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Subject: experimental study × WGL Institute: IKZ ×

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    Gate controlled valley polarizer in bilayer graphene
    ([London] : Nature Publishing Group UK, 2020) Chen, Hao; Zhou, Pinjia; Liu, Jiawei; Qiao, Jiabin; Oezyilmaz, Barbaros; Martin, Jens
    Sign reversal of Berry curvature across two oppositely gated regions in bilayer graphene can give rise to counter-propagating 1D channels with opposite valley indices. Considering spin and sub-lattice degeneracy, there are four quantized conduction channels in each direction. Previous experimental work on gate-controlled valley polarizer achieved good contrast only in the presence of an external magnetic field. Yet, with increasing magnetic field the ungated regions of bilayer graphene will transit into the quantum Hall regime, limiting the applications of valley-polarized electrons. Here we present improved performance of a gate-controlled valley polarizer through optimized device geometry and stacking method. Electrical measurements show up to two orders of magnitude difference in conductance between the valley-polarized state and gapped states. The valley-polarized state displays conductance of nearly 4e2/h and produces contrast in a subsequent valley analyzer configuration. These results pave the way to further experiments on valley-polarized electrons in zero magnetic field.
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    Violation of a Leggett-Garg inequality with ideal non-invasive measurements
    (London : Nature Publishing Group, 2012) Knee, G.C.; Simmons, S.; Gauger, E.M.; Morton, J.J.L.; Riemann, H.; Abrosimov, N.V.; Becker, P.; Pohl, H.-J.; Itoh, K.M.; Thewalt, M.L.W.; Briggs, G.A.D.; Benjamin, S.C.
    The quantum superposition principle states that an entity can exist in two different states simultaneously, counter to our 'classical' intuition. Is it possible to understand a given system's behaviour without such a concept? A test designed by Leggett and Garg can rule out this possibility. The test, originally intended for macroscopic objects, has been implemented in various systems. However to date no experiment has employed the 'ideal negative result' measurements that are required for the most robust test. Here we introduce a general protocol for these special measurements using an ancillary system, which acts as a local measuring device but which need not be perfectly prepared. We report an experimental realization using spin-bearing phosphorus impurities in silicon. The results demonstrate the necessity of a non-classical picture for this class of microscopic system. Our procedure can be applied to systems of any size, whether individually controlled or in a spatial ensemble.