arXiv subjects

Francisco Guinea

Publications and source records attributed to Francisco Guinea.

At least 145 records · Page 8Linked to original sources

arXiv2013

Artificial graphene as a tunable Dirac material

Artificial honeycomb lattices offer a tunable platform to study massless Dirac quasiparticles and their topological and correlated phases. Here we review recent progress in the design and fabrication of such synthetic structures focusing on nanopatterning of two-dimensional electron gases in semiconductors, molecule-by-molecule assembly by scanning probe methods, and optical trapping of ultracold atoms in crystals of light. We also discuss photonic crystals with Dirac cone dispersion and topologically protected edge states. We emphasize how the interplay between single-particle band structure engineering and cooperative effects leads to spectacular manifestations in tunneling and optical spectroscopies.

cond-mat.mes-hall↗

arXiv2013

Generation of pure bulk valley current in graphene

The generation of valley current is a fundamental goal in graphene valleytronics but no practical ways of its realization are known yet. We propose a workable scheme for the generation of bulk valley current in a graphene mechanical resonator through adiabatic cyclic deformations of the strains and chemical potential in the suspended region. The accompanied strain gauge fields can break the spatial mirror symmetry of the problem within each of the two inequivalent valleys, leading to a fnite valley current due to quantum pumping. An all-electrical measurement configuration is designed to detect the novel state with pure bulk valley currents.

cond-mat.mes-hall↗

arXiv2012

Electric field screening in atomically thin layers of MoS2: the role of interlayer coupling

The aim of this work is to study the electrostatic screening by single and few-layer MoS2 sheets by means of electrostatic force microscopy in combination with a non-linear Thomas-Fermi Theory to interpret the experimental results. We find that a continuum model of decoupled layers, which satisfactorily reproduces the electrostatic screening for graphene and graphite, cannot account for the experimental observations. A three-dimensional model with an interlayer hopping parameter can on the other hand successfully account for the observed electric field screening by MoS2 nanolayers, pointing out the important role of the interlayer coupling in the screening of MoS2.

cond-mat.mes-hall↗

arXiv2012

Coulomb drag in graphene - boron nitride heterostructures: the effect of virtual phonon exchange

For a system of two spatially separated monoatomic graphene layers encapsulated in hexagonal boron nitride, we consider the drag effect between charge carriers in the Fermi liquid regime. Commonly, the phenomenon is described in terms of an interlayer Coulomb interaction. We show that if an additional electron - electron interaction via exchange of virtual substrate phonons is included in the model, the predicted drag resistivity is modified considerably at temperatures above 150 K. The anisotropic crystal structure of boron nitride, with strong intralayer and comparatively weak interlayer bonds, is found to play an important role in this effect.

cond-mat.mes-hall↗

arXiv2012

Mid-infrared plasmons in scaled graphene nanostructures

Plasmonics takes advantage of the collective response of electrons to electromagnetic waves, enabling dramatic scaling of optical devices beyond the diffraction limit. Here, we demonstrate the mid-infrared (4 to 15 microns) plasmons in deeply scaled graphene nanostructures down to 50 nm, more than 100 times smaller than the on-resonance light wavelength in free space. We reveal, for the first time, the crucial damping channels of graphene plasmons via its intrinsic optical phonons and scattering from the edges. A plasmon lifetime of 20 femto-seconds and smaller is observed, when damping through the emission of an optical phonon is allowed. Furthermore, the surface polar phonons in SiO2 substrate underneath the graphene nanostructures lead to a significantly modified plasmon dispersion and damping, in contrast to a non-polar diamond-like-carbon (DLC) substrate. Much reduced damping is realized when the plasmon resonance frequencies are close to the polar phonon frequencies. Our study paves the way for applications of graphene in plasmonic waveguides, modulators and detectors in an unprecedentedly broad wavelength range from sub-terahertz to mid-infrared.

cond-mat.mes-hall↗

arXiv2012

Odd momentum pairing and superconductivity in vertical graphene heterostructures

Vertical graphene heterostructures made up of graphene layers separated by BN spacers allow for novel ways of tuning the interactions between electrons. We study the possibility of electron pairing mediated by modified repulsive interactions. Long range intravalley and short range intervalley interactions give rise to different anisotropic phases. We show that a superconducting state with gaps of opposite signs in different valleys, an odd momentum pairing state, can exist above carrier densities of $5-10 \times 10^{13} {\rm cm}^{-2}$. The dependence of the transition temperature on the different parameters of the devices is studied in detail.

cond-mat.supr-con↗

arXiv2012

Non-Abelian gauge potentials in graphene bilayers

We study the effect of spatial modulations in the interlayer hopping of graphene bilayers, such as those that arise upon shearing or twisting. We show that their single-particle physics, characterized by charge accumulation and recurrent formation of zero-energy bands as the pattern period L increases, is governed by a non-Abelian gauge potential arising in the low-energy electronic theory due to the coupling between layers. We show that such gauge-type couplings give rise to a potential that, for certain discrete values of L, spatially confines states at zero energy in particular regions of the Moiré patterns. We also draw the connection between the recurrence of the flat zero-energy bands and the non-Abelian character of the potential.

cond-mat.mes-hall↗

arXiv2012

Electron-hole puddles in the absence of charged impurities

It is widely believed that carrier-density inhomogeneities ("electron-hole puddles") in single-layer graphene on a substrate like quartz are due to charged impurities located close to the graphene sheet. Here we demonstrate by using a Kohn-Sham-Dirac density-functional scheme that corrugations in a real sample are sufficient to determine electron-hole puddles on length scales that are larger than the spatial resolution of state-of-the-art scanning tunneling microscopy.

cond-mat.mes-hall↗

arXiv2012

Temperature dependence of the conductivity of graphene on boron nitride

The substrate material of monolayer graphene influences the charge carrier mobility by various mechanisms. At room temperature, the scattering of conduction electrons by phonon modes localized at the substrate surface can severely limit the charge carrier mobility. We here show that for substrates made of the piezoelectric hexagonal boron nitride (hBN), in comparison to the widely used SiO$_2$, this mechanism of remote phonon scattering is --at room temperature-- weaker by almost an order of magnitude, and causes a resistivity of approximately 3\,$Ω$. This makes hBN an excellent candidate material for future graphene based electronic devices operating at room temperature.

cond-mat.mtrl-sci↗

arXiv2012

Electron pumping in graphene mechanical resonators

The combination of high frequency vibrations and metallic transport in graphene makes it a unique material for nano-electromechanical devices. In this letter, we show that graphene-based nano-electromechanical devices are extremely well suited for charge pumping, due to the sensitivity of its transport coefficients to perturbations in electrostatic potential and mechanical deformations, with the potential for novel small scale devices with useful applications.

cond-mat.mes-hall↗

arXiv2011

Geometrical and topological aspects of graphene and related materials

Graphene, a two-dimensional crystal made of carbon atoms, provides a new and unexpected bridge between low and high-energy physics. The field has evolved very fast and very good reviews are already available in the literature. Graphene constitutes a condensed matter realization of lower dimensional quantum field theory models that were proposed to confront important -- still unresolved -- puzzles of the area: Chiral symmetry breaking and quark confinement. The new materials named topological insulators, closely related to graphene, are physical realizations of topological field theory. This article reviews some of these topics with the aim of bridging the gap and making these condensed matter issues accessible to high energy readers. The electronic interactions in the monolayer are analyzed with special emphasis on the recent experimental confirmation of some theoretical predictions. The issue of spontaneous chiral symmetry breaking in the model materials is also reviewed. Finally we give an extensive description of some recent topological aspects of graphene that allow to understand the main aspects of topological insulators.

cond-mat.mes-hall↗

arXiv2011

Quenching of the quantum Hall effect in graphene with scrolled edges

Edge nanoscrolls are shown to strongly influence transport properties of suspended graphene in the quantum Hall regime. The relatively long arc length of the scrolls in combination with their compact transverse size results in formation of many nonchiral transport channels in the scrolls. They short-circuit the bulk current paths and inhibit the observation of the quantized two-terminal resistance. Unlike competing theoretical proposals, this mechanism of disrupting the Hall quantization in suspended graphene is not caused by ill-chosen placement of the contacts, singular elastic strains, or a small sample size.

cond-mat.mes-hall↗

arXiv2011

Effect of Coulomb interactions on the physical observables of graphene

We give an update of the situation concerning the effect of electron-electron interactions on the physics of a neutral graphene system at low energies. We revise old renormalization group results and the use of 1/N expansion to address questions of the possible opening of a low-energy gap, and the magnitude of the graphene fine structure constant. We emphasize the role of Fermi velocity as the only free parameter determining the transport and electronic properties of the graphene system and revise its renormalization by Coulomb interactions in the light of recent experimental evidence.

cond-mat.mes-hall↗

arXiv2011

Skipping and snake orbits of electrons: singularities and catastrophes

Near the sample edge, or a sharp magnetic field step the drift of two-dimensional electrons in a magnetic field has the form of skipping/snake orbits. We show that families of skipping/snake orbits of electrons injected at one point inside a 2D metal generically exhibit caustics folds, cusps and cusp triplets, and, in one extreme case, a section of the batterfly bifurcation. Periodic appearance of singularities along the $\pm B$-interface leads to the magneto-oscillations of nonlocal conductance in multi-terminal electronic devices.

cond-mat.mes-hall↗

arXiv2010

Gauge fields and interferometry in folded graphene

Folded graphene flakes are a natural byproduct of the micromechanical exfoliation process. In this Letter we show by a combination of analytical and numerical methods that such systems behave as intriguing interferometers due to the interplay between an externally applied magnetic field and the gauge field induced by the deformations in the region of the fold.

cond-mat.mes-hall↗

arXiv2010

Singular elastic strains and magnetoconductance of suspended graphene

Graphene membranes suspended off electric contacts or other rigid supports are prone to elastic strain, which is concentrated at the edges and corners of the samples. Such a strain leads to an algebraically varying effective magnetic field that can reach a few Tesla in sub-micron wide flakes. In the quantum Hall regime the interplay of the effective and the physical magnetic fields causes backscattering of the chiral edge channels, which can destroy the quantized conductance plateaus.

cond-mat.mes-hall↗

arXiv2010

Low-energy instability of flexural phonons in graphene

We study the effect exerted by the electrons on the flexural phonons in graphene, accounting for the attractive interaction created by the exchange of electron-hole excitations. Combining the self-consistent computation of the phonon self-energy with renormalization group methods, we show that graphene has two different phases corresponding to soft and strong renormalization of the bending rigidity in the long-wavelength limit. In the first case, the system may have an intermediate scale in which the phonon dispersion is softened, but it manages finally to become increasingly rigid over large distance scales. The strongly renormalized phase is closer however to critical behavior, with an effective rigidity that becomes pinned in practice at very small values, implying a very large susceptibility for the development of a condensate of the flexural phonon field.

cond-mat.mes-hall↗

arXiv2008

Stacking Faults, Bound States, and Quantum Hall Plateaus in Crystalline Graphite

We analyze the electronic properties of a simple stacking defect in Bernal graphite. We show that a bound state forms, which disperses as $|\bfk-\bfK|^3$ in the vicinity of either of the two inequivalent zone corners $\bfK$. In the presence of a strong c-axis magnetic field, this bound state develops a Landau level structure which for low energies behaves as $E\nd_n\propto |n B|^{3/2}$. We show that buried stacking faults have observable consequences for surface spectroscopy, and we discuss the implications for the three-dimensional quantum Hall effect (3DQHE). We also analyze the Landau level structure and chiral surface states of rhombohedral graphite, and show that, when doped, it should exhibit multiple 3DQHE plateaus at modest fields.

cond-mat.mes-hall↗