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Quantum effects in molecularly-tailored magnetic graphene
Lapo Bogani - Department of Materials, University of Oxford, Oxford, UK
Mercredi 20/02/2019, 11h00-12h00
SPEC Salle Itzykson, Bât.774, Orme des Merisiers

Controlling the dynamics of spins in carbon nanosystems is pivotal to the design of spintronic and quantum computing devices. Graphene, a single-layer network of carbon atoms, shows outstanding electrical and mechanical properties, and graphene ribbons with nanometer-scale widths should exhibit half-metallicity, quantum confinement and edge effects. Magnetic edges in graphene nanoribbons have undergone intense theoretical scrutiny, because their coherent manipulation would be a milestone for spintronic and quantum computing devices. Experimental investigations are however hampered by the fact that most nanoribbons do not have the required atomic control of the edges, and that the proposed graphene terminations are chemically unstable. Several questions remain unsolved: how can molecular spins be assembled into hybrid structures? What is the influence of the graphene environment on the spin? Can molecules be used to control coherent currents in graphene devices? Here we try to provide an answer to these questions, exploring spin-graphene interactions by using molecular magnetic materials.

Here we show our results using bottom-up shaping of molecular graphene quantum systems. We then show that, while the static spin response remains unaltered, the quantum spin dynamics and associated selection rules are profoundly modulated. The couplings to graphene phonons, to other spins, and to Dirac fermions are quantified using a newly-developed model. Coupling to Dirac electrons introduces a dominant quantum-relaxation channel that, by driving the spins over Villain's threshold, gives rise to fully-coherent, resonant spin tunneling.[1] We then show how graphene nanoribbons made via molecular routes can be functionalized to create almost-ideal magnetic structures to test a decade of theoretical work. We observe the predicted delocalized magnetic edge states, and comparison with a non-graphitized reference material allows clear identification of fingerprint behaviours. We quantify the spin-orbit coupling parameters, define the interaction patterns, and unravel the spin decoherence channels. Even without any optimization, the spin coherence time is in the µs range at room temperature, and we perform quantum inversion operations between edge and radical spins.[2]

Eventually we show how molecular synthesis can lead to the rational, precise inclusion of five-membered sits in graphene dots, thus realizing one of the earliest theoretical concepts for magnetic functionalization of graphene. We show that coherence times of almost ms can be achieved above liquid nitrogen temperature, and we discuss the effect of nuclear bath and the environment.[3]

[1] C. Cervetti et al., Nature Materials, 2016, doi:10.1038/nmat4490;

[2] M. Slota et al., Nature, 2018, https://doi.org/10.1038/s41586-018-0154-7

[3] F. Lombardi et al., in preparation


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