We describe the synthesis of bilayer graphene thin films deposited on insulating silicon carbide and report the characterization of their electronic band structure using angle-resolved photoemission. By selectively adjusting the carrier concentration in each layer, changes in the Coulomb potential led to control of the gap between valence and conduction bands. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic-scale electronic devices

No abstract

We investigated electronic structure of 5d transition-metal oxide Sr 2 IrO 4 using angle-resolved photoemission, optical conductivity, and x-ray absorption measurements and first-principles band calculations. The system was found to be well described by novel effective total angular momentum J eff states, in which relativistic spin-orbit (SO) coupling is fully taken into account under a large crystal field. Despite of delocalized Ir 5d states, the J eff -states form so narrow bands that even a small correlation energy leads to the J eff = 1/2 Mott ground state with unique electronic and magnetic behaviors, suggesting a new class of the J eff quantum spin driven correlated-electron phenomena.

With the recent discovery of superconductivity in carbon nanotubes (CNTs), 1, 2 alkaline-metal-doped C 60 crystals, 3 and graphite intercalation compounds 4-6 (GICs) with relatively high transition temperatures, there is a strong interest in the influence of many-body interactions on the electron dynamics in these systems. Graphene is a sheet of carbon atoms distributed in a honeycomb lattice and is the building block for all of these materials;therefore it is a model system for this entire family. Recently, graphene has been isolated using exfoliation from graphite 7, 8 and graphitization of SiC, 9, 10 enabling, for the first time, the direct measurement of the manybody interactions fundamental to all of these carbon systems. These interactions could be especially interesting owing to the effectively massless, relativistic nature of the charge carriers, which follows from the linearity of the bands at the Dirac crossing energy E D = ħω D and the formal equivalence of the Schrödinger wave equation with the relativistic Dirac equation for graphene. 7,8,11

The unusual transport properties of graphene are the direct consequence of a peculiar band structure near the Dirac point. We determine the shape of the bands and their characteristic splitting, and find the transition from two-dimensional to bulk character for 1 to 4 layers of graphene by angle-resolved photoemission. By detailed measurements of the bands we derive the stacking order, layer-dependent electron potential, screening length, and strength of interlayer interaction by comparison with tight binding calculations, yielding a comprehensive description of multilayer graphene's electronic structure. DOI: 10.1103/PhysRevLett.98.206802 PACS numbers: 73.21.ÿb, 73.22.ÿf, 73.90.+f, 79.60.ÿi Much recent attention has been given to the electronic structure of multilayer films of graphene, the honeycomb carbon sheet which is the building block of graphite, carbon nanotubes, C 60 , and other mesoscopic forms of carbon [1]. Recent progress in synthesizing or isolating multilayer graphene films [2 -4] has provided access to their physical properties, and revealed many interesting transport phenomena, including an anomalous quantum Hall effect [5,6], ballistic electron transport at room temperature [7], micronscale coherence length [7,8], and novel many-body couplings [9].These effects originate from the effectively massless Dirac fermion character of the carriers derived from graphene's valence bands, which exhibit a linear dispersion degenerate near the so-called Dirac point energy E D [10].These unconventional properties of graphene offer a new route to room temperature, molecular-scale electronics capable of quantum computing [6,7]. For example, a possible switching function in bilayer graphene has been suggested by reversibly lifting the band degeneracy at the Fermi level (E F ) upon application of an electric field [11,12]. This effect is due to a unique sensitivity of the band structure to the charge distribution brought about by the interplay between strong interlayer hopping and weak interlayer screening, neither of which is currently well understood [13,14].In order to evaluate the interlayer screening, stacking order,and interlayer coupling, we have systematically studied the evolution of the band structure of one to four layers of graphene using angle-resolved photoemission spectroscopy (ARPES). We demonstrate experimentally that the interaction between layers and the stacking sequence affect the topology of the bands, the former inducing an electronic transition from 2D to 3D (bulk) character when going from one layer to multilayer graphene. The interlayer hopping integral and screening length are determined as a function of the number of graphene layers by exploiting the sensitivity of states to the Coulomb potential, and the layer-dependent carrier concentration is estimated.The films were synthesized on n-type (nitrogen, 1 10 18 cm ÿ3 ) 6H-SiC(0001) substrates (SiCrystal AG) that were etched in hydrogen at 1550 C. Annealing in a vacuum first removes the resulting silicate adlayer and then causes t...

The kagome lattice is a two-dimensional network of corner-sharing triangles that is known to host exotic quantum magnetic states. Theoretical work has predicted that kagome lattices may also host Dirac electronic states that could lead to topological and Chern insulating phases, but these states have so far not been detected in experiments. Here we study the d-electron kagome metal FeSn, which is designed to support bulk massive Dirac fermions in the presence of ferromagnetic order. We observe a temperature-independent intrinsic anomalous Hall conductivity that persists above room temperature, which is suggestive of prominent Berry curvature from the time-reversal-symmetry-breaking electronic bands of the kagome plane. Using angle-resolved photoemission spectroscopy, we observe a pair of quasi-two-dimensional Dirac cones near the Fermi level with a mass gap of 30 millielectronvolts, which correspond to massive Dirac fermions that generate Berry-curvature-induced Hall conductivity. We show that this behaviour is a consequence of the underlying symmetry properties of the bilayer kagome lattice in the ferromagnetic state and the atomic spin-orbit coupling. This work provides evidence for a ferromagnetic kagome metal and an example of emergent topological electronic properties in a correlated electron system. Our results provide insight into the recent discoveries of exotic electronic behaviour in kagome-lattice antiferromagnets and may enable lattice-model realizations of fractional topological quantum states.

Self-assembled indium linear chains on the Si(111) surface are found to exhibit instability of the metallic phase and 1D charge density wave (CDW). The room-temperature metallic phase of these chains undergoes a temperature-induced, reversible transition into a semiconducting phase. The 1D CDW along the chains is observed directly in real space by scanning tunneling microscopy at low temperature. The Fermi contours of the metallic phase measured by angle-resolved photoemission exhibit a perfect nesting predicting precisely the CDW periodicity. [S0031-9007(99)09330-8]

The rotation of the polarization of light after passing a medium in a magnetic field, discovered by Faraday, is an optical analogue of the Hall effect, which combines sensitivity to the carrier type with access to a broad energy range. Up to now the thinnest structures showing the Faraday rotation were several-nanometre-thick two-dimensional electron gases. As the rotation angle is proportional to the distance travelled by the light, an intriguing issue is the scale of this effect in two-dimensional atomic crystals or films—the ultimately thin objects in condensed matter physics. Here we demonstrate that a single atomic layer of carbon—graphene—turns the polarization by several degrees in modest magnetic fields. Such a strong rotation is due to the resonances originating from the cyclotron effect in the classical regime and the inter-Landau-level transitions in the quantum regime. Combined with the possibility of ambipolar doping, this opens pathways to use graphene in fast tunable ultrathin infrared magneto-optical devices