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We investigate the effect of one-dimensional periodic potentials on the low energy band structure of ABC trilayer graphene first by assuming that all the three layers have the same potential. Extra Dirac points having the same electron hole crossing energy as that of the original Dirac point are generated by superlattice potentials with equal well and barrier widths. When the potential height is increased, the numbers of extra Dirac points are increased. The dispersions around the Dirac points are not isotropic. It is noted that the dispersion along the ky direction for kx=0 oscillates between a non-linear dispersion and a linear dispersion when the potential height is increased. When the well and barrier widths are not identical, the symmetry of the conduction and valence bands is broken. The extra Dirac points are shifted either upward or downward depending on the barrier and well widths from the zero energy, while the position of the central Dirac point oscillates with the superlattice potential height. By considering different potentials for different layers, extra Dirac points are generated not from the original Dirac points but from the valleys formed in the energy spectrum. Two extra Dirac points appear from each pair of touched valleys, so four Dirac points appeared in the spectrum at particular barrier height. By increasing the barrier height of superlattice potential two Dirac points merge into the original Dirac point. This emerging and merging of extra Dirac points is different from the equal potential case. [ABSTRACT FROM AUTHOR]

Using the Dirac-Weyl model, we study the low-lying spectra of a massless Dirac electron bound to a positively charged Coulomb impurity in a graphene magnetic dot, for which the inhomogeneous magnetic field is created by different magnetic fields inside and outside the dot with a field ratio α. Numerical results show that the zero energy states, in the presence of the impurity, are converted into hole-like states. Additionally, for positive and negative α values, the states are, in general, located inside and outside the dot, respectively, causing the spectra to exhibit different features. [ABSTRACT FROM AUTHOR]

We report a theoretical study of the equilibrium spin current (ESC) flowing in a ferromagnet/ferromagnet (FM/FM) graphene junction with noncollinear magnetizations, in which the FM correlation in graphene is induced by depositing an FM insulator on it. Using the scattering matrix method, we derived an analytical expression of the ESC which is formally the same as that of a normal FM/FM junction. The ESC comes from the exchange coupling between the two FM magnetizations. Owing to the linear dispersion of graphene, the ESC disappears in the one-dimensional case, while it exists in the two-dimensional case. It exhibits damped oscillations with change in the Fermi energy, the layer length between the two FM regions, and the FM exchange splitting. It is also found that ESC in FM/FM graphene junctions has electron-hole inversion symmetry whereas it has electron-hole inversion antisymmetry in normal FM/FM junctions. [ABSTRACT FROM AUTHOR]

We investigate theoretically the Hartman effect in quantum tunneling through single and double barriers in a single graphene layer. The numerical results indicate that the Hartman effect in graphene depends heavily on the incident angle and the energy of the carrier in the tunneling process through single and double barriers. We find that the Hartman effect disappears for normal incidence and appears when the incident angle and energy are larger than some critical values. [ABSTRACT FROM AUTHOR]

Using the massless Dirac-Weyl model for monolayer graphene sheet, we study the low-lying spectra of a single Dirac electron system bound to an on-center positively charged Coulomb impurity, under both electrostatic potential and magnetic field. Numerical results obtained from diagonalization show that, the increase of the electrostatic potential causes the low-lying states to evolve from one Landau-type plateau to the higher ones, and the whole spectra exhibit similar features with slight shifts in eigenenergies when the on-center impurity is considered. Electrostatic-potential dependent optical spectra with their corresponding absorption coefficients as functions of incident photon energies for transitions between low-lying states are presented. [ABSTRACT FROM AUTHOR]

We investigate theoretically spin injection efficiency from the ferromagnetic graphene to normal graphene (FG/NG) based on electron optics, where the magnetization in the FG is assumed from the magnetic proximity effect. Based on a graphene lattice model, we demonstrated that one spin-species electron flow from a point source could be nearly suppressed through the FG-NG interface, when the total internal reflection effect occurs with the help of an additional barrier masking the Klein tunneling, while the opposite spin-species electron flow could even be collimated due to the negative refraction under suitable parameters. Not only at the focusing point is the efficient spin injection achieved, but in the whole NG region the spin injection efficiency can also be maintained at a high level. It is also shown that the nonideal FG-NG interface could reduce the spin injection efficiency since the electron optics phenomena are weakened owing to the interfacial backscattering. Our findings may shed light on making graphene-based spin devices in the spintronics field. [ABSTRACT FROM AUTHOR]

We propose a method of generating spin currents in monolayer graphene through adiabatic quantum pumping by two oscillating potentials. Spin splitting is induced in the graphene layer by ferromagnetic proximity. The pumped charge and spin currents are sensitive functions of the Fermi energy, which can thus be used to control the degree of polarization. The predicted pumped currents are measurable using the current technology. The proposed method is useful in the development of graphene spintronics. [ABSTRACT FROM AUTHOR]

We study adiabatic quantum pumping of Dirac fermions in a monolayer graphene in the large pumping amplitude regime in order to study how to generate a large spin polarized current. Spin current is generated by applying two periodic oscillating gate voltages to a monolayer graphene with exchange splitting induced by ferromagnetic proximity. We find that pumped charge and spin currents are sensitive functions of the Fermi energy and pure spin current and spin current with different degrees of polarization and large magnitudes are obtained in our scheme. We also study the effects of parameters of the system, such as the barrier separation and the exchange splitting, on the pumped currents. The spin current generated by this method can be as high as 80-100 nA. This shows the method can be used in the study of spin physics in graphene and the realization of graphene spintronic devices. [ABSTRACT FROM AUTHOR]

We theoretically explore possible π-state Josephson junctions made from graphene-like two-dimensional materials (G) with the honeycomb lattice structure. It is shown that the valley polarization in the G sheet could lead to a 0-π state transition of the Josephson junction because of the valley-singlet Cooper pairs acquiring a nonzero momentum. When the valley-mixing scattering exists in the interfaces of the junction due to lattice mismatch, an odd-frequency valley-triplet supercurrent flows in the system even though the G sheet is fully valley polarized, and the supercurrent is characterized by a rapid atomic-scale oscillation with a periodicity of three lattice constants. [ABSTRACT FROM AUTHOR]

The zigzag graphene nanoribbon (ZGNR) has very peculiar electronic structure and transport properties such as the edge state, spontaneous magnetization, and the even-odd effect. In this work, we report a theoretical study on the interplay between the magnetization and pseudoparity of the particle, and their combined effect on the tunneling conductance spectra of the ZGNR/superconductor junction. It is shown that the magnetization in ZGNR can significantly modify the original even-odd effect in nonmagnetic ZGNR due to the definite pseudoparity of the particles. For the ferromagnetic ZGNR that can be obtained by an external magnetic field or the proximity effect through a ferromagnet on ZGNR, the Andreev reflection (AR), which is entirely prohibited in the nonmagnetic ZGNR with an even zigzag chain number, is now allowed only for one kind of spin; thus the system resembles a spin-diode device in which only one spin species AR can occur under positive bias while the other spin species AR occurs under negative bias. For the antiferromagnetic ZGNR with weak magnetization, the conductance gap appears at Fermi energy due to the insulating property of ZGNR; in addition, AR is also possible for the even ZGNR, and two conductance peaks appear in the superconductor energy gap, which is attributed to the pseudoparity of the edge state destroyed by the antiferromagnetic ordering in ZGNR. For the odd ZGNR, either ferromagnetic or antiferromagnetic magnetization has no qualitative influence on the conductance spectra of the junction. Our findings may shed light on devising spin devices based on magnetized graphene nanoribbons. [ABSTRACT FROM AUTHOR]

We propose an approach to detect spin currents in an even zigzag graphene nanoribbon (ZR) junction, based on the peculiar intervalley selection rule in the ribbon when only zero-energy modes are involved in transport. Spatial spin separation arises from the bipolar property of the undoped ZR and opposite pseudoparities of electrons in the conduction and valence bands, and does not involve any magnetic field or spin orbit interaction. A measurable fully spin-polarized charge current or potential can thus be converted from either coherent or incoherent spin resources. Our findings may shed light on making spintronics devices based on graphene nanoribbons. [ABSTRACT FROM AUTHOR]