Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long wavelength optical phonons of graphene. The EFE in this two dimensional honeycomb lattice of carbon atoms creates large density modulations of carriers with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display the interactions of lattice vibrations with these unusual carriers. The changes of phonon frequency and line-width demonstrate optically the particle-hole symmetry about the charge-neutral Dirac-point. The linear dependence of the phonon frequency on the EFE-modulated Fermi energy is explained as the electron-phonon coupling of mass-less Dirac fermions.The interaction between electrons and quantized lattice vibrations in a solid is one of the most fundamental realms of study in condensed matter physics. In particular, the electron-phonon interaction in graphene and its derivatives plays an important role in understanding anomalies of photoemission spectra observed in graphite [1] and graphene [2], the non-linear high energy electron transport in carbon nanotubes [3,4,5,6,7], as well as phonon structures in graphite [8,9] and carbon nanotubes [9,10,11].Traditionally, electron-phonon interactions are investigated through chemical doping, in which the charge carrier density is varied by introduction of impurities. The electric field effect (EFE) is an alternative method for changing the charge carrier density effectively in lowdimensional systems. The EFE has proven very successful in graphene, a single atomic sheet of graphite, where unconventional integer quantum Hall effect [12,13] has revealed physics linked to the uniqueness of the electronic band structure near the charge neutral Dirac points ( Fig. 1(a)).We measured Raman spectra of optical phonons in graphene where large densities of free electrons or free holes are modulated by the EFE. We discovered that the even parity long wavelength optical phonon (the graphene G band) has marked dependence on gate voltage and the induced charge density. The dependence of phonon frequency and line-width on the EFE induced charge density demonstrates that the intriguing physics of mass-less Dirac fermions with particle-hole symmetry is encoded in the electron-phonon interaction.Raman studies of graphite [14] are at the forefront of research on carbon based materials. The recent availability of few-layer and single-layer graphene [15,16], has stimulated great interest in Raman scattering in such novel and exciting systems. For example, dimensional crossover was observed in Raman spectra of thin graphitic films as a function of multilayer thickness [17,18,19]. In the work reported here, Raman spectroscopy emerges as an insightful method to probe the EFE in a single atomic layer and the phonon dynamics that are associated with the two dimensional (2D) Dirac fermions.We focus on the doubly degenerate optical phonon of E 2g symmetry at ∼1580 cm −1 , known as the G band. We also report on the s...

In monolayer graphene, substitutional doping during growth can be used to alter its electronic properties. We used scanning tunneling microscopy, Raman spectroscopy, x-ray spectroscopy, and first principles calculations to characterize individual nitrogen dopants in monolayer graphene grown on a copper substrate. Individual nitrogen atoms were incorporated as graphitic dopants, and a fraction of the extra electron on each nitrogen atom was delocalized into the graphene lattice. The electronic structure of nitrogen-doped graphene was strongly modified only within a few lattice spacings of the site of the nitrogen dopant. These findings show that chemical doping is a promising route to achieving high-quality graphene films with a large carrier concentration.

At low energy, electrons in doped graphene sheets behave like massless Dirac fermions with a Fermi velocity, which does not depend on carrier density. Here we show that modulating a two-dimensional electron gas with a long-wavelength periodic potential with honeycomb symmetry can lead to the creation of isolated massless Dirac points with tunable Fermi velocity. We provide detailed theoretical estimates to realize such artificial graphenelike system and discuss an experimental realization in a modulation-doped GaAs quantum well. Graphene is a one-atom-thick two-dimensional ͑2D͒ electron system composed of carbon atoms on a honeycomb lattice.1 The lattice has two inequivalent sites in the unit cell that are analogous to the two spin orientations of a spin-1/2 particle. This observation opens the way to an elegant description of electrons in graphene as particles endowed with a pseudospin degree-of-freedom.1 At low energy, electrons in graphene are described by a 2D massless Dirac fermion ͑MDF͒ Hamiltonian, H D = v F · p, where v F is the bare Fermi velocity, which does not depend on carrier density, p is the 2D momentum measured from the corners of the Brillouin zone, and is the pseudospin operator constructed with two Pauli matrices ͕ i , i = x , y͖, which act on the sublattice pseudospin degree-of-freedom. It follows that the energy eigenstates are chiral, i.e., for a given p have pseudospins oriented either parallel ͑conduction band͒ or antiparallel ͑valence band͒ to p. The Dirac-like wave equation and the chirality of its eigenstates have a number of very intriguing implications.1 It would be highly desirable to have other materials with Dirac-like spectrum and a pseudospin degree-offreedom. One candidate is represented by HgTe/Hg͑Cd͒Te quantum wells ͑QWs͒ where MDFs are predicted to arise at a critical QW thickness.2 More recently, Park and Louie 3 proposed that MDFs can arise in any 2D electron gas ͑2DEG͒ if appropriately nanopatterned.Here we present an independent approach to the realization of "artificial graphene" in a nanopatterned 2DEG. We provide theoretical evidence for the occurrence of linearly dispersing energy bands in an artificially engineered honeycomb lattice, and we demonstrate a remarkable dependence of the Fermi velocity on the strength of the external potential in this system. We also define the conditions that the external periodic potential and the electron density must satisfy in order to achieve artificial MDFs. Finally we present the photoluminescence ͑PL͒ of the 2DEG confined in a highmobility modulation-doped GaAs/AlGaAs QW where a nanopatterning with honeycomb symmetry is achieved by dry etching. We believe that the development of patterned 2DEGs with tunable parameters will offer unprecedented opportunities to study fundamental interactions of MDFs in high-mobility semiconductor structures.We start our analysis by considering a 2DEG consisting of electrons with band mass m b = 0.067m ͑m is the bare electron mass in vacuum͒ confined in a GaAs/AlGaAs QW. The 2DEG is subjected to a...