Graphene on transition-metal dichalcogenides: a platform for proximity spin-orbit physics and optospintronics

Hybrids of graphene and two dimensional transition metal dichalcogenides (TMDC) have the potential to bring graphene spintronics to the next level. As we show here by performing first-principles calculations of graphene on monolayer MoS$_2$, there are several advantages of such hybrids over pristine graphene. First, Dirac electrons in graphene exhibit a giant global proximity spin-orbit coupling, without compromising the semimetallic character of the whole system at zero field. Remarkably, these spin-orbit effects can be very accurately described by a simple effective Hamiltonian. Second, the Fermi level can be tuned by a transverse electric field to cross the MoS$_2$ conduction band, creating a system of coupled massive and massles electron gases. Both charge and spin transport in such systems should be unique. Finally, we propose to use graphene/TMDC structures as a platform for optospintronics, in particular for optical spin injection into graphene and for studying spin transfer between TMDC and graphene.Comment: 7 pages, 6 figure

Five abstraction rules to remove transitions while preserving compositional synthesis results

This working paper investigates under which conditions transitions can be removed from an automaton while preserving important synthesis properties. The work is part of a framework for compositional synthesis of least restrictive controllable and nonblocking supervisors for modular discrete event systems. The method for transition removal complements previous results, which are largely focused on state merging. Issues concerning transition removal in synthesis are discussed, and redirection maps are introduced to enable a supervisor to process an event, even though the corresponding transition is no longer present in the model. Based on the results, different techniques are proposed to remove controllable and uncontrollable transitions, and an example shows the potential of the method for practical problems

Anisotropic optical properties of Fe/GaAs(001) nanolayers from first principles

We investigate the anisotropy of the optical properties of thin Fe films on GaAs(001) from first-principles calculations. Both intrinsic and magnetization-induced anisotropy are covered by studying the system in the presence of spin-orbit coupling and external magnetic fields. We use the linearized augmented plane wave method, as implemented in the WIEN2k density functional theory code, to show that the $C_{2v}$ symmetric anisotropy of the spin-orbit coupling fields at the Fe/GaAs(001) interface manifests itself in the corresponding anisotropy of the optical conductivity and the polar magneto-optical Kerr effect. While their magnetization-induced anisotropy is negligible, the intrinsic anisotropy of the optical properties is significant and reflects the underlying $C_{2v}$ symmetry of the Fe/GaAs(001) interface. This suggests that the effects of anisotropic spin-orbit coupling fields in experimentally relevant Fe/GaAs(001) slabs can be studied by purely optical means.Comment: 8 pages, 11 figure

Synthesis observation equivalence and weak synthesis observation equivalence

This working paper proposes an algorithm to simplify automata in such a way that compositional synthesis results are preserved in every possible context. It relaxes some requirements of synthesis observation equivalence from previous work, so that better abstractions can be obtained. The paper describes the algorithm, adapted from known bisimulation equivalence algorithms, for the improved abstraction method. The algorithm has been implemented in the DES software tool Supremica and has been used to compute modular supervisors for several large benchmark examples. It successfully computes modular supervisors for systems with more than 10¹² reachable states

Spin relaxation mechanism in graphene: resonant scattering by magnetic impurities

It is proposed that the observed small (100 ps) spin relaxation time in graphene is due to resonant scattering by local magnetic moments. At resonances, magnetic moments behave as spin hot spots: the spin-flip scattering rates are as large as the spin-conserving ones, as long as the exchange interaction is greater than the resonance width. Smearing of the resonance peaks by the presence of electron-hole puddles gives quantitative agreement with experiment, for about 1 ppm of local moments. While the local moments can come from a variety of sources, we specifically focus on hydrogen adatoms. We perform first-principles supercell calculations and introduce an effective Hamiltonian to obtain realistic input parameters for our mechanism.Comment: 5 pages, 3 figures + Suppl. material (3 pages, 5 figures

An algorithm for weak synthesis observation equivalence for compositional supervisor synthesis

This paper proposes an algorithm to simplify automata in such a way that compositional synthesis results are preserved in every possible context. It relaxes some requirements of synthesis observation equivalence from previous work, so that better abstractions can be obtained. The paper describes the algorithm, adapted from known bisimulation equivalence algorithms, for the improved abstraction method. The algorithm has been implemented in the DES software tool Supremica and has been used to compute modular supervisors for several large benchmark examples. It successfully computes modular supervisors for systems with more than 1012 reachable states

Partial unfolding for compositional nonblocking verification of extended finite-state machines

This working paper describes a framework for compositional nonblocking verification of reactive systems modelled as extended finite-state machines. The nonblocking property can capture the absence of livelocks and deadlocks in concurrent systems. Compositional verification is shown in previous work to be effective to verify this property for large discrete event systems. Here, these results are applied to extended finite-state machines communicating via shared memory. The model to be verified is composed gradually, simplifying components through abstraction at each step, while conflict equivalence guarantees that the final verification result is the same as it would have been for the non-abstracted model. The working paper concludes with an example showing the potential of compositional verification to achieve substantial state-space reduction