Single or multi-flavor Kondo effect in graphene

Based on the tight-binding formalism, we investigate the Anderson and the Kondo model for an adaom magnetic impurity above graphene. Different impurity positions are analyzed. Employing a partial wave representation we study the nature of the coupling between the impurity and the conducting electrons. The components from the two Dirac points are mixed while interacting with the impurity. Two configurations are considered explicitly: the adatom is above one atom (ADA), the other case is the adatom above the center the honeycomb (ADC). For ADA the impurity is coupled with one flavor for both A and B sublattice and both Dirac points. For ADC the impurity couples with multi-flavor states for a spinor state of the impurity. We show, explicitly for a 3d magnetic atom, $d_{z^{2}}$, ($d_{xz}$,$d_{yz}$), and ($d_{x^{2}-y^{2}}$,$d_{xy}$) couple respectively with the $\Gamma_{1}$, $\Gamma_{5} (E_{1})$, and $\Gamma_{6} (E_{2})$ representations (reps) of $C_{6v}$ group in ADC case. The basses for these reps of graphene are also derived explicitly. For ADA we calculate the Kondo temperature.Comment: 11 pages, 1 fures, 2 tables, accepted by EP

Laser-assisted spin-polarized transport in graphene tunnel junctions

Keldysh nonequilibrium Green's function method is utilized to study theoretically the spin polarized transport through a graphene spin valve irradiated by a monochromatic laser field. It is found that the bias dependence of the differential conductance exhibits successive peaks corresponding to the resonant tunneling through the photon-assisted sidebands. The multi photon processes originate from the combined effects of the radiation field and the graphene tunneling properties, and are shown to be substantially suppressed in a graphene spin valve which results in a decrease of the differential conductance for a high bias voltage. We also discussed the appearance of a dynamical gap around zero bias due to the radiation field. The gap width can be tuned by changing the radiation electric field strength and the frequency. This leads to a shift of the resonant peaks in the differential conductance. We also demonstrate numerically the dependencies of the radiation and spin valve effects on the parameters of the external fields and those of the electrodes. We find that the combined effects of the radiation field, the graphene, and the spin valve properties bring about an oscillatory behavior in the tunnel magnetoresistance (TMR), and this oscillatory amplitude can be changed by scanning the radiation field strength and/or the frequency.Comment: 31 pages, 5 figures, corrected version to the paper in J. Phys.: Condens. Matter 24 (2012) 26600

Magneto-transport in impurity-doped few-layer graphene spin valve

Using Keldysh nonequilibrium Green's function method we study the spin-dependent transport through impurity-doped few layer graphene sandwiched between two magnetic leads with an arbitrary mutual orientations of the magnetizations. We find for parallel electrodes magnetizations that the differential conductance possesses two resonant peaks as the applied bias increases. These peaks are traced back to a buildup of a magnetic moment on the impurity due to the electrodes spin polarization. For a large mutual angle of the electrodes magnetization directions, the two resonant peaks approach each others and merge into a single peak for antiparallel orientation of the electrodes magnetizations. We point out that the tunneling magnetoresistance (TMR) may change sign for relatively small changes in the values of the polarization parameters. Furthermore, we inspect the behaviour of the differential conductance and TMR upon varying the temperature.Comment: 8 pages, 7 figures, accepted by Phys. Rev.

Imprints of Schwinger Effect on Primordial Spectra

We study the Schwinger effect during inflation and its imprints on the primordial power spectrum and bispectrum. The produced charged particles by Schwinger effect during inflation can leave a unique angular dependence on the primordial spectra.Comment: 27 pages, 13 figures, JHEP accepted versio