Quasi-Ferromagnet Spintronics in Graphene Nanodisk-Lead System

A zigzag graphene nanodisk can be interpreted as a quantum dot with an internal degree of freedom. It is well described by the infinite-range Heisenberg model. We have investigated its thermodynamical properties. There exists a quasi-phase transition between the quasi-ferromagnet and quasi-paramagnet states, as signaled by a sharp peak in the specific heat and in the susceptability. We have also analyzed how thermodynamical properties are affected when two leads are attached to the nanodisk. It is shown that lead effects are described by the many-spin Kondo Hamiltonian. There appears a new peak in the specific heat, and the multiplicity of the ground state becomes just one half of the system without leads. Another lead effect is to enhance the ferromagnetic order. Being a ferromagnet, a nanodisk can be used as a spin filter. Furthermore, since the relaxation time is finite, it is possible to control the spin of the nanodisk by an external spin current. We then propose a rich variety of spintronic devices made of nanodisks and leads, such as spin memory, spin amplifier, spin valve, spin-field-effect transistor, spin diode and spin logic gates such as spin-XNOR gate and spin-XOR gate. Graphene nanodisks could well be basic components of future nanoelectronic and spintronic devices.Comment: 12 pages, 13 figures, invited paper to "focus on graphene

Proximity effect, quasiparticle transport, and local magnetic moment in ferromagnet-d-wave superconductor junctions

The proximity effect, quasiparticle transport, and local magnetic moment in ferromagnet--d-wave superconductor junctions with {110}-oriented interface are studied by solving self-consistently the Bogoliubov-de Gennes equations within an extended Hubbard model. It is found that the proximity induced order parameter oscillates in the ferromagnetic region. The modulation period is shortened with the increased exchange field while the oscillation amplitude is depressed by the interfacial scattering. With the determined superconducting energy gap, a transfer matrix method is proposed to compute the subgap conductance within a scattering approach. Many novel features including the zero-bias conductance dip and splitting are exhibited with appropriate values of the exchange field and interfacial scattering strength. The conductance spectrum can be influenced seriously by the spin-flip interfacial scattering. In addition, a sizable local magnetic moment near the {110}-oriented surface of the d-wave superconductor is discussed.Comment: 10 pages, 16 ps-figures, to appear in Phys. Rev.

Electronic properties of bulk and thin film SrRuO3_3: a search for the metal-insulator transition

We calculate the properties of the 4$d$ ferromagnet SrRuO$_3$ in bulk and thin film form with the aim of understanding the experimentally observed metal to insulator transition at reduced thickness. Although the spatial extent of the 4$d$ orbitals is quite large, many experimental results have suggested that electron-electron correlations play an important role in determining this material's electronic structure. In order to investigate the importance of correlation, we use two approaches which go beyond the conventional local density approximation to density functional theory (DFT): the local spin density approximation + Hubbard $U$ (LSDA+$U$) and the pseudopotential self-interaction correction (pseudo-SIC) methods. We find that the details of the electronic structure predicted with the LSDA do not agree with the experimental spectroscopic data for bulk and thin film SrRuO$_3$. Improvement is found by including electron-electron correlations, and we suggest that bulk orthorhombic SrRuO$_3$ is a {\it weakly strongly-correlated} ferromagnet whose electronic structure is best described by a 0.6 eV on-site Hubbard term, or equivalently with corrections for the self-interaction error. We also perform {\it ab initio} transport calculations that confirm that SrRuO$_3$ has a negative spin polarization at the Fermi level, due to the position of the minority Ru 4$d$ band center. Even with correlations included in our calculations we are unable to reproduce the experimentally observed metal-insulator transition, suggesting that the electronic behavior of SrRuO$_3$ ultra-thin films might be dominated by extrinsic factors such as surface disorder and defects.Comment: 15 pages, 12 figures, 3 table

Spin Dynamics and Spin Transport

Spin-orbit (SO) interaction critically influences electron spin dynamics and spin transport in bulk semiconductors and semiconductor microstructures. This interaction couples electron spin to dc and ac electric fields. Spin coupling to ac electric fields allows efficient spin manipulating by the electric component of electromagnetic field through the electric dipole spin resonance (EDSR) mechanism. Usually, it is much more efficient than the magnetic manipulation due to a larger coupling constant and the easier access to spins at a nanometer scale. The dependence of the EDSR intensity on the magnetic field direction allows measuring the relative strengths of the competing SO coupling mechanisms in quantum wells. Spin coupling to an in-plane electric field is much stronger than to a perpendicular field. Because electron bands in microstructures are spin split by SO interaction, electron spin is not conserved and spin transport in them is controlled by a number of competing parameters, hence, it is rather nontrivial. The relation between spin transport, spin currents, and spin populations is critically discussed. Importance of transients and sharp gradients for generating spin magnetization by electric fields and for ballistic spin transport is clarified.Comment: Invited talk at the 3rd Intern. Conf. on Physics and Applications of Spin-Related Phenomena in Semiconductors, Santa Barbara (CA), July 21 - 23. To be published in the Journal of Superconductivity. 7 pages, 2 figure

Quantum Well Based on Graphene and Narrow-Gap Semiconductors

We consider the energy spectrum of the planar quantum well which consisted of two ribbons of narrow-gap semiconductors and a graphene ribbon between ones. It is shown that the gapless mode appears only in case of inverted narrow-gap semiconductors. Spin splitting of the energy spectrum for a nonsymmetric quantum well is calculated taking into account a specificity of graphene. We investigate interface states and optical transitions. It is shown that the optical transitions are possible only with a conservation of a parity.Comment: 13 pages, 2 figures, 1 tabl