Anomalous orbital magnetism in Dirac-electron systems: Role of pseudo-spin paramagnetism

The orbital diamagnetic susceptibility is calculated in monolayer and bilayer graphenes with band gap as well as in three-dimensional Dirac systems. It is demonstrated that the pseudo-spin degree of freedom such as valleys produces paramagnetic susceptibility in an equal manner as the real spin dominating over the Landau diamagnetism. The pseudo-spin paramagnetism explains the origin of a singular diamagnetism which is present only in the band-gap region and disappears rapidly inside the conduction and valence bands.Comment: 9 pages, 5 figure

Interface Landau levels in graphene monolayer-bilayer junction

Electronic structure of graphene monolayer-bilayer junction in a magnetic field is studied within an effective-mass approximation. The energy spectrum is characterized by interface Landau levels, i.e., the locally flat bands appearing near the boundary region, resulting in a series of characteristic peaks in the local density of states. Their energies are independent of boundary types such as zigzag or armchair. In the atomic scale, the local density of states shows a Kekul\'{e} pattern due to the valley mixing in the armchair boundary, while does not in the zigzag boundary.Comment: 12 pages, 9 figure

Transmission through a boundary between monolayer and bilayer graphene

The electron transmission between monolayer and bilayer graphene is theoretically studied for zigzag and armchair boundaries within an effective-mass scheme. Due to the presence of an evanescent wave in the bilayer graphene, traveling modes are well connected to each other. The transmission through the boundary is strongly dependent on the incident angle and the dependence is opposite between the K and K' points, leading to valley polarization of transmitted wave.Comment: 14 pages, 7 figure

Transport in Bilayer Graphene: Calculations within a self-consistent Born approximation

The transport properties of a bilayer graphene are studied theoretically within a self-consistent Born approximation. The electronic spectrum is composed of $k$-linear dispersion in the low-energy region and $k$-square dispersion as in an ordinary two-dimensional metal at high energy, leading to a crossover between different behaviors in the conductivity on changing the Fermi energy or disorder strengths. We find that the conductivity approaches $2e^2/\pi^2\hbar$ per spin in the strong-disorder regime, independently of the short- or long-range disorder.Comment: 8 pages, 5 figure

Hall plateau diagram for the Hofstadter butterfly energy spectrum

We extensively study the localization and the quantum Hall effect in the Hofstadter butterfly, which emerges in a two-dimensional electron system with a weak two-dimensional periodic potential. We numerically calculate the Hall conductivity and the localization length for finite systems with the disorder in general magnetic fields, and estimate the energies of the extended levels in an infinite system. We obtain the Hall plateau diagram on the whole region of the Hofstadter butterfly, and propose a theory for the evolution of the plateau structure with increasing disorder. There we show that a subband with the Hall conductivity $n e^2/h$ has $|n|$ separated bunches of extended levels, at least for an integer $n \leq 2$. We also find that the clusters of the subbands with identical Hall conductivity, which repeatedly appear in the Hofstadter butterfly, have a similar localization property.Comment: 9 pages, 12 figure

Magneto-optical properties of multilayer graphenes

The magneto-optical absorption properties of graphene multilayers are theoretically studied. It is shown that the spectrum can be decomposed into sub-components effectively identical to the monolayer or bilayer graphene, allowing us to understand the spectrum systematically as a function of the layer number. Odd-layered graphenes always exhibit absorption peaks which shifts in proportion to sqrt(B), with B being the magnetic field, due to the existence of an effective monolayer-like subband. We propose a possibility of observing the monolayer-like spectrum even in a mixture of multilayer graphene films with various layers numbers.Comment: 9 pages, 7 figure

Magnetic field screening and mirroring in graphene

The orbital magnetism in spatially varying magnetic fields is studied in monolayer graphene within the effective mass approximation. We find that, unlike the conventional two-dimensional electron system, graphene with small Fermi wave number k_F works as a magnetic shield where the field produced by a magnetic object placed above graphene is always screened by a constant factor on the other side of graphene. The object is repelled by a diamagnetic force from the graphene, as if there exists its mirror image with a reduced amplitude on the other side of graphene. The magnitude of the force is much greater than that of conventional two-dimensional system. The effect disappears with the increase of k_F.Comment: 5 pages, 3 figure

Magnetophonon Resonance in Monolayer Graphene

The conductivity describing magnetophonon resonances is calculated in monolayer graphene, with the Fermi level located near the Dirac point. Intervalley scattering due to zone-edge phonons gives dominant contribution to the conductivity compared to intravalley scattering due to zone-center optical phonons mainly because of lower frequency. Resonances are classified into three types, i.e., principal, symmetric, and asymmetric transitions. The magnetophonon oscillations due to the principal and symmetric transitions are periodic in inverse magnetic field, while those due to the asymmetric transitions are not precisely periodic. The amplitude of the oscillation is shown to be weakly dependent on magnetic field

Impurity driven inter-tube conductance in double-wall carbon nanotubes

Abstract The inter-tube conductance of double-wall carbon nanotubes with impurities is numerically studied. Same impurities lead to significantly different inter-tube conductance depending on the tube where they are located