arXiv subjects

Francisco Guinea

Publications and source records attributed to Francisco Guinea.

At least 127 records · Page 7Linked to original sources

arXiv2015

Strain engineering in semiconducting two-dimensional crystals

One of the fascinating properties of the new families of two-dimensional crystals is their high stretchability and the possibility to use external strain to manipulate, in a controlled manner, their optical and electronic properties. Strain engineering, understood as the field that study how the physical properties of materials can be tuned by controlling the elastic strain fields applied to it, has a perfect platform for its implementation in the atomically thin semiconducting materials. The object of this review is to give an overview of the recent progress to control the optical and electronics properties of 2D crystals, by means of strain engineering. We will concentrate on semiconducting layered materials, with especial emphasis in transition metal dichalcogenides (MoS$_2$, WS$_2$, MoSe$_2$ and WSe$_2$). The effect of strain in other atomically thin materials like black phosphorus, silicene, etc., is also considered. The benefits of strain engineering in 2D crystals for applications in nanoelectronics and optoelectronics will be revised, and the open problems in the field will be discussed.

cond-mat.mes-hall↗

arXiv2015

Magneto-electronic properties of multilayer black phosphorus

We examine the electronic properties of 2D electron gas in black phosphorus multilayers in the presence of a perpendicular magnetic field, highlighting the role of in-plane anisotropy on various experimental quantities such as ac magneto-conductivity, screening, and magneto-plasmons. We find that resonant structures in the ac conductivity exhibits a red-shift with increasing doping due to inter-band coupling, $γ$. This arises from an extra correction term in the Landau energy spectrum proportional to $n^2γ^2$ ($n$ is Landau index), up to second order in $γ$. We found also that Coulomb interaction leads to highly anisotropic magneto-excitons.

cond-mat.mes-hall↗

arXiv2015

Strain dependent elastic modulus of graphene

Indentation experiments on graphene membranes pre-stressed by hydrostatic pressure show an increase in effective elastic modulus from 300 N/m in non pressurized membranes to 700 N/m for pre-strains above 0.5 %. This pronounced dependence of the stiffness of graphene with strain is attributed to its high anharmonicity and the great influence of out of plane corrugations of this atomic thick membrane in its mechanical properties. Our experimental findings imply that graphene measured stiffness is highly influenced by the presence of corrugations and that the in plane elastic modulus corresponding to atomic bond stretching is more akin to 700 N/m, instead of the commonly accepted 340 N/m.

cond-mat.mtrl-sci↗

arXiv2014

Spin-valley relaxation and quantum transport regimes in two-dimensional transition metal dichalcogenides

Quantum transport and spintronics regimes are studied in p- and n-doped atomic layers of hexagonal transition metal dichalcogenides (TMDCs), subject to the interplay between the valley structure and spin-orbit coupling. We find how spin relaxation of carriers depends on their areal density and show that it vanishes for holes near the band edge, leading to the density-independent spin diffusion length, and we develop a theory of weak localisation/antilocalisation, describing the crossovers between the orthogonal, double-unitary and symplectic regimes of quantum transport in TMDCs.

cond-mat.mes-hall↗

arXiv2014

Stiffening graphene by controlled defect creation

Graphene extraordinary strength, stiffness and lightness have generated great expectations towards its application in flexible electronics and as mechanical reinforcement agent. However, the presence of lattice defects, unavoidable in sheets obtained by scalable routes, might degrade its mechanical properties. Here we report a systematic study on the elastic modulus and strength of graphene with controlled density of defects. Counter intuitively, the in-plane Young modulus increases with increasing defect density up to almost twice the initial value for vacancy content of ~0.2%, turning it into the stiffest material ever reported. For higher density of vacancies, elastic modulus decreases with defect inclusion. The initial increase in Young modulus is explained in terms of a dependence of the elastic coefficients with the momentum of flexural modes predicted for 2D membranes. In contrast, the fracture strength decreases with defect density according to standard fracture continuum models. These quantitative structure-property relationships, measured in atmospheric conditions, are of fundamental and technological relevance and provide guidance for applications in which graphene mechanics represents a disruptive improvement.

cond-mat.mtrl-sci↗

arXiv2014

Random strain fluctuations as dominant disorder source for high-quality on-substrate graphene devices

We have performed systematic investigations of transport through graphene on hexagonal boron nitride (hBN) substrates, together with confocal Raman measurements and a targeted theoretical analysis, to identify the dominant source of disorder in this system. Low-temperature transport measurements on many devices reveal a clear correlation between the carrier mobility $μ$ and the width $n^*$ of the resistance peak around charge neutrality, demonstrating that charge scattering and density inhomogeneities originate from the same microscopic mechanism. The study of weak-localization unambiguously shows that this mechanism is associated to a long-ranged disorder potential, and provides clear indications that random pseudo-magnetic fields due to strain are the dominant scattering source. Spatially resolved Raman spectroscopy measurements confirm the role of local strain fluctuations, since the line-width of the Raman 2D-peak --containing information of local strain fluctuations present in graphene-- correlates with the value of maximum observed mobility. The importance of strain is corroborated by a theoretical analysis of the relation between $μ$ and $n^*$ that shows how local strain fluctuations reproduce the experimental data at a quantitative level, with $n^*$ being determined by the scalar deformation potential and $μ$ by the random pseudo-magnetic field (consistently with the conclusion drawn from the analysis of weak-localization). Throughout our study, we compare the behavior of devices on hBN substrates to that of devices on SiO$_2$ and SrTiO$_3$, and find that all conclusions drawn for the case of hBN are compatible with the observations made on these other materials. These observations suggest that random strain fluctuations are the dominant source of disorder for high-quality graphene on many different substrates, and not only on hexagonal boron nitride.

cond-mat.mes-hall↗

arXiv2014

Substrate Sensitive Mid-Infrared Photoresponse in Graphene

We report mid-infrared photocurrent spectra of graphene nanoribbon arrays on SiO2 dielectrics showing dual signatures of the substrate interaction. First, hybrid polaritonic modes of graphene plasmons and dielectric surface polar phonons produce a thermal photocurrent in graphene with spectral features that are tunable by gate voltage, nanoribbon width, and light polarization. Secondly, phonon-polaritons associated with the substrate are excited, which indirectly heat up the graphene leading to a graphene photocurrent with fixed spectral features. Models for other commonly used substrates show that the responsivity of graphene infrared photodetectors can be tailored to specific mid-IR frequency bands by the choice of the substrate.

cond-mat.mtrl-sci↗

arXiv2014

Novel mid-infrared plasmonic properties of bilayer graphene

We study the mid-infrared plasmonic response in Bernal-stacked bilayer graphene. Unlike its monolayer counterpart, bilayer graphene accommodates optically active phonon modes and a resonant interband transition at infrared frequencies. They strongly modifies the plasmonic properties of bilayer graphene, leading to Fano-type resonances, giant plasmonic enhancement of infrared phonon absorption, narrow window of optical transparency, and a new plasmonic mode at higher energy than the classical plasmon.

cond-mat.mes-hall↗

arXiv2014

Generation and morphing of plasmons in graphene superlattices

Recent experimental studies on graphene on hexagonal Boron Nitride (hBN) have demonstrated that hBN is not only a passive substrate that ensures superb electronic properties of graphene's carriers, but that it actively modifies their massless Dirac fermion character through a periodic moiré potential. In this work we present a theory of the plasmon excitation spectrum of massless Dirac fermions in a moiré superlattice. We demonstrate that graphene-hBN stacks offer a rich platform for plasmonics in which control of plasmon modes can occur not only via electrostatic gating but also by adjusting e.g. the relative crystallographic

cond-mat.mes-hall↗

arXiv2014

Electronic structure of spontaneously strained graphene on hexagonal Boron Nitride

Hexagonal Boron Nitride substrates have been shown to dramatically improve the electric properties of graphene. Recently, it has been observed that when the two honeycomb crystals are close to perfect alignment, strong lattice distortions develop in graphene due to the moiré adhesion landscape. Simultaneously a gap opens at the Dirac point. Here we derive a simple low energy model for graphene carriers close to alignment with the substrate, taking into account spontaneous strains at equilibrium, pseudogauge fields and deformation potentials. We carry out a detailed characterisation of the modified band structure, gap, local and global density of states, and band topology in terms of physical parameters. We show that the overall electronic structure is strongly modified by the spontaneous strains.

cond-mat.mes-hall↗

arXiv2014

Spontaneous Strains and Gap in Graphene on Boron Nitride

The interaction between a graphene layer and a hexagonal Boron Nitride (hBN) substrate induces lateral displacements and strains in the graphene layer. The displacements lead to the appearance of commensurate regions and the existence of an average gap in the electronic spectrum of graphene. We present a simple, but realistic model, by which the displacements, strains and spectral gap can be derived analytically from the adhesion forces between hBN and graphene. When the lattice axes of graphene and the substrate are aligned, strains reach a value of order 2\%, leading to effective magnetic fields above 100T. The combination of strains and induced scalar potential gives a sizeable contribution to the electronic gap. Commensuration effects are negligible, due to the large stiffness of graphene.

cond-mat.mes-hall↗

arXiv2014

Plasmons and screening in monolayer and multilayer black phosphorus

Black phosphorus exhibits a high degree of band anisotropy. However, we found that its in-plane static screening remains relatively isotropic for momenta relevant to elastic long-range scattering processes. On the other hand, the collective electronic excitations in the system exhibit a strong anisotropy. Band non-parabolicity leads to a plasmon frequency which scales as $n^β$, where $n$ is the carrier concentration, and $β<\tfrac{1}{2}$. Screening and charge distribution in the out-of-plane direction are also studied using a non-linear Thomas-Fermi model.

cond-mat.mtrl-sci↗

arXiv2013

Collective excitations in a large-d model for graphene

We consider a model of Dirac fermions coupled to flexural phonons to describe a graphene sheet fluctuating in dimension $2+d$. We derive the self-consistent screening equations for the quantum problem, exact in the limit of large $d$. We first treat the membrane alone, and work out the quantum to classical, and harmonic to anharmonic crossover. For the coupled electron-membrane problem we calculate the dressed two-particle propagators of the elastic and electron interactions and find that it exhibits a collective mode which becomes unstable at some wave-vector $q_{\rm c}$ for large enough coupling $g$. The saddle point analysis, exact at large $d$, indicates that this instability corresponds to spontaneous and simultaneous appearance of gaussian curvature and electron puddles. The relevance to ripples in graphene is discussed.

cond-mat.mes-hall↗

arXiv2013

Local strain engineering in atomically thin MoS2

Tuning the electronic properties of a material by subjecting it to strain constitutes an important strategy to enhance the performance of semiconducting electronic devices. Using local strain, confinement potentials for excitons can be engineered, with exciting possibilities for trapping excitons for quantum optics and for efficient collection of solar energy. Two-dimensional materials are able to withstand large strains before rupture, offering a unique opportunity to introduce large local strains. Here, we study atomically thin MoS2 layers with large local strains of up to 2.5% induced by controlled delamination from a substrate. Using simultaneous scanning Raman and photoluminescence imaging, we spatially resolve a direct bandgap reduction of up to 90 meV induced by local strain. We observe a funnel effect in which excitons drift hundreds of nanometers to lower bandgap regions before recombining, demonstrating exciton confinement by local strain. The observations are supported by an atomistic tight-binding model developed to predict the effect of inhomogeneous strain on the local electronic states in MoS2. The possibility of generating large strain-induced variations in exciton trapping potentials opens the door for a variety of applications in atomically thin materials including photovoltaics, quantum optics and two-dimensional optoelectronic devices.

cond-mat.mes-hall↗

arXiv2013

Tunable phonon-induced transparency in bilayer graphene nanoribbons

In the phenomenon of electromagnetically induced transparency1 (EIT) of a three-level atomic system, the linear susceptibility at the dipole-allowed transition is canceled through destructive interference of the direct transition and an indirect transition pathway involving a meta-stable level, enabled by optical pumping. EIT not only leads to light transmission at otherwise opaque atomic transition frequencies, but also results in the slowing of light group velocity and enhanced optical nonlinearity. In this letter, we report an analogous behavior, denoted as phonon-induced transparency (PIT), in AB-stacked bilayer graphene nanoribbons. Here, light absorption due to the plasmon excitation is suppressed in a narrow window due to the coupling with the infrared active Γ-point optical phonon, whose function here is similar to that of the meta-stable level in EIT of atomic systems. We further show that PIT in bilayer graphene is actively tunable by electrostatic gating, and estimate a maximum slow light factor of around 500 at the phonon frequency of 1580 cm-1, based on the measured spectra. Our demonstration opens an avenue for the exploration of few-photon non-linear optics and slow light in this novel two-dimensional material, without external optical pumping and at room temperature.

cond-mat.mes-hall↗

arXiv2013

Flexural mode of graphene on a substrate

Out of plane vibrations are suppressed in graphene layers placed on a substrate. These modes, in suspended samples, are relevant for the understanding of properties such as the resistivity, the thermal expansion coefficient, and other. We present a general framework for the study of the properties of out of plane modes in graphene on different substrates. We use the model to estimate the substrate induced changes in the thermal expansion coefficient, or in the temperature dependence of the resistivity.

cond-mat.mes-hall↗

arXiv2013

Coupling light into graphene plasmons through surface acoustic waves

We propose a scheme for coupling laser light into graphene plasmons with the help of electrically generated surface acoustic waves. The surface acoustic wave forms a diffraction grating which allows to excite the long lived phonon-like branch of the hybridized graphene plasmon-phonon dispersion with infrared laser light. Our approach avoids patterning the graphene sheet, does not rely on complicated optical near-field techniques, and allows to electrically switch the coupling between far field radiation and propagating graphene plasmons.

cond-mat.mes-hall↗

arXiv2013

Electron-phonon interaction on the surface of a 3D topological insulator

We analyze the role of a Rayleigh surface phonon mode in the electron-phonon interaction at the surface of a 3D topological insulator. A strong renormalization of the phonon dispersion, leading eventually to the disappearance of Rayleigh phonons, is ruled out in the ideal case of a continuum long-wavelength limit, which is only justified if the surface is clean and defect free. The absence of backscattering for Dirac electrons at the Fermi surface is partly responsible for the reduced influence of the electron-phonon interaction. A pole in the dielectric response function due to the Rayleigh phonon dispersion could drive the electron-electron interaction attractive at low frequencies. However, the average pairing interaction within the weak coupling approach is found to be too small to induce a surface superconducting instability.

cond-mat.mes-hall↗