Yuan-Cheng Huang

Magnetoconductance of graphene nanoribbons

The electronic and transport properties of monolayer and AB-stacked bilayer zigzag graphene nanoribbons subject to the influences of a magnetic field are investigated theoretically. We demonstrate that the magnetic confinement and the size effect affect the electronic properties competitively. In the limit of a strong magnetic field, the magnetic length is much smaller than the ribbon width, and the bulk electrons are confined solely by the magnetic potential. Their properties are independent of the width, and the Landau levels appear. On the other hand, the size effect dominates in the case of narrow ribbons. In addition, the dispersion relations rely sensitively on the interlayer interactions. Such interactions will modify the subband curvature, create additional band-edge states, change the subband spacing or the energy gap, and separate the partial flat bands. The band structures are symmetric or asymmetric about the Fermi energy for monolayer or bilayer nanoribbons, respectively. The chemical-potential-dependent electrical and thermal conductance exhibits a stepwise increase behaviour. The competition between the magnetic confinement and the size effect will also be reflected in the transport properties. The features of the conductance are found to be strongly dependent on the field strength, number of layers, interlayer interactions, and temperature.

Effects of Transverse Electric Fields on Quasi-Landau Levels in Zigzag Graphene Nanoribbons

The magnetoelectronic properties of one-dimensional zigzag graphene nanoribbons are investigated using the Peierls tight-binding model with uniform magnetic and electric fields. They are mainly determined by external fields and quantum confinement effects. Magnetic fields lead to quasi-Landau levels (QLLs), enhance partial flat bands, and result in Landau wave functions. Electric fields significantly distort dispersionless QLLs, change the band symmetry, induce more band-edge states, split partial flat bands, and drastically alter the distribution of wave functions. The density of states directly reflects the main features of energy bands, such as the numbers, frequencies, heights, and divergence forms of prominent peaks, which can be confirmed experimentally. Magneto-optical absorption spectra are predicted to be markedly changed under the influence of external electric fields.

Excitation spectra of ABC-stacked graphene superlattice

The excitation spectra of ABC-stacked graphene superlattices are calculated by the gradient approximation. Based on the selection rule, only excitations between the conduction and valence bands of the same pairs are allowed. These excitations occur at the critical points in the energy-wave vector space. Different polarization directions induce different spectra. The spectra of a parallel polarization are stronger than those of a vertical polarization. The strong electron-hole excitations cause special structures in the electron-hole excitation and reflectance spectra. However, only one plasmon peak exists in the loss spectra of the parallel polarization. Some properties are consistent with the experimental measurements.

Critical optical properties of AA-stacked multilayer graphenes

The band structures and optical properties of AA-stacked multilayer graphenes are calculated by the tight-binding model and gradient approximation. For a nL-layer AA-stacked graphene, there are nL peaks at both low and middle frequencies. The threshold energy of odd-layer graphene is much lower than that of even-layer graphene for nL<10. The differences in the electronic structures and optical properties between the odd and even layers are reduced with increasing nL. When nL grows to 30 (200), the spectra of 2D graphene are almost identical to those of 3D graphite at middle (low) frequencies.

Electric-field induced modification of electronic properties of few-layer graphene nanoribbons

In the presence of electric fields, the low-energy electronic properties of AB-stacked few-layer graphene nanoribbons are studied by using the tight-binding model. They are strongly dependent on the geometric structures (the interlayer interactions, the ribbon edges, the ribbon width Ny, and the ribbon number Nz) and the field strength. The interlayer interactions significantly affect density of states (DOS), energy gap (Eg), band structure, and free carriers. DOS exhibits many special structures including plateau, discontinuities, and divergent peaks. The effective electric field modifies the energy dispersions, alters the subband spacing, changes the subband curvature, produces the new edge state, switches the band gap, and causes the metal-semiconductor (or semiconductor-metal) transitions. In gapless zigzag ribbons, electric fields not only lifts the degeneracy of partial flatbands at EF but also induces an energy gap. Eg is dependent on the ribbon width, ribbon edges, and the field strength. The semi...

Influence of anisotropic dipole matrix element on optical response of AB-stacked graphene superlattice

When the polarization direction of the laser beam Ê lies on the graphene plane, the absorption spectrum A(ω)is isotropic and includes one sharp peak and some shoulders. As for Ê along the stacking direction, A(ω) is much weaker, and shows only one broadened peak. Because of the dipole matrix element M(cv), the optical excitations do not fully reflect the features of electronic structures [or the joint density of states (JDOS)]. M(cv) plays an important role in the relationship between A(ω) and JDOS. It is strongly dependent on Ê, showing an anisotropic property.

Low-frequency electronic and optical properties of rhombohedral graphite

Low-energy electronic and optical properties of ABC-stacked graphite are respectively studied by the tight-binding model and gradient approximation. The band structures include linear and parabolic bands with and without degeneracy. They show strongly anisotropic dispersions. ABC-stacked graphite is a semimetal due to the slight overlap near the Fermi level between the conduction and valence bands. The interlayer interactions change the energy dispersion, state degeneracy, and the positions of band-crossings and band-edge states. When the state energy is higher than the degenerate energy of the conduction band (E(2d)(c)) or lower than that of the valence bands (E(2d)(v)), a greater number of states might exist. The special band structures would be reflected in the density of states (DOS), the joint density of states (JDOS), and the absorption spectra (A(ω)). For example, the DOS exhibits a cave-like structure at ω = E(2d)(c) and E(2d)(v). Both a special jump in the JDOS and a turning point in the A(ω) occur at ω = E(2d)(c) - E(2d)(v). The DOS and A(ω) could be respectively verified by scanning tunneling spectroscopy and optical absorption spectroscopy.

Conductance of bilayer graphene nanoribbons with different widths

The electronic and transport properties of bilayer graphene nanoribbons with different width are investigated theoretically by using the tight-binding model. The energy dispersion relations are found to exhibit significant dependence on the interlayer interactions and the geometry of the bilayer graphene nanoribbons. The energy gaps are oscillatory with the upper ribbon displacement. For all four types of bilayer graphene nanoribbons, the bandgaps touch the zero value and exhibit semiconductor–metal transitions. Variations in the electronic structures with the upper ribbon displacement will be reflected in the electrical and thermal conductance. The chemical-potential-dependent electrical and thermal conductances exhibit a stepwise increase and spike behavior. These conductances can be tuned by varying the upper ribbon displacement. The peak and trench structures of the conductance will be stretched out as the temperature rises. In addition, quantum conductance behavior in bilayer graphene nanoribbons can be observed experimentally at temperature below 10 K.

Low-energy Landau levels of Bernal zigzag graphene ribbons

Low-energy Landau levels of AB-stacked zigzag graphene ribbons in the presence of a uniform perpendicular magnetic field (B) are investigated by the Peierls coupling tight-binding model. State energies and associated wave functions are dominated by the B-field strength and the kzdependent interribbon interactions. The occupied valence bands are asymmetric to the unoccupied conduction bands about the Fermi level. Many doubly degenerate Landau levels and singlet curving magnetobands exist along kx and kz directions, respectively. Such features are directly reflected in density of states, which exhibits a lot of asymmetric prominent peaks because of 1D curving bands. The kz-dependent interribbon interactions dramatically modify the magnetobands, such as the lift of double degeneracy, the change of state energies, and the production of two groups of curving magnetobands. They also change the characteristics of the wave functions and cause the redistribution of the charge carrier density. The kz-dependent wave functions are further used to predict the selection rule of the optical transition. Electronic address: t00252@mail.tut.edu.tw Electronic address: mflin@mail.ncku.edu.tw