arXiv subjects

Federico Escudero

Publications and source records attributed to Federico Escudero.

At least 19 recordsLinked to original sources

arXiv2026

Twist and strain identification in moir\'e heterostructures

The geometrical and electronic properties of moir\'e materials are highly sensitive to the twist and strain in the samples due to the moir\'e magnifying effect. Accurate identification of twist and strain in moir\'e materials is therefore essential. In this work, we establish a general framework to extract the twist and strain configurations from moir\'e images with either atomic or moir\'e scale resolution. With only moir\'e-wavelength information, we show that there is a continuous family of possible twist and strain configurations, each one accounting for different orientations of the moir\'e pattern. To estimate the most likely twist-strain configuration, we discuss additional constraints and methods involving the minimum elastic energy and the electronic spectra. The minimum elastic energy, in particular, reflects that shear configurations become much more favorable as the strain increases. As an example of the developed methodology, we discuss the formation and identification of strained triangular moir\'e patterns. Our framework provides a comprehensive approach to identify the twist and strain configurations in systems with moir\'e-scale resolution.

cond-mat.mtrl-sci↗

arXiv2026

Tunable topological narrow bands in twisted bilayer-trilayer graphene

We investigate the low-energy band structure and topology of twisted bilayer--trilayer graphene with four stacking configurations: AB--ABC, BA--ABC, AB--ABA, and BA--ABA. Using both tight-binding and continuum models, we first establish that the two approaches show good agreement in the band structure in low-energy regime. We then study the evolution of the flat bands and their valley Chern numbers as functions of twist angle, perpendicular electric field, and the self-consistent Hartree potential. At relatively large twist angles and under electric field, we find a topological transition between the narrow bands, with the total Chern number of the flat bands following the Chern number sum rules derived from the chiral-limit description. We also observe another type of topological transition when the flat bands hybridize with adjacent remote bands, where gap closing and reopening processes lead to Chern number and charge density transfer. By constructing topological phase diagrams in the space of twist angle and electric field, we show that the perpendicular electric field provides an efficient tuning knob for controlling the stability and transitions of the Chern bands. Finally, we find that the Hartree potential mainly induce weak band shifts and reshaping in the narrow bands. However, with a combination of Hartree potential and the electric fields, the narrow bands show rich topological phase diagram. Our results clarify the interplay between the stacking, twist angle and electric field in manipulating the narrow bands and their topology in twisted bilayer--trilayer graphene, and provide guidance for engineering topological narrow bands with tunable Chern numbers in realistic twisted multilayer graphene systems.

cond-mat.mes-hall↗

arXiv2026

Entanglement (1+2) QED in a double layer of Dirac Materials

We investigate the momentum-space entanglement between two Dirac quasiparticles in a double-layer honeycomb lattice coupled via a planar electromagnetic cavity. We model the low-energy excitations as massive Dirac fermions in $(1+2)$ dimensions and derive the Bethe-Salpeter equation using the ladder approximation. We use a Born-level approximation around a free two-body quasiparticle state, where the interaction is mediated by the cavity photon propagator. From the reduced sublattice density matrix, we compute a momentum-resolved von Neumann entropy. Within the perturbatively controlled regime, the entropy remains small, while phenomenological self-energy dressing drives a crossover to strong enhancement of the entanglement entropy. Stationary entanglement is obtained only when the quasiparticle coherence time exceeds the photon propagation time between the layers. The maximum-entropy regime appears to be a viable method for achieving Bell-like states. These results demonstrate how self-energy renormalization, virtual particle exchange, and spinor geometry combine to reshape the entanglement landscape of Dirac materials.

quant-ph↗

arXiv2026

Straintronics and twistronics in bilayer graphene

The interplay of twist and strain in bilayer graphene enables the formation of moir\'e patterns and narrow bands that host correlated and topological phases. While magic-angle twisted bilayer graphene has been widely studied, strain provides an additional and realistic control knob for band engineering. In this work, we first generate a global method to construct commensurate supercells for arbitrary twist and heterostrain. Then, using atomistic tight-binding and strain-extended continuum models to study the commensurate structures, we identify configurations that minimize the bandwidth beyond the magic angle. The results reveal a strong dependence of band narrowing and topology on strain type, magnitude, direction and lattice relaxation. Particularly, shear strain produces a stronger distortion than uniaxial strain. Including electron-electron interactions through a self-consistent Hartree potential shows that strain broadens the bare bands while reducing electrostatic renormalization. Strain also drives topological transitions as the narrow and remote bands hybridize, establishing twisted and strained bilayer graphene as a tunable platform for flat-band and topological phenomena.

cond-mat.mes-hall↗

arXiv2026

Moir\'e-driven equilibrium of perturbations in moir\'e systems

Perturbations in moir\'e materials, such as due to substrates or strain, are common in many experiments and can significantly modify the electronic properties of the system. Here, we show that perturbations in twisted bilayer graphene tend to be transferred between the coupled Dirac cones, eventually reaching an equilibrium near the magic angle. We connect our results to experiments and show that this equilibrium behavior remains robust even when the moir\'e potential itself is perturbed. Our findings extend the notion of the magic angle to a more general regime governed by moir\'e-driven equilibrium.

cond-mat.mes-hall↗

arXiv2025

Geometrical properties of strained and twisted moir\'e heterostructures

The experimental observations of many interaction-driven electronic phases in moir\'e superlattices have stimulated intense theoretical and experimental efforts to understand and engineer these correlated physics. Strain is a powerful tool for manipulating and controlling the geometrical and electronic structures of moir\'e superlattices. This review provides a comprehensive introduction to the geometry of strained moir\'e superlattices. First, starting from the linear elasticity theory, we briefly introduce the general formalism of small deformations in two-dimensional materials, and discuss the particular cases of uniaxial, shear and biaxial strain. Then, we apply the theory to twisted and strained moir\'e materials, mainly focusing on the hexagonal homobilayers, hexagonal heterobilayers and monoclinic lattices. Special moir\'e geometries, like the quasi-unidimensional patterns, square patterns and hexagonal, are theoretically predicted by manipulating the strain and twist. Finally, we review recently developed strain techniques and the special moir\'e geometries realized via these approaches. This review aims at equipping the reader with a robust understanding on the description and implementation of strain in moir\'e materials, as well as highlight some major breakthroughs in this active field.

cond-mat.mes-hall↗

arXiv2025

Review of the tight-binding method applicable to the properties of moir\'e superlattices

Moir\'e superlattices have emerged as a versatile platform for exploring a wide range of ex- otic quantum phenomena. Unlike angstrom-scale materials, the moir\'e length-scale system contains a large number of atoms, and its electronic structure is significantly modulated by the lattice relaxation. These features pose a huge theoretical challenge. Among the available theoretical approaches, tight-binding (TB) methods are widely employed to predict the electronic, transport, and optical properties of systems such as twisted graphene, twisted transition-metal dichalcogenides (TMDs), and related moir\'e materials. In this review, we pro- vide a comprehensive overview of atomistic TB Hamiltonians and the numerical techniques commonly used to model graphene-based, TMD-based and hBN-based moir\'e superlattices. We also discuss the connection between atomistic TB descriptions and effective low-energy continuum models. Two examples of different moir\'e materials and geometries are provided to emphasize the advantages of the TB methods. This review is intended to serve as a theoretical and practical guide for those seeking to apply TB methods to the study of various properties of moir\'e superlattices.

cond-mat.mtrl-sci↗

arXiv2025

Twistraintronics in Square Moire Superlattices of Stacked Graphene Layers

We report the first observation of controlled, strain-induced square moire patterns in stacked graphene. By selectively displacing native wrinkles, we drive a reversible transition from the usual trigonal to square moire order. Scanning tunneling microscopy reveals elliptically shaped AA domains, while spectroscopy shows strong electronic correlation in the form of narrow bands with split Van Hove singularities near the Fermi level. A continuum model with electrostatic interactions reproduces these features under the specific twist-strain combination that minimizes elastic energy. This work demonstrates that the combination of twist and strain, or twistraintronics, enables the realization of highly correlated electronic states in moire heterostructures with geometries that were previously inaccessible.

cond-mat.mes-hall↗

arXiv2024

Entanglement harvesting in buckled honeycomb lattices by vacuum fluctuations in a microcavity

We study the entanglement harvesting between two identical buckled honeycomb lattices placed inside a planar microcavity. By applying time dependent perturbation theory, we obtain quantum correlations between both layers induced by the cavity field. Considering the vacuum state as the initial state of the cavity field and tracing out the time-evolved degrees of freedom, we analyze the entanglement formation using the concurrence measure. We show that the concurrence depends on the virtual photon exchanged and the positions of the layer through the interlayer photon propagator. Furthermore, we find that the formation of entanglement between equal energy electrons tends to be enhanced when they move in perpendicular directions. Our results indicate that a buckled honeycomb structure and a large spin-orbit interaction favor the entanglement harvesting.

cond-mat.mes-hall↗

arXiv2024

Diagrammatic perturbation approach to moiré bands in twisted bilayer graphene

We develop a diagrammatic perturbation theory to account for the emergence of moiré bands in the continuum model of twisted bilayer graphene. Our framework is build upon treating the moiré potential as a perturbation that transfers electrons from one layer to another through the exchange of the three wave vectors that define the moiré Brillouin zone. By working in the two-band basis of each monolayer, we analyze the one-particle Green's function and introduce a diagrammatic representation for the scattering processes. We then identify the moiré-induced self-energy, relate it to the quasiparticle weight and velocity of the moiré bands, and show how it can be obtained by summing irreducible diagrams. We also connect the emergence of flat bands to the behavior of the static self-energy at the magic angle. In particular, we show that a vanishing Dirac velocity is a direct consequence of the relative orientation of the momentum transfer vectors, suggesting that the origin of magic angles in twisted bilayer graphene is intrinsically connected to its geometrical properties. Our approach provides a diagrammatic framework that highlights the physical properties of the moiré bands.

cond-mat.mes-hall↗

arXiv2024

Designing Moiré Patterns by Strain

Experiments conducted on two-dimensional twisted materials have revealed a plethora of moiré patterns with different forms and shapes. The formation of these patterns is usually attributed to the presence of small strains in the samples, which typically arise during their fabrication. In this work we find that the superlattice structure of such systems actually depends crucially on the interplay between twist and strain. For systems composed of honeycomb lattices, we show that this can lead to the formation of practically any moiré geometry, even if each lattice is only slightly distorted. As a result, we show that under strain the moiré Brillouin zone is not a stretched irregular hexagon, but rather a primitive cell that changes according to the geometry of the strained moiré vectors. We identify the conditions for the formation of hexagonal moiré patterns arising solely due to shear or biaxial strain, thus opening the possibility of engineering moiré patterns solely by strain. Moreover, we study the electronic properties in such moiré patterns and find that the strain tends to suppress the formation of the flat moiré bands, even in the strain-induced hexagonal patterns analogous to those obtained by the twist only. Our work explains the plethora of moiré patterns observed in experiments, and provides a solid theoretical foundation from which one can design moiré patterns by strain.

cond-mat.mes-hall↗

arXiv2023

Coulomb drag in metallic twisted bilayer graphene

Strongly correlated phases in twisted bilayer graphene (TBG) typically arise as transitions from a state in which the system behaves as a normal metal. In such metallic regime, electron-electron interactions usually only play a subleading role in transport measurements, compared to the dominant scattering mechanism. Here, we propose and theoretically study an exception to this based on a Coulomb drag setup between two metallic TBG, separated so that they only couple through many-body interactions. We find that by solely varying the twist angle equally in both TBG, the drag resistivity exhibits a unique maximum as the system crossovers from a degenerate to a nondegenerate regime. When the twist angles in each TBG differ, we find an anomalous drag resistivity characterized by the appearance of multiple peaks. We show that this behavior can be related to the dependence of the rectification function on the twist angle.

cond-mat.mes-hall↗

arXiv2022

Coulomb drag between two graphene layers at different temperatures

We theoretically study the Coulomb drag in graphene when there is a temperature difference between the layers. Within the degenerate limit for equal layer densities, we find that this can lead to significant deviations from the usual quadratic temperature dependence of the drag resistivity. The exact behavior depends strongly on the phase space available for intraband scattering, and is not symmetrical when the temperatures of the layers are interchanged. In particular, when one layer is at a much higher temperature $T$ than the other, the drag resistivity behaves as $ρ_D\sim T/d^5$, where $d$ is the interlayer separation. The magnitude of the drag in this limit is always larger when the active layer is at the higher temperature.

cond-mat.mes-hall↗

arXiv2021

A general formulation for the magnetic oscillations in two dimensional systems

We develop a general formalism for the magnetic oscillations (MO) in two dimensional (2D) systems. We consider general 2D Landau levels, which may depend on other variable or indices, besides the perpendicular magnetic field. In the ground state, we obtain expressions for the MO phase and amplitude. From this we use a Fourier expansion to write the MO, with the first term being a sawtooth oscillation. We also consider the effects of finite temperature, impurities or lattice imperfections, assuming a general broadening of the Landau levels. We develop two methods for describing these damping effects in the MO. One in terms of the occupancy of the Landau levels, the other in terms of reduction factors, which results in a generalization of the Lifshits-Kosevich (LK) formula. We show that the first approach is particularly useful at very low damping, when only the states close to the Fermi energy are excited. In contrast, the LK formula may be more convenient at higher damping, when only few terms are needed in its harmonic expansion. We compare different damping situations, showing how the MO are broadened in each case. The general formulation presented allows to relate the properties of the MO with those of the 2D systems.

cond-mat.mtrl-sci↗

arXiv2020

Influence of temperature on the magnetic oscillations in graphene with spin splitting: a new approach

We analyze the magnetic oscillations (MO) in pristine graphene, under a perpendicular magnetic field, taking into account the Zeeman effect. We consider a constant Fermi energy, such that the valence band is always full and only the conduction band is available. At zero temperature the MO consist of two sawtooth peaks, one for each spin. Both peaks have the same frequency, but different amplitude and phase. We show that, in order to observe the spin splitting in the MO, Fermi energy of about 0.1 eV is required. At low temperatures we obtain that the MO can be expressed as the MO at zero temperature, plus small Fermi-Dirac like functions, each centered around the MO peaks. Using this expression, we show that the spin splitting is observable in the MO only when the thermal energy is smaller than the Zeeman energy. We also analyze the shift of the MO extrema as the temperature increases. We show that it depends on the magnetic field, which implies a broken periodicity at nonzero temperature. Finally, we obtain an analytical expression for the MO envelope.

cond-mat.mtrl-sci↗

arXiv2020

Temperature effect on the magnetic oscillations in 2D materials

We study the magnetic oscillations (MO) in 2D materials with a buckled honeycomb lattice, considering a perpendicular electric and magnetic field. At zero temperature the MO consist of the sum of four sawtooth oscillations, with two unique frequencies and phases. The values of these frequencies depend on the Fermi energy and electric field, which in turn determine the condition for a beating phenomenon in the MO. We analyse the temperature effect in the MO by considering its local corrections over each magnetization peak, given by Fermi-Dirac like functions. We show that the width of these functions is related to the minimum temperature necessary to observe the spin and valley properties in the MO. In particular, we find that in order to observe the spin splitting, the width must be lower than the MO phase difference. Likewise, in order to observe valley mixing effects, the width must be lower than the MO period. We also show that at high temperatures, all the maxima and minima in the MO are shift to a constant value, in which case we obtain a simple expression for the MO and its envelope. The results obtained show unique features in the MO in 2D materials, given by the interplay between the valley and spin.

cond-mat.mes-hall↗

arXiv2018

Magnetic oscillations in silicene

In this work the magnetic oscillations (MO) in pristine silicene at $T=0$ K are studied. Considering a constant electron density we obtain analytical expressions for the ground state internal energy and magnetization, under a perpendicular electric and magnetic field, taking in consideration the Zeeman effect. It is found that the MO are sawtooth-like, depending on the change in the last occupied energy level. This leads us to a classification of the MO peaks in terms of the Landau level (LL), valley or spin changes. Using this classification we analyze the MO for different values of the electric field $E_{z}$. When $E_{z}=0$, the energy levels have a valley degeneracy and the MO peaks occur only whenever the last energy level changes its LL and/or spin. When $E_{z}\neq0$, the valley degeneracy is broken and new MO peaks appear, associated with the valley change in the last energy level. By analyzing the MO peaks amplitude it is possible to extract information about the Fermi velocity and the spin-orbit interaction strength. Finally we analyze the MO frequencies, which can also be associated with the change of LL, valley or spin in the last energy level.

cond-mat.mtrl-sci↗

arXiv2017

Magnetization in pristine graphene with Zeeman splitting and variable spin-orbit coupling

The aim of this work is to describe the spin magnetization of graphene with Rashba spinorbit coupling and Zeeman effect. It is shown that the magnetization depends critically on the spin-orbit coupling l that is controlled with an external electric field. In turn, by manipulating the density of charge carriers, it is shown that spin up and down Landau levels mix introducing jumps in the spin magnetization. Two magnetic oscillations phases are described that can be tunable through the applied external fields. The maximum and minimum of the oscillations can be alternated by taking into account how the energy levels are filled when the Rashba-spin-orbit coupling is turned on. The results obtained are of importance to design superlattices with variable spin-orbit coupling with different configurations in which spin oscillations and spin filters can be developed.

cond-mat.str-el↗