Showing papers by "Nuno M. R. Peres published in 2011"
TL;DR: In this article, it is shown that by enclosing graphene in an optical cavity, giant Faraday rotations in the infrared regime are generated and measurable Faraday rotation angles in the visible range become possible.
Abstract: We show that by enclosing graphene in an optical cavity, giant Faraday rotations in the infrared regime are generated and measurable Faraday rotation angles in the visible range become possible. Explicit expressions for the Hall steps of the Faraday rotation angle are given for relevant regimes. In the context of this problem we develop an equation of motion (EOM) method for calculation of the magneto-optical properties of metals and semiconductors. It is shown that properly regularized EOM solutions are fully equivalent to the Kubo formula.
151 citations
TL;DR: In this article, the ground-state energies of the different five stackings are computed using DFT in order to determine the most stable configuration for bilayers, and the interlayer distance for the five different types of stacking in which boron-nitride bilayers can be found is determined.
Abstract: We have studied boron nitride monolayer and bilayer band structures. For bilayers, the ground-state energies of the different five stackings are computed using DFT in order to determine the most stable configuration. Also, the interlayer distance for the five different types of stacking in which boron-nitride bilayers can be found is determined. Using a minimal tight-binding model for the band structures of boron nitride bilayers, the hopping parameters and the on-site energies have been extracted by fitting a tight-binding empirical model to the DFT results.
150 citations
TL;DR: In this paper, a coherent picture of the dc conductivity of monolayer and bilayer graphene at finite electronic densities emerges upon considering that strong short-range potentials are the main source of scattering in these two systems.
Abstract: We show that a coherent picture of the dc conductivity of monolayer and bilayer graphene at finite electronic densities emerges upon considering that strong short-range potentials are the main source of scattering in these two systems. The origin of the strong short-range potentials may lie in adsorbed hydrocarbons at the surface of graphene. The equivalence among results based on the partial-wave description of scattering, the Lippmann-Schwinger equation, and the $T$-matrix approach is established. Scattering due to resonant impurities close to the neutrality point is investigated via a numerical computation of the Kubo formula using a kernel polynomial method. We find that relevant adsorbate species originate impurity bands in monolayer and bilayer graphene close to the Dirac point. In the midgap region, a plateau of minimum conductivity of about ${e}^{2}/h$ (per layer) is induced by the resonant disorder. In bilayer graphene, a large adsorbate concentration can develop an energy gap between midgap and high-energy states. As a consequence, the conductivity plateau is supressed near the edges and a ``conductivity gap'' takes place. Finally, a scattering formalism for electrons in biased bilayer graphene, taking into account the degeneracy of the spectrum, is developed and the dc conductivity of that system is studied.
139 citations
TL;DR: In this paper, the optical conductivity of freely suspended graphene was examined under non-equilibrium conditions using femtosecond pump-probe spectroscopy, and the authors observed a conductivity transient that varied strongly with the electronic temperature, exhibiting a crossover from enhanced to decreased absorbance with increasing pump fluence.
Abstract: The optical conductivity of freely suspended graphene was examined under non-equilibrium conditions using femtosecond pump-probe spectroscopy. We observed a conductivity transient that varied strongly with the electronic temperature, exhibiting a crossover from enhanced to decreased absorbance with increasing pump fluence. The response arises from a combination of bleaching of the inter-band transitions by Pauli blocking and induced absorption from the intra-band transitions of the carriers. The latter dominates at low electronic temperature, but, despite an increase in Drude scattering rate, is overwhelmed by the former at high electronic temperature. The time-evolution of the optical conductivity in all regimes can described in terms of a time-varying electronic temperature.
72 citations
TL;DR: In this article, the authors explore the characteristics of the electronic band structure with a focus on the nature of edge states, and trace this curious effect to the doubling of the unit cell along the edge, brought about by the edge reconstruction.
Abstract: In this paper, we study zigzag graphene nanoribbons with edges reconstructed with Stone-Wales defects, by means of an empirical (first-neighbor) tight-binding method, with parameters determined by ab initio calculations of very narrow ribbons. We explore the characteristics of the electronic band structure with a focus on the nature of edge states. Edge reconstruction allows the appearance of a new type of edge states. They are dispersive, with nonzero amplitudes in both sublattices; furthermore, the amplitudes have two components that decrease with different decay lengths with the distance from the edge; at the Dirac points one of these lengths diverges, whereas the other remains finite, of the order of the lattice parameter. We trace this curious effect to the doubling of the unit cell along the edge, brought about by the edge reconstruction. In the presence of a magnetic field, the zero-energy Landau level is no longer degenerate with edge states as in the case of the pristine zigzag ribbon.
62 citations
TL;DR: In this paper, the effect of extended charge defects in electronic transport properties of graphene was studied and it was shown that at low energies these defects interact quite strongly with the 2D Dirac fermions and have an important effect in the DC-conductivity of these materials.
Abstract: We study the effect of extended charge defects in electronic transport properties of graphene. Extended defects are ubiquitous in chemically and epitaxially grown graphene samples due to internal strains associated with the lattice mismatch. We show that at low energies these defects interact quite strongly with the 2D Dirac fermions and have an important effect in the DC-conductivity of these materials.
51 citations
TL;DR: In this paper, Coulomb drag in ultra-clean graphene double layers can be used for controlling the on-and-off ratio for current flow by tuning the external gate voltage, which can be tuned from conductive to a highly resistive state.
Abstract: We show that Coulomb drag in ultra-clean graphene double layers can be used for controlling the on-and-off ratio for current flow by tuning the external gate voltage. Hence, although graphene remains semi-metallic, the double-layer graphene system can be tuned from conductive to a highly resistive state. We show that our results explain previous data of Coulomb drag in double-layer graphene samples in disordered SiO2 substrates.
39 citations
TL;DR: In this paper, a phenomenological analysis of the problem of the electronic doping of a graphene sheet by deposited transition metal atoms, which aggregate in clusters, is presented, which allows us to explain the thermodynamic properties of the device, such as the level of doping of graphene and the ionization potential of the metal clusters, in terms of the chemical interaction between graphene and metal clusters.
Abstract: We perform a phenomenological analysis of the problem of the electronic doping of a graphene sheet by deposited transition metal atoms, which aggregate in clusters. The sample is placed in a capacitor device such that the electronic doping of graphene can be varied by the application of a gate voltage and such that transport measurements can be performed via the application of a (much smaller) voltage along the graphene sample, as reported in the work of Pi et al. [Phys. Rev. B 80, 075406 (2009)]. The analysis allows us to explain the thermodynamic properties of the device, such as the level of doping of graphene and the ionization potential of the metal clusters, in terms of the chemical interaction between graphene and the clusters. We are also able, by modeling the metallic clusters as perfectly conducting spheres, to determine the scattering potential due to these clusters on the electronic carriers of graphene and hence the contribution of these clusters to the resistivity of the sample. The model presented is able to explain the measurements performed by Pi et al. on Pt-covered graphene samples at the lowest metallic coverages measured, and we also present a theoretical argument based on the above model that explains why significant deviations from such a theory are observed at higher levels of coverage.
32 citations
TL;DR: In this paper, the energy levels associated with the even-parity wavefunctions of the harmonic oscillator with an additional delta-function potential at the origin were derived, and the authors brought to the attention of students a non-trivial and analytical example of a modification of the usual harmonic oscillators potential.
Abstract: We derive the energy levels associated with the even-parity wavefunctions of the harmonic oscillator with an additional delta-function potential at the origin. Our results bring to the attention of students a non-trivial and analytical example of a modification of the usual harmonic oscillator potential, with emphasis on the modification of the boundary conditions at the origin. This problem calls the attention of the students to an inaccurate statement in quantum mechanics textbooks often found in the context of the solution of the harmonic oscillator problem.
26 citations
TL;DR: In this article, the energy levels associated with the even-parity wave functions of the harmonic oscillator with an additional delta-function potential at the origin were derived, and the authors brought to the attention of students a non-trivial and analytical example of a modification of the usual harmonic oscillators potential, with emphasis on the modification of boundary conditions.
Abstract: We derive the energy levels associated with the even-parity wave functions of the harmonic oscillator with an additional delta-function potential at the origin. Our results bring to the attention of students a non-trivial and analytical example of a modification of the usual harmonic oscillator potential, with emphasis on the modification of the boundary conditions at the origin. This problem calls the attention of the students to an inaccurate statement in quantum mechanics textbooks often found in the context of solution of the harmonic oscillator problem.
23 citations
TL;DR: In this paper, Coulomb drag in ultra-clean graphene double layers can be used for controlling the on/off ratio for current flow by tuning the external gate voltage, which can be tuned from conductive to a highly resistive state.
Abstract: We show that Coulomb drag in ultra-clean graphene double layers can be used for controlling the on/off ratio for current flow by tunning the external gate voltage. Hence, although graphene remains semi-metallic, the double layer graphene system can be tuned from conductive to a highly resistive state. We show that our results explain previous data of Coulomb drag in double layer graphene samples in disordered SiO2 substrates.
TL;DR: In this article, the formation of local resonances in graphene in the presence of magnetic adatoms containing localized orbitals of arbitrary symmetry, corresponding to any given angular momentum state, is described.
Abstract: In this paper, we describe the formation of local resonances in graphene in the presence of magnetic adatoms containing localized orbitals of arbitrary symmetry, corresponding to any given angular momentum state. We show that quantum interference effects which are naturally inbuilt in the honeycomb lattice in combination with the specific orbital symmetry of the localized state lead to the formation of fingerprints in differential conductance curves. In the presence of Jahn-Teller distortion effects, which lift the orbital degeneracy of the adatoms, the orbital symmetries can lead to distinctive signatures in the local density of states. We show that those effects allow scanning tunneling probes to characterize adatoms and defects in graphene.