J. Cayssol

Contact resistance and shot noise in graphene transistors

Potential steps naturally develop in graphene near metallic contacts. We investigate the influence of these steps on the transport in graphene field effect transistors. We give simple expressions to estimate the voltage-dependent contribution of the contacts to the total resistance and noise in the diffusive and ballistic regimes.

Floquet topological insulators

Topological insulators represent unique phases of matter with insulating bulk and conducting edge or surface states, immune to small perturbations such as backscattering due to disorder. This stems from their peculiar band structure, which provides topological protections. While conventional tools (pressure, doping etc.) to modify the band structure are available, time periodic perturbations can provide tunability by adding time as an extra dimension enhanced to the problem. In this short review, we outline the recent research on topological insulators in non-equilibrium situations. Firstly, we introduce briefly the Floquet formalism that allows to describe steady states of the electronic system with an effective time-independent Hamiltonian. Secondly, we summarize recent theoretical work on how light irradiation drives semi-metallic graphene or a trivial semiconducting system into a topological phase. Finally, we show how photons can be used to probe topological edge or surface states. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Quasiballistic transport of Dirac fermions in a Bi2Se3 nanowire.

Quantum coherent transport of surface states in a mesoscopic nanowire of the three-dimensional topological insulator Bi(2}Se(3) is studied in the weak-disorder limit. At very low temperatures, many harmonics are evidenced in the Fourier transform of Aharonov-Bohm oscillations, revealing the long phase coherence length of spin-chiral Dirac fermions. Remarkably, from their exponential temperature dependence, we infer an unusual 1/T power law for the phase coherence length L(φ)(T). This decoherence is typical for quasiballistic fermions weakly coupled to their environment.

Spin-orbit effects in a graphene bipolar pn junction

A graphene pn junction is studied theoretically in the presence of both intrinsic and Rashba spin-orbit couplings. We show that a crossover from perfect reflection to perfect transmission is achieved at normal incidence by tuning the perpendicular electric field. By further studying angular-dependent transmission, we demonstrate that perfect reflection at normal incidence can be clearly distinguished from trivial band gap effects. We also investigate how spin-orbit effects modify the conductance and the Fano factor associated with a potential step in both nn and np cases.

Probing the helical edge states of a topological insulator by Cooper-pair injection

We consider the proximity effect between a singlet

Josephson coupling through ferromagnetic heterojunctions with noncollinear magnetizations

s

Diffusion at the surface of topological insulators

-wave superconductor and the edge of a quantum spin Hall (QSH) topological insulator. We establish that Andreev reflection at a QSH edge state/superconductor interface is perfect while nonlocal Andreev processes through the superconductor are totally suppressed. We compute the corresponding conductance and noise.

Nonsinusoidal current-phase relation in strongly ferromagnetic and moderately disordered SFS junctions

We study the Josephson effect in clean heterojunctions that consist of superconductors connected through two metallic ferromagnets with insulating interfaces. We solve the scattering problem based on the Bogoliubov-de Gennes equation for arbitrary relative orientation of in-plane magnetizations, transparency of interfaces, and mismatch of Fermi wave vectors. Both spin-singlet and -triplet superconducting correlations are taken into account, and the Josephson current is calculated as a function of thickness of ferromagnetic layers and of the angle

Persistent currents in Dirac fermion rings

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Interfacial charge and spin transport in Z2 topological insulators

between their magnetizations. For relatively weak ferromagnets, we find that the critical Josephson current