Cyril Petitjean

Semiclassical Gaps in the Density of States of Chaotic Andreev Billiards

The connection of a superconductor to a chaotic ballistic quantum dot leads to interesting phenomena, most notably the appearance of a hard gap in its excitation spectrum. Here we treat such an Andreev billiard semiclassically where the density of states is expressed in terms of the classical trajectories of electrons (and holes) that leave and return to the superconductor. We show how classical orbit correlations lead to the formation of the hard gap, as predicted by random matrix theory in the limit of negligible Ehrenfest time tau{E}, and how the influence of a finite tau{E} causes the gap to shrink. Furthermore, for intermediate tau{E} we predict a second gap below E=pi variant Plancks/2pi/2tau{E} which would presumably be the clearest signature yet of tau{E} effects.

Graphene-Based Heterojunction between Two Topological Insulators

Quantum Hall (QH) and quantum spin Hall (QSH) phases have very different edge states and, when going from one phase to the other, the direction of one edge state must be reversed. We study this phenomena in graphene in presence of a strong perpendicular magnetic field on top of a spin-orbit (SO) induced QSH phase. We show that, below the SO gap, the QSH phase is virtually unaffected by the presence of the magnetic field. Above the SO gap, the QH phase is restored. An electrostatic gate placed on top of the system allows to create a QSH-QH junction which is characterized by the existence of a spin-polarized chiral state, propagating along the topological interface. We find that such a setup naturally provides an extremely sensitive spin-polarized current switch.

Displacement echoes: Classical decay and quantum freeze

Motivated by neutron scattering experiments, we investigate the decay of the fidelity with which a wave packet is reconstructed by a perfect time-reversal operation performed after a phase-space displacement. In the semiclassical limit, we show that the decay rate is generically given by the Lyapunov exponent of the classical dynamics. For small displacements, we additionally show that, following a short-time Lyapunov decay, the decay freezes well above the ergodic value because of quantum effects. Our analytical results are corroborated by numerical simulations.

Interplay between nonequilibrium and equilibrium spin torque using synthetic ferrimagnets.

We discuss the current induced magnetization dynamics of spin valves F(0)|N|SyF where the free layer is a synthetic ferrimagnet SyF made of two ferromagnetic layers F(1) and F(2) coupled by RKKY exchange coupling. When the magnetic moment of the outer layer F(2) dominates the magnetization of the SyF, the sign of the effective spin torque exerted on the layer F(1) is controlled by the couplings strength: for weak coupling the spin torque tends to antialign F(1)s magnetization with respect to the pinned layer F(0). At large coupling the situation is reversed and tends to align F(1) with respect to F(0). At intermediate coupling, numerical simulations reveal that the competition between these two incompatible limits leads generically to spin torque oscillator (STO) behavior. The STO is found at zero magnetic field, with very significant amplitude of oscillations and frequencies up to 50 GHz or higher.

Dephasing in quantum chaotic transport: A semiclassical approach

We investigate the effect of dephasing/decoherence on quantum transport through open chaotic ballistic conductors in the semiclassical limit of small Fermi wavelength to system size ratio,

Lyapunov Generation of Entanglement and the Correspondence Principle

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Multiscale approach to spin transport in magnetic multilayers

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Quantum Reversibility and Echoes in Interacting Systems

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Weak localization with nonlinear bosonic matter waves

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