Gaped graphene bilayer: disorder and magnetic field effects

Double layer graphene is a gapless semiconductor which develops a finite gap when the layers are placed at different electrostatic potentials. We study, within the tight-biding approximation, the electronic properties of the gaped graphene bilayer in the presence of disorder, perpendicular magnetic field, and transverse electric field. We show that the gap is rather stable in the presence of diagonal disorder. We compute the cyclotron effective mass in the semi-classical approximation, valid at low magnetic fields. Landau level formation is clearly seen in zigzag and armchair ribbons of the gaped bilayer at intermediate magnetic fields.Comment: 5 pages, 4 figure

A study of charm hadron production in e^+e^- annihilation

The processes of D^{(*)}-mesons and \Lambda_c-baryons production in e^+e^--annihilation at 10.58 GeV and 91.18 GeV energies are concerned. At the 10.58 GeV energy the production of charmed particles via the B-mesons decays is also concerned. Scaling violation of the fragmentation functions is calculated at the NLL-accuracy. Nonperturbative fragmentation functions are retrieved from the experimental data of B-factories and are approximated by simple analytic expressions. It is proved, that the difference between nonperturbative fragmentation functions of mesons and baryons can be easily explained by quark counting.Comment: 21 pages, 15 figure

Quantum Hall activation gaps in bilayer graphene

We have measured the quantum Hall activation gaps in bilayer graphene at filling factors $\nu=\pm4$ and $\nu=\pm8$ in high magnetic fields up to 30 T. We find that energy levels can be described by a 4-band relativistic hyperbolic dispersion. The Landau level width is found to contain a field independent background due to an intrinsic level broadening and a component which increases linearly with magnetic field.Comment: 4 pages, accepted version (just removed a few typos), will appear as Fast Track Communication in Solid State Commu

Supersymmetry and Correlated Electrons in Graphene Quantum Hall Effect

We present a supersymmetric description of the quantum Hall effect (QHE) in graphene. The noninteracting system is supersymmetric separately at the so-called K and K' points of the Brillouin zone corners. Its essential consequence is that the energy levels and the Landau levels are different objects in graphene QHE. Each energy level has a four-fold degeneracy within the noninteracting theory. With the Coulomb interaction included, an excitonic gap opens in the zero-energy state, while each nonzero energy level splits into two levels since up-spin and down-spin electrons come from different Landau levels. We argue the emergence of the plateaux at $\nu =\pm (4n-2)$ for small magnetic field $B$ and at $\nu =0$, $\pm 1$, $\pm 2n$ for large $B$ with $n$ natural numbers.Comment: 5 pages, 2 figure

Minimal conductivity in bilayer graphene

Using the Landauer formula approach, it is proven that minimal conductivity of order of $e^{2}/h$ found experimentally in bilayer graphene is its intrinsic property. For the case of ideal crystals, the conductivity turns our to be equal to $e^{2}/2h$ per valley per spin. A zero-temperature shot noise in bilayer graphene is considered and the Fano factor is calculated. Its value $1-2/\pi$ is close to the value 1/3 found earlier for the single-layer graphene.Comment: 3 pages, 1 figur

Edge States at the Interface between Monolayer and Bilayer Graphene

The electronic property of monolayer-bilayer hybrid graphene with a zigzag interface is studied by both the Dirac equation and numerical calculation. There are two types of zigzag interface stacks. The dispersion and local density of states behave quit differently along the interface at the Fermi energy due to the different locations of the edge state. We hope our study can give some insights in the understanding of the transport and STM experiments.Comment: 8 pages, 8 figure

Zitterbewegung, chirality, and minimal conductivity in graphene

It has been recently demonstrated experimentally that graphene, or single-layer carbon, is a gapless semiconductor with massless Dirac energy spectrum. A finite conductivity per channel of order of $e^{2}/h$ in the limit of zero temperature and zero charge carrier density is one of the striking features of this system. Here we analyze this peculiarity based on the Kubo and Landauer formulas. The appearance of a finite conductivity without scattering is shown to be a characteristic property of Dirac chiral fermions in two dimensions.Comment: final version; 4 pages, 1 eps figur

Energy bands of atomic monolayers of various materials: Possibility of energy gap engineering

The mobility of graphene is very high because the quantum Hall effects can be observed even at room temperature. Graphene has the potential of the material for novel devices because of this high mobility. But the energy gap of graphene is zero, so graphene can not be applied to semiconductor devices such as transistors, LEDs, etc. In order to control the energy gaps, we propose atomic monolayers which consist of various materials besides carbon atoms. To examine the energy dispersions of atomic monolayers of various materials, we calculated the electronic states of these atomic monolayers using density functional theory with structural optimizations. The quantum chemical calculation software "Gaussian 03" was used under periodic boundary conditions. The calculation method is LSDA/6-311G(d,p), B3LYP/6-31G(d), or B3LYP/6-311G(d,p). The calculated materials are C (graphene), Si (silicene), Ge, SiC, GeC, GeSi, BN, BP, BAs, AlP, AlAs, GaP, and GaAs. These atomic monolayers can exist in the flat honeycomb shapes. The energy gaps of these atomic monolayers take various values. Ge is a semimetal; AlP, AlAs, GaP, and GaAs are indirect semiconductors; and others are direct semiconductors. We also calculated the change of energy dispersions accompanied by the substitution of the atoms. Our results suggest that the substitution of impurity atoms for monolayer materials can control the energy gaps of the atomic monolayers. We conclude that atomic monolayers of various materials have the potential for novel devices.Comment: This paper was first presented at the 14th International Conference on Modulated Semiconductor Structures (MSS14) held in Kobe, Japan, on 23 July 200