arXiv subjects

Francisco Guinea

Publications and source records attributed to Francisco Guinea.

At least 91 records · Page 5Linked to original sources

arXiv2019

Large-scale critical behavior of the rippling phase transition for graphene membranes

We analyze the spontaneous rippling of graphene membranes as function of the coupling between lattice deformations and electrons. We numerically study a model of an elastic membrane coupled to Dirac fermions. We identify a phase transition from a flat to a rippled configuration of the membrane when increasing the coupling and propose a scaling procedure that allows us to effectively reach arbitrary large system sizes. We find that the critical value of the coupling rapidly decays as the system increases its size, in agreement with the experimental observation of an unavoidable stable rippled state for suspended graphene membranes. This decay turns out to be controlled by a power law with a critical exponent $\sim 1/2$.

cond-mat.soft↗

arXiv2019

Twists and The Electronic Structure of Graphitic Materials

We analyze the effect of twists on the electronic structure of configurations of infinite stacks of graphene layers. We focus on three different cases: an infinite stack where each layer is rotated with respect to the previous one by a fixed angle, two pieces of semi-infinite graphite rotated with respect to each other, and finally a single layer of graphene rotated with respect to a graphite surface. In all three cases we find a rich structure, with sharp resonances and flat bands for small twist angles. The method used can be easily generalized to more complex arrangements and stacking sequences.

cond-mat.str-el↗

arXiv2019

Electronic band structure and pinning of Fermi energy to van Hove singularities in twisted bilayer graphene: a self consistent approach

The emergence of flat bands in twisted bilayer graphene leads to an enhancement of interaction effects, and thus to insulating and superconducting phases at low temperatures, even though the exact mechanism is still widely debated. The position and splitting of the flat bands is also very sensitive to the residual interactions. Moreover, the low energy bands of twisted graphene bilayers show a rich structure of singularities in the density of states, van Hove singularities, which can enhance further the role of interactions. We study the effect of the long-range interactions on the band structure and the van Hove singularities of the low energy bands of twisted graphene bilayers. Reasonable values of the long-range electrostatic interaction lead to a band dispersion with a significant dependence on the filling. The change of the shape and position of the bands with electronic filling implies that the van Hove singularities remain close to the Fermi energy for a broad range of fillings. This result can be described as an effective pinning of the Fermi energy at the singularity. The sensitivity of the band structure to screening by the environment may open new ways of manipulating the system.

cond-mat.str-el↗

arXiv2019

Direct and inverse spin Hall effect: Lorentz force and Zeeman energy

It is shown that magnetic forces as the Lorentz force, exerted on electric currents, and the force μDiv(B), exerted on electron spins at rest, account for both the transverse spin imbalance typical of spin Hall effect and the transverse charge imbalance associated with pure spin currents (inverse spin Hall effect). Considering that for stationary currents the laboratory reference frame and those for which the spin up and spin down carriers are at rest are inertial systems, one can easily find the forces exerted by the lattice on both spin sub-bands, as well as the force between sub-bands.

cond-mat.mes-hall↗

arXiv2019

The emergence of one-dimensional channels in marginal-angle twisted bilayer graphene

We generalize the continuum model for Moiré structures made from twisted graphene layers, in order to include lattice relaxation and the formation of channels at very small (marginal) twist angles. We show that a precise description of the electronic structure at such small angles can be achieved by i) calculating first the relaxed atomic structure, ii) projecting the interlayer electronic hopping parameters using a suitable basis of Bloch states, and iii) increasing the number of harmonics in the continuum approximation to interlayer hopping. The results show a complex structure of quasi one dimensional states when a finite bias is applied.

cond-mat.str-el↗

arXiv2019

Electron heating and mechanical properties of graphene

The heating of electrons in graphene by laser irradiation, and its effects on the lattice structure, are studied. Values for the temperature of the electron system in realistic situations are obtained. For sufficiently high electron temperatures, the occupancy of the states in the $σ$ band of graphene is modified. The strength of the carbon-carbon bonds changes, leading to the emergence of strains, and to buckling in suspended samples. While most applications of `strain engineering' in two dimensional materials focus on the effects of strains on electronic properties, the effect studied here leads to alterations of the structure induced by light. This novel optomechanical coupling can induce deflections in the order of $\sim 50$ nm in micron size samples.

cond-mat.mes-hall↗

arXiv2019

Continuum models for twisted bilayer graphene: the effects of lattice deformation and hopping parameter

We analyze a description of twisted graphene bilayers, that incorporates deformation of the layers due to the nature modern interlayer potentials, and a modification of the hopping parameters between layers in the light of the classic Slonczewski-Weiss-McClure parametrisation. We shall show that flat bands result in all cases, but that their nature can be rather different. We will show how to construct a more general reduction to a continuum model, and show that even though such a model can be constructed, its complexity increases, requiring more coupling parameters to be included, and the full in-layer dispersion to be taken into account. We conclude that the combination of all these effects will have a large impact on the wave functions of the flat bands, and that changes in the detail of the underlying models can lead to significant changes. A robust conclusion is that the natural strength of the interlayer couplings is higher than usually assumed, which causes additional Dirac points to appear for the standard magic angles. This gives rise to a degeneracy at the $Γ$ point. Since the appearance of a gap at the $Γ$ point is crucial for the construction of the Wannier states which are used in the standard descriptions of superconductivity, such an approach not be robust.

cond-mat.str-el↗

arXiv2019

Lattice Deformation, Low Energy Models and Flat Bands in Twisted Graphene Bilayers

Twisted graphene bilayers show a complex electronic structure, further modified by interaction effects. The main features can be obtained from effective models, which make use a few phenomenological parameters. We analyze the influence of effects at the atomic scale, such as interlayer hopping and lattice relaxation, on the electronic bands. We assume that the twist angle and the size of the Moiré pattern is fixed, as it is usually the case in experiments. We obtain a strong dependence of the electronic structure on details of the models at the atomic scale. We discuss how to incorporate this dependence on effective models

cond-mat.str-el↗

arXiv2019

Strained bubbles in van der Waals heterostructures as local emitters of photoluminescence with adjustable wavelength

The possibility to tailor photoluminescence (PL) of monolayer transition metal dichalcogenides (TMDCs) using external factors such as strain, doping and external environment is of significant interest for optoelectronic applications. Strain in particular can be exploited as a means to continuously vary the bandgap. Micrometer-scale strain gradients were proposed for creating 'artificial atoms' that can utilize the so-called exciton funneling effect and work, for example, as exciton condensers. Here we describe room-temperature PL emitters that naturally occur whenever monolayer TMDC is deposited on an atomically flat substrate. These are hydrocarbon-filled bubbles which provide predictable, localized PL from well-separated submicron areas. Their emission energy is determined by the built-in strain controlled only by the substrate material, such that both the maximum strain and the strain profile are universal for all bubbles on a given substrate, i.e., independent of the bubble size. We show that for bubbles formed by monolayer MoS2, PL can be tuned between 1.72 to 1.81 eV by choosing bulk PtSe2, WS2, MoS2 or graphite as a substrate and its intensity is strongly enhanced by the funneling effect. Strong substrate-dependent quenching of the PL in areas of good contact between MoS2 and the substrate ensures localization of the luminescence to bubbles only; by employing optical reflectivity measurements we identify the mechanisms responsible for the quenching. Given the variety of available monolayer TMDCs and atomically flat substrates and the ease of creating such bubbles, our findings open a venue for making and studying the discussed light-emitting 'artificial atoms' that could be used in applications.

cond-mat.mes-hall↗

arXiv2019

Hund nodal line semimetals: The case of twisted magnetic phase in the double-exchange model

We propose a class of topological metals, which we dub \emph{Hund nodal line semimetals}, arising from the strong Coulomb interaction encoded in the Hund's coupling between itinerant electrons and localized spins. We here consider a particular twisted spin configuration, which is realized in the double exchange model which describes the manganite oxides. The resulting effective tetragonal lattice of electrons with hoppings tied to the local spin features an antiunitary \emph{non-symmorphic} symmetry that in turn, together with another non-symmorphic but unitary, glide mirror symmetry, protects crossings of a double pair of bands along a high-symmetry line on the Brillouin zone boundary. We also discuss symmetry breaking arising from various perturbations of the twisted phase. Our results may motivate further studies of other realizations of this state of matter, for instance in different spin backgrounds, properties of its drumhead surface states, as well as its stability to disorder and interactions among the itinerant electrons.

cond-mat.mes-hall↗

arXiv2019

Signatures of surface Majorana modes in the magnetic response of topological superconductors

We study the magnetic susceptibility of a two-dimensional cone of Majorana modes localised at the surface of a three-dimensional time-reversal invariant topological superconductor belonging to class DIII. A field parallel to the surface tilts the surface Majorana cone along the supercurrent direction. For fields larger than a critical threshold field $H^*$, $H>H^*$, a transition from type I to type II Dirac cone occurs and a finite current carried by the Majorana modes start to flow, leading to an additional diamagnetic contribution to the surface magnetization. On a curved surface interband transitions are promoted by the Majorana spin connection that couples to the external field, giving rise to a finite frequency magnetic susceptibility.

cond-mat.supr-con↗

arXiv2018

Dimensional reduction, quantum Hall effect and layer parity in graphite films

The quantum Hall effect (QHE) originates from discrete Landau levels forming in a two-dimensional (2D) electron system in a magnetic field. In three dimensions (3D), the QHE is forbidden because the third dimension spreads Landau levels into multiple overlapping bands, destroying the quantisation. Here we report the QHE in graphite crystals that are up to hundreds of atomic layers thick - thickness at which graphite was believed to behave as a 3D bulk semimetal. We attribute the observation to a dimensional reduction of electron dynamics in high magnetic fields, such that the electron spectrum remains continuous only in the direction of the magnetic field, and only the last two quasi-one-dimensional (1D) Landau bands cross the Fermi level. In sufficiently thin graphite films, the formation of standing waves breaks these 1D bands into a discrete spectrum, giving rise to a multitude of quantum Hall plateaux. Despite a large number of layers, we observe a profound difference between films with even and odd numbers of graphene layers. For odd numbers, the absence of inversion symmetry causes valley polarisation of the standing-wave states within 1D Landau bands. This reduces QHE gaps, as compared to films of similar thicknesses but with even layer numbers because the latter retain the inversion symmetry characteristic of bilayer graphene. High-quality graphite films present a novel QHE system with a parity-controlled valley polarisation and intricate interplay between orbital, spin and valley states, and clear signatures of electron-electron interactions including the fractional QHE below 0.5 K.

cond-mat.mes-hall↗

arXiv2018

Electrostatic effects, band distortions and superconductivity in twisted graphene bilayers

Bilayer graphene twisted by a small angle shows a significant charge modulation away from neutrality, as the charge in the narrow bands near the Dirac point is mostly localized in the regions of the Moiré pattern with $AA$ stacking. The resulting electrostatic potential gives rise to the dominant contribution to the electron-electron interaction within this low energy band, which becomes significantly distorted. The changes in the band structure are described by new local interactions, and lead to an assisted electron-hopping process. These couplings favor superconductivity at certain fillings.

cond-mat.mes-hall↗

arXiv2018

Magnetic tilting and emergent Majorana spin connection in topological superconductors

Due to the charge neutral and localized nature of surface Majorana modes, detection schemes usually rely on local spectroscopy or interference through the Josephson effect. Here, we theoretically study the magnetic response of a two-dimensional cone of Majorana fermions localized at the surface of class DIII Topological Superconductors. For a field parallel to the surface the Zeeman term vanishes and the orbital term induces a Doppler shift of the Andreev levels resulting in a tilting of the surface Majorana cone. For fields larger than a critical threshold field $H^*$ the system undergoes a transition from type I to type II Dirac-Majorana cone. In a spherical geometry the surface curvature leads to the emergence of the Majorana spin connection in the tilting term via an interplay between orbital and Zeeman, that generates a finite non-trivial coupling between negative and positive energy states. Majorana modes are thus expected to show a finite response to the applied field, that acquires a universal character in finite geometries and opens the way to detection of Majorana modes via time-dependent magnetic fields.

cond-mat.supr-con↗

arXiv2018

Piezoelectricity and valley Chern number in inhomogeneous hexagonal 2D crystals

Conversion of mechanical forces to electric signal is possible in non-centrosymmetric materials due to linear piezoelectricity. The extraordinary mechanical properties of two-dimensional materials and their high crystallinity make them exceptional platforms to study and exploit the piezoelectric effect. Here, the piezoelectric response of non-centrosymmetric hexagonal two-dimensional crystals is studied using the modern theory of polarization and ${\bm k} \cdot {\bm p}$ model Hamiltonians. An analytical expression for the piezoelectric constant is obtained in terms of topological quantities such as the {\it valley Chern number}. The theory is applied to semiconducting transition metal dichalcogenides and hexagonal Boron Nitride. We find good agreement with available experimental measurements for MoS$_2$. We further generalise the theory to study the polarization of samples subjected to inhomogeneous strain (e.g.~nanobubbles). We obtain a simple expression in terms of the strain tensor, and show that charge densities $\gtrsim 10^{11} {\rm cm}^{-2}$ can be induced by realistic inhomogeneous strains, $ε\approx 0.01 - 0.03$.

cond-mat.mtrl-sci↗

arXiv2018

Suppressing backscattering of helical edge modes with a spin bath

In this paper, we address the question of stability of protected chiral modes (e.g., helical edge states at the boundary of two-dimensional topological insulators) upon interactions with the external bath. Namely, we study how backscattering amplitude changes when different interaction channels between the system and the environment are present. Depending on the relative strength of the Coulomb and spin-spin channels, we discover three different possible regimes. While the Coulomb interaction on its own naturally amplifies the backscattering and destroys the protection of chiral modes, and the spin-spin channel marginally suppresses backscattering, their interplay can make the backscattering process strictly irrelevant, opening the possibility to use the external spin bath as a stabilizer that alleviates destructive effects and restores the chirality protection.

cond-mat.mes-hall↗

arXiv2018

Topological $π$-junctions from crossed Andreev reflection in the Quantum Hall regime

We consider a two-dimensional electron gas (2DEG) in the Quantum Hall regime in the presence of a Zeeman field, with the Fermi level tuned to filling factor $ν=1$. We show that, in the presence of spin-orbit coupling, contacting the 2DEG to a narrow strip of an s-wave superconductor produces a topological superconducting gap along the contact as a result of crossed Andreev reflection (CAR) processes across the strip. The sign of the topological gap, controlled by the CAR amplitude, depends periodically on the Fermi wavelength and strip width and can be externally tuned. An interface between two halves of a long strip with topological gaps of opposite sign implements a robust $π$-junction, hosting a pair of Majorana zero modes that do not split despite their overlap. We show that such a situation can be exploited to perform protected non-Abelian tunnel-braid operations without any fine tuning.

cond-mat.mes-hall↗

arXiv2017

Edge modes and non local conductance in graphene superlattices

We study the existence of edge modes in gapped Moiré superlattices in graphene monolayer ribbons. We find that the superlattice bands acquire finite Chern numbers, which lead to a Valley Hall Effect. The presence of dispersive edge modes is confirmed by calculations of the band structure of realistic nanoribbons using tight binding methods. These edge states are only weakly sensitive to disorder, as short-range scattering processes lead to mean free paths of the order of microns. The results explain the existence of edge currents when the chemical potential lies within the bulk superlattice gap, and offer an explanation for existing non-local resistivity measurements in graphene ribbons on boron nitride.

cond-mat.mes-hall↗