustness of type-II Dirac cones in biphenylene-based structures

The electronic properties of one- and two-dimensional biphenylene-based systems, such as nanoribbons and bilayers, are studied within a unified approach. Besides the bilayer with direct (AA) stacking, we present two additional symmetric stackings for bilayer biphenylene that we denote by AB, by analogy with bilayer graphene, and AX, which can be derived by a small translation (slip) from the AA bilayer, with distinct band structures. We combine first-principles calculations with a tight-binding model to provide a realistic effective description of these structures. Our approach provides a global framework to characterize and analyze the robustness of the type-II Dirac cone within these structures, captures the variations caused by different stackings, and highlights important symmetries inherent in the biphenylene nanoribbon Dirac cones and edge states.

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arXiv2023

Fractional corner charges in threefold-symmetric two-dimensional materials with fragile topology

We perform a systematic study of the signatures of fragile topology in over 50 nonmagnetic two-dimensional materials with formula AB$_2$, belonging to space group $P$-$3m1$. Using group theory analysis in the framework of topological quantum chemistry, we find fragile bands near the Fermi level for all the materials studied. Since stable topological bands are also present in these systems, the interplay of both phases is discussed, showing that corner charges appear in over 80% of the materials and are linked to fragile topology. Using first-principles calculations, we predict fractionally-charged corner charges protected by $C_3$ symmetry. Our work aims to broaden the scope of materials with experimentally accessible fragile bands.

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arXiv2023

Polarization-tuneable excitonic spectral features in the optoelectronic response of atomically thin ReS2

The low crystal symmetry of rhenium disulphide (ReS2) leads to the emergence of dichroic optical and optoelectronic response, absent in other layered transition metal dichalcogenides, which could be exploited for device applications requiring polarization resolution. To date, spectroscopy studies on the optical response of ReS2 have relied almost exclusively in characterization techniques involving optical detection, such as photoluminescence, absorbance, or reflectance spectroscopy. However, to realize the full potential of this material, it is necessary to develop knowledge on its optoelectronic response with spectral resolution. In this work, we study the polarization-dependent photocurrent spectra of few-layer ReS2 photodetectors, both in room conditions and at cryogenic temperature. Our spectral measurements reveal two main exciton lines at energies matching those reported for optical spectroscopy measurements, as well as their excited states. Moreover, we also observe an additional exciton-like spectral feature with a photoresponse intensity comparable to the two main exciton lines. We attribute this feature, not observed in earlier photoluminescence measurements, to a non-radiative exciton transition. The intensities of the three main exciton features, as well as their excited states, modulate with linear polarization of light, each one acquiring maximal strength at a different polarization angle. We have performed first-principles exciton calculations employing the Bethe-Salpeter formalism, which corroborate our experimental findings. Our results bring new perspectives for the development of ReS2-based nanodevices.

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arXiv2023

Impact of vacancies on twisted bilayer graphene quantum point contacts

We carry out an extensive numerical study of low-temperature electronic transport in quantum point contacts based on twisted bilayer graphene. Assuming ballistic electron dynamics, quantized plateaus in the conductance are observed in defect-free samples when the twisting angle is large enough. However, plateaus are smeared out and hardly noticeable on decreasing the angle. Close to the magic angle, the conductance around the charge neutrality point drops significantly and the quantization steps visible at higher angles are no longer appreciable. Furthermore, we consider the effects of a random distribution of vacancies on the quantum point contact. Whereas the electron-hole symmetry is broken in pristine samples, we find that this symmetry is restored upon increasing the concentration of vacancies. We explain this effect by a reduction of the effective interlayer coupling due to the presence of the vacancies.

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arXiv2023

Topological bands in the PdSe$_2$ pentagonal monolayer

The electronic structure of monolayer pentagonal palladium diselenide (PdSe2) is analyzed from the topological band theory perspective. Employing first-principles calculations, effective models and symmetry indicators, we find that the low-lying conduction bands are topologically nontrivial, protected by time reversal and crystalline symmetries. Numerical evidence supporting the nontrivial character of the bands is presented. Furthermore, we obtain a relevant physical response from the topological viewpoint, such as the spin Hall conductivity.

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arXiv2023

Universality of moiré physics in collapsed chiral carbon nanotubes

We report the existence of moiré patterns and magic angle physics in all families of chiral collapsed carbon nanotubes. A detailed study of the electronic structure of all types of chiral nanotubes, previously collapsed via molecular dynamics, has been performed. We find that each family possesses a unique geometry and moiré disposition, as well as a characteristic number of flat bands. Remarkably, all kinds of nanotubes behave the same with respect to magic angle tuning, showing a monotonic behavior that gives rise to magic angles in full agreement with those of twisted bilayer graphene. Therefore, magic angle behavior is universally found in chiral collapsed nanotubes with a small chiral angle, giving rise to moiré patterns. Our approach comprises first-principles and semi-empirical calculations of the band structure, density of states and spatial distribution of the localized states signaled by flat bands.

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arXiv2022

Persistence of symmetry-protected Dirac points at the surface of the topological crystalline insulator SnTe upon impurity doping

We investigate the effect of a non-magnetic donor impurity located at the surface of the SnTe topological crystalline insulator. In particular, the changes on the surface states due to a Sb impurity atom are analyzed by means of ab initio simulations of pristine and impurity-doped SnTe. Both semi-infinite and slab geometries are considered within the first-principles approach. Furthermore, minimal and Green's function continuum models are proposed with the same goal. We find that the Dirac cones are shifted down in energy upon doping; this shift strongly depends on the position of the impurity with respect to the surface. In addition, we observe that the width of the impurity band presents an even-odd behavior by varying the position of the impurity. This behavior is related to the position of the nodes of the wave function with respect to the surface, and hence it is a manifestation of confinement effects. We compare slab and semi-infinite geometries within the ab initio approach, demonstrating that the surface states remain gapless and their spin textures are unaltered in the doped semi-infinite system. In the slab geometry, a gap opens due to hybridization of the states localized at opposite surfaces. Finally, by means of a continuum model, we extrapolate our results to arbitrary positions of the impurity, clearly showing a non-monotonic behavior of the Dirac cone.

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arXiv2022

Impact of electron-electron interactions on the thermoelectric efficiency of graphene quantum point contacts

Thermoelectric materials open a way to harness dissipated energy and make electronic devices less energy-demanding. Heat-to-electricity conversion requires materials with a strongly suppressed thermal conductivity but still high electronic conduction. This goal is largely achieved with the help of nanostructured materials, even if the bulk counterpart is not highly efficient. In this work, we investigate how thermoelectric efficiency is enhanced by many-body effects in graphene nanoribbons at low temperature. To this end, starting from the Kane-Mele-Hubbard model within a mean-field approximation, we carry out an extensive numerical study of the impact of electron-electron interactions on the thermoelectric efficiency of graphene nanoribbons with armchair or zigzag edges. We consider two different regimes, namely trivial and topological insulator. We find that electron-electron interactions are crucial for the appearance of interference phenomena that give rise to an enhancement of the thermoelectric efficiency of the nanoribbons. Lastly, we also propose an experimental setup that would help to test the validity of our conclusions.

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arXiv2021

Charge-spin interconversion in graphene-based systems from density functional theory

We present a methodology to address, from first principles, charge-spin interconversion in two-dimensional materials with spin-orbit coupling. Our study relies on an implementation of density functional theory based quantum transport formalism adapted to such purpose. We show how an analysis of the $k$-resolved spin polarization gives the necessary insight to understand the different charge-spin interconversion mechanisms. We have tested it in the simplest scenario of isolated graphene in a perpendicular electric field where effective tight-binding models are available to compare with. Our results show that the flow of an unpolarized current across a single layer of graphene produces, as expected, a spin separation perpendicular to the current for two of the three spin components (out-of-plane and longitudinal), which is the signature of the spin Hall effect. Additionally, it also yields an overall spin accumulation for the third spin component (perpendicular to the current), which is the signature of the Rashba-Edelstein effect. Even in this simple example, our results reveal an unexpected competition between the Rashba and the intrinsic spin-orbit coupling. Remarkably, the sign of the accumulated spin density does not depend on the electron or hole nature of the injected current for realistic values of the Rashba coupling.

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arXiv2021

Fast response photogating in monolayer MoS2 phototransistors

Two-dimensional transition metal dichalcogenide (TMD) phototransistors have been object of intensive research during the last years due to their potential for photodetection. Photoresponse in these devices is typically caused by a combination of two physical mechanisms: photoconductive effect (PCE) and photogating effect (PGE). In earlier literature for monolayer (1L) MoS2 phototransistors PGE is generally attributed to charge trapping by polar molecules adsorbed to the semiconductor channel, giving rise to a very slow photoresponse. Thus, the photoresponse of 1L-MoS2 phototransistors at high-frequency light modulation is assigned to PCE alone. Here we investigate the photoresponse of a fully h-BN encapsulated monolayer (1L) MoS2 phototransistor. In contrast with previous understanding, we identify a rapidly responding PGE mechanism that becomes the dominant contribution to photoresponse under high-frequency light modulation. Using a Hornbeck-Haynes model for the photocarrier dynamics, we fit the illumination power dependence of this PGE and estimate the energy level of the involved traps. The resulting energies are compatible with shallow traps in MoS2 caused by the presence of sulfur vacancies.

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arXiv2021

One-dimensional moiré superlattices and flat bands in collapsed chiral carbon nanotubes

We demonstrate that one-dimensional moiré patterns, analogous to those found in twisted bilayer graphene, can arise in collapsed chiral carbon nanotubes. Resorting to a combination of approaches, namely, molecular dynamics to obtain the relaxed geometries and tight-binding calculations validated against ab initio modeling, we find that magic angle physics occur in collapsed carbon nanotubes. Velocity reduction, flat bands and localization in AA regions with diminishing moiré angle are revealed, showing a magic angle close to 1$^{\rm o}$. From the spatial extension of the AA regions and the width of the flat bands, we estimate that many-body interactions in these systems are stronger than in twisted bilayer graphene. Chiral collapsed carbon nanotubes stand out as promising candidates to explore many-body effects and superconductivity in low dimensions, emerging as the one-dimensional analogues of twisted bilayer graphene.

cond-mat.mtrl-sci↗

arXiv2020

Two-dimensional Weyl points and nodal lines in pentagonal materials and their optical response

Two-dimensional pentagonal structures based on the Cairo tiling are the basis of a family of layered materials with appealing physical properties. In this work we present a theoretical study of the symmetry-based electronic and optical properties of these pentagonal materials. We provide a complete classification of the space groups that support pentagonal structures for binary and ternary systems. By means of first-principles calculations, their electronic band structures and the local spin textures in momentum space are analyzed. Our results show that pentagonal structures can be realized in chiral and achiral lattices with Weyl nodes pinned at high-symmetry points and nodal lines along the Brillouin zone boundary; these degeneracies are protected by the combined action of crystalline and time-reversal symmetries. Additionally, we discuss the linear and nonlinear optical features of some penta-materials, such as the shift current, which shows an enhancement due to the presence of nodal lines and points, and their possible applications.

cond-mat.mtrl-sci↗

arXiv2020

Spin-polarized currents in corrugated graphene nanoribbons

We investigate the production of spin-polarized currents in corrugated graphene nanoribbons. Such corrugations are modeled as multiple regions with Rashba spin-orbit interactions, where concave and convex curvatures are treated as Rashba regions with opposite signs. Numerical examples for different separated Rashba-zone geometries calculated within the tight-binding approximation are provided. Remarkably, the spin-polarized current in a system with several Rashba areas can be enhanced with respect to the case with a single Rashba part of the same total area. The enhancement is larger for configurations with multiple regions with the same Rashba sign. This indicates that the increase of the spin polarization is due to the scattering of the electrons traversing regions with and without Rashba interaction. Additionally, we relate the appearance of the spin-polarized currents to novel symmetry relations between the spin-dependent conductances. These symmetries turn out to be a combination of different symmetry operations in real and spin spaces, as those occurring in non-planar systems like carbon nanotubes. Our results show that two-dimensional devices with Rashba spin-orbit interaction can be used as excellent spintronic devices in an all-electrical or mechanical setup.

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arXiv2019

Flat bands and gaps in twisted double bilayer graphene

We present electronic structure calculations of twisted double bilayer graphene (TDBG): A tetralayer graphene structure composed of two AB-stacked graphene bilayers with a relative rotation angle between them. Using first-principles calculations, we find that TDBG is semiconducting with a band gap that depends on the twist angle, that can be tuned by an external electric field. The gap is consistent with TDBG symmetry and its magnitude is related to surface effects, driving electron transfer from outer to inner layers. The surface effect competes with an energy upshift of localized states at inner layers, giving rise to the peculiar angle dependence of the band gap, which reduces at low angles. For these low twist angles, the TDBG develops flat bands, in which electrons in the inner layers are localized at the AA regions, as in twisted bilayer graphene.

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