Strain-induced switching of magnetoresistance and perfect spin-valley filtering in graphene

We study spin-dependent valley currents and magnetoresistance (TMR) in double magnetic layer graphene-based N/F₁/St/F₂/N junction where N, F_1,2 and St are normal, ferromagnetic and strain-engineered graphene, respectively. Local strain in the St region causes a pseudo-vector potential with the same magnitude for the K- and K′-valleys but different sign, leading to valley polarization when applying real vector potential into the junction. The F layers cause the Zeeman field for controlling spin current and can be orientated either parallel (P) or anti parallel (AP). As an effect of the interplay of the Zeeman field and the strain field, the current in the junction is split into four current groups, I_k↑, I_k↓, I_k′↑ and I_k′↓, called spin-valley currents. We find that, the interplay of the Zeeman and strain field causes a perfect spin-valley filtering in the angular space only for the P configuration. Large TMR and switching of TMR triggered by a very small strain are also predicted. Our work reveals the potential of the interplay of the Zeeman and the strain field for application of spin-valley-based nanoelectronics and the switching effect induced by a very small strain should be applicable for strain sensor device.     

Spin currents in graphene under tension

Based on the Tight-Binding model, we have asymmetric massless Dirac fermions as the carriers in graphene under tension. Because of uniaxial strain, the velocities of Dirac fermions depend on their directions. This work studies the effect of the uniaxial strain on the spin transport through a single magnetic barrier of the strained graphene system. The result shows that graphene has a great potential for applications in nano-mechanical spintronic devices. This is because of strain in graphene can induce the spin-dependent pseudo-potentials at the barrier to control the spin currents of the junction.     

Tunneling conductance in a gapped graphene-based superconducting structure: Case of massive Dirac electrons

The tunneling conductance in a NG/SG graphene junction in which the graphene was grown on a SiC substrate is simulated. The carriers in the normal graphene (NG) and the superconducting graphene (SG) are treated as massive relativistic particles. It is assumed that the Fermi energy in the NG and SG are E_FN400 meV and E_FS400 meV+U, respectively. Here U is the electrostatic potential from the superconducting gate electrode. It is seen that the Klein tunneling disappears in the case where a gap exist in the energy spectrum. As U→∞, the zero bias normalized conductance becomes persistent at a minimal value of G/G₀1.2. The normalized conductance G/G₀ is found to depend linearly on U with constant slope of , where is the size of the gap Δ opening up in the energy spectrum of the graphene grown on the SiC substrate. It is found that G/G₀2+αU for potentials in the range −270 meV<U<0 meV and G=0 for potentials U<−270 meV. As α→∞, the conductance for eV=Δ (V is the bias voltage placed across the NG/SG junction) can be approximated by a unit step function G(eV=Δ,U)/G₀2Θ(U). This last behavior indicates that a NG/SG junction made with gapped graphene could be used as a nano switch having excellent characteristics.     

Dirac quasiparticle tunneling in a NG/ferromagnetic barrier/SG graphene junction

We study the tunneling conductance in a spin dependent barrier NG/F_B/SG graphene junction, where NG, F_B and SG are normal graphene, gate ferromagnetic graphene barrier with thickness d and the graphene s-wave superconductor, respectively. In our work, the quasiparticle scattering process at the interfaces is based on quasi particles governed by the Dirac Bogoliubov–de Gennes equation with effective speed of light v_F ∼ 10⁶ m/s. The conductance of the junction is calculated based on Blonder–Tinkham–Klapwijk (BTK) formalism. The oscillatory conductance under varying gate potential and exchange energy in F_B and the conductance induced by specular Andreev reflection are studied. By taking into account both effects of barrier strengths due to the gate potential χ_G∼V_Gd/?v_F${\chi }_{\text{G}}\sim V_{\text{G}}d/?v_{\text{F}}$ and the exchange energy χ_ex∼E_exd/?v_F${\chi }_{\text{ex}}\sim E_{\text{ex}}d/?v_{\text{F}}$ in the F_B region, we find that the zero bias conductance of junction depends only on the ferromagnetic barrier strength χ_ex in F_B, when the Fermi energy in SG is very much larger than that the Fermi energy in NG (E_FS ? E_FN). The oscillatory conductance peaks can be controlled by either varying χ_ex or χ_G. In the limiting case, by setting E_ex = 0, the conductance in a NG/N_B/SG graphene junction, where SG is the s-wave superconductor, is also studied in order to compare with two earlier contradicted data. Our result agrees with what was obtained by Linder and Sudbo [J. Linder, A. Sudbo, Phys. Rev. B 77 (2008) 64507], which confirms the contradiction to what was given by Bhattacharjee and Sengupta [S. Bhattacharjee, K. Sengupta, Phys. Rev. Lett. 97 (2006) 217001].     

Similarities between normal- and super-currents in topological insulator magnetic tunnel junctions

This work compares the normal-current in a NM/Fi/NM junction with the super-current in a SC/Fi/SC junction, where both are topological insulator systems. NM and Fi are normal region and ferromagnetic region of thickness d with exchange energy m playing a role of the mass of the Dirac electrons and with the gate voltage V_G, respectively. SC is superconducting region induced by a s-wave superconductor. We show that, interestingly, the critical super-current passing through a SC/Fi/SC junction behaves quite similar to the normal-current passing through a NM/Fi/NM junction. The normal-current and super-current exhibit N-peak oscillation, found when currents are plotted as a function of the magnetic barrier strength χ ∼ md  /?$?$v_F. With the barrier strength Z ∼ V_Gd  /?$?$v_F, the number of peaks N is determined through the relation Z ∼ Nπ + σπ (with 0 < σ?1$\sigma ?1$ for χ < Z). The normal- and the super-currents also exhibit oscillating with the same height for all of peaks, corresponding to the Dirac fermion tunneling behavior. These anomalous oscillating currents due to the interplay between gate voltage and magnetic field in the barrier were not found in graphene-based NM/Fi/NM and SC/Fi/SC junctions. This is due to the different magnetic effect between the Dirac fermions in topological insulator and graphene.