Monolayer graphene with an energy gap presents a pseudospin symmetry broken ferromagnet with a perpendicular pseudomagnetization whose direction is switched by altering the type of doping between n and p. We demonstrate an electrical current switching effect in pseudospin version of a spin valve in which two pseudoferromagnetic regions are contacted through a normal graphene region. The proposed structure exhibits a pseudomagnetoresistance, defined as the relative difference of resistances of parallel and antiparallel alignments of the pseudomagnetizations, which can be tuned to unity. This perfect pseudomagnetic switching is found to show a strong robustness with respect to increasing of the contact length, the effect which we explain in terms of an unusually long range penetration of an equilibrium pseudospin polarization into the normal region by proximity to a pseudoferromagnet. Our results reveals the potential of gapped graphene for realization of pseudospin-based nanoelectronics.Comment: 6 pages, 5 figure
Andreev reflection in a monolayer molybdenum disulfide superconducting-normal (S/N) hybrid junction is investigated. We find, by using a modified-Dirac Hamiltonian and the scattering formalism, that the perfect Andreev reflection happens at normal incidence with p-doped S and N regions. The probability of the Andreev reflection and the resulting Andreev conductance, in this system, are demonstrated to be large in comparison with the corresponding gapped graphene structure. We further investigate the effect of a topological term (β) in the Hamiltonian and show that it results in an enhancement of the Andreev conductance with p-doped S and N regions, while in the corresponding structure with n-doped S region it is strongly reducible in comparison. This effect can be explained in terms of the dependence of the Andreev reflection probability on the sign of β and the chemical potential in the superconducting region.
We propose a hole-doped molybdenum disulfide (MoS2) superconducting spin valve (F/S/F) hybrid structure in which the Andreev reflection process is suppressed for all incoming waves with a determined range of the chemical potential in ferromagnetic (F) region and the cross-conductance in the right F region depends crucially on the configuration of magnetizations in the two F regions. Using the scattering formalism, we find that the transport is mediated purely by elastic electron cotunneling (CT) process in a parallel configuration and changes to the pure crossed Andreev reflection (CAR) process in the low-energy regime, without fixing of a unique parameter, by reversing the direction of magnetization in the right F region. This suggests both valley-and spin-switch effects between the perfect elastic CT and perfect CAR processes and makes the nonlocal charge current to be fully valley-and spin-polarized inside the right F region where the type of the polarizations can be changed by reversing the magnetization direction in the right F region. We further demonstrate that the presence of the strong spin-orbit interaction λ and an additional topological term (β) in the Hamiltonian of MoS2 result in an enhancement of the charge conductance of the CT and CAR processes and make them to be present for long lengths of the superconducting region. Besides, we find that the thermal conductance of the structure with a small length of the highly doped superconducting region exhibits linear dependence on the temperature at low temperatures whereas it enhances exponentially at higher temperatures. In particular, we demonstrate that the thermal conductance versus the strength of the exchange field (h) in F region displays a maximum value at h < λ, which moves towards larger exchange fields by increasing the temperature.
We theoretically reveal the possibility of specular Andreev reflection in a thin film topological insulator normal-superconductor (N/S) junction in the presence of a gate electric field. The probability of specular Andreev reflection increases with the electric field, and electron-hole conversion with unit efficiency happens in a wide experimentally accessible range of the electric field. We show that perfect specular Andreev reflection can occur for all angles of incidence with a particular excitation energy value. In addition, we find that the thermal conductance of the structure displays exponential dependence on the temperature. Our results reveal the potential of the proposed topological insulator thin-film-based N/S structure for the realization of intraband specular Andreev reflection.
We theoretically demonstrate unusual features of superconducting proximity effect in gapped graphene which presents a pseudospin symmetry-broken ferromagnet with a net pseudomagnetization. We find that the presence of a band gap makes the Andreev conductance of graphene superconductor/pseudoferromagnet (S/PF) junction to behave similar to that of a graphene ferromagnet-superconductor junction. The energy gap $\Delta_N$ enhance the pseudospin inverted Andreev conductance of S/PF junction to reach a limiting maximum value for $\Delta_N\gg \mu$, which depending on the bias voltage can be larger than the value for the corresponding junction with no energy gap. We further demonstrate a damped-oscillatory behavior for the local density of states of the PF region of S/PF junction and a long-range crossed Andreev reflection process in PF/S/PF structure with antiparallel alignment of pseudomagnetizations of PFs, which confirm that, in this respect, the gapped normal graphene behaves like a ferromagnetic graphene.Comment: 7.2 pages, 5 figures, accepted for publication in Phys. Rev.
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