Browsing by Author "Taniguchi, Takashi"

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    Interference of chiral Andreev edge states
    (Nature Physics, 2020-08-01) Zhao, Lingfei; Arnault, Ethan G; Bondarev, Alexey; Seredinski, Andrew; Larson, Trevyn; Draelos, Anne W; Li, Hengming; Watanabe, Kenji; Taniguchi, Takashi; Amet, François; Baranger, Harold U; Finkelstein, Gleb
    © 2020, The Author(s), under exclusive licence to Springer Nature Limited. The search for topological excitations such as Majorana fermions has spurred interest in the boundaries between distinct quantum states. Here, we explore an interface between two prototypical phases of electrons with conceptually different ground states: the integer quantum Hall insulator and the s-wave superconductor. We find clear signatures of hybridized electron and hole states similar to chiral Majorana fermions, which we refer to as chiral Andreev edge states (CAESs). These propagate along the interface in the direction determined by the magnetic field and their interference can turn an incoming electron into an outgoing electron or hole, depending on the phase accumulated by the CAESs along their path. Our results demonstrate that these excitations can propagate and interfere over a significant length, opening future possibilities for their coherent manipulation.
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    Loss and Decoherence at the Quantum Hall-Superconductor Interface.
    (Physical review letters, 2023-10) Zhao, Lingfei; Iftikhar, Zubair; Larson, Trevyn FQ; Arnault, Ethan G; Watanabe, Kenji; Taniguchi, Takashi; Amet, François; Finkelstein, Gleb
    We perform a systematic study of Andreev conversion at the interface between a superconductor and graphene in the quantum Hall (QH) regime. We find that the probability of Andreev conversion from electrons to holes follows an unexpected but clear trend: the dependencies on temperature and magnetic field are nearly decoupled. We discuss these trends and the role of the superconducting vortices, whose normal cores could both absorb and dephase the individual electrons in a QH edge. Our Letter may pave the road to engineering a future generation of hybrid devices for exploiting superconductivity proximity in chiral channels.
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    Multiterminal Inverse AC Josephson Effect.
    (Nano letters, 2021-11-15) Arnault, Ethan G; Larson, Trevyn FQ; Seredinski, Andrew; Zhao, Lingfei; Idris, Sara; McConnell, Aeron; Watanabe, Kenji; Taniguchi, Takashi; Borzenets, Ivan; Amet, François; Finkelstein, Gleb
    When a Josephson junction is exposed to microwave radiation, it undergoes the inverse AC Josephson effect─the phase of the junction locks to the drive frequency. As a result, the I-V curves of the junction acquire "Shapiro steps" of quantized voltage. If the junction has three or more superconducting contacts, coupling between different pairs of terminals must be taken into account and the state of the junction evolves in a phase space of higher dimensionality. Here, we study the multiterminal inverse AC Josephson effect in a graphene sample with three superconducting terminals. We observe robust fractional Shapiro steps and correlated switching events, which can only be explained by considering the device as a completely connected Josephson network. We successfully simulate the observed behaviors using a modified two-dimensional RCSJ model. Our results suggest that multiterminal Josephson junctions are a playground to study highly connected nonlinear networks with novel topologies.
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    Quantum Hall-based superconducting interference device.
    (Science Advances, 2019-09-13) Seredinski, Andrew; Draelos, Anne W; Arnault, Ethan G; Wei, Ming-Tso; Li, Hengming; Fleming, Tate; Watanabe, Kenji; Taniguchi, Takashi; Amet, François; Finkelstein, Gleb
    We present a study of a graphene-based Josephson junction with dedicated side gates carved from the same sheet of graphene as the junction itself. These side gates are highly efficient and allow us to modulate carrier density along either edge of the junction in a wide range. In particular, in magnetic fields in the 1- to 2-T range, we are able to populate the next Landau level, resulting in Hall plateaus with conductance that differs from the bulk filling factor. When counter-propagating quantum Hall edge states are introduced along either edge, we observe a supercurrent localized along that edge of the junction. Here, we study these supercurrents as a function of magnetic field and carrier density.
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    Supercurrent Flow in Multiterminal Graphene Josephson Junctions.
    (Nano letters, 2019-02) Draelos, Anne W; Wei, Ming-Tso; Seredinski, Andrew; Li, Hengming; Mehta, Yash; Watanabe, Kenji; Taniguchi, Takashi; Borzenets, Ivan V; Amet, François; Finkelstein, Gleb
    We investigate the electronic properties of ballistic planar Josephson junctions with multiple superconducting terminals. Our devices consist of monolayer graphene encapsulated in boron nitride with molybdenum-rhenium contacts. Resistance measurements yield multiple resonant features, which are attributed to supercurrent flow among adjacent and nonadjacent Josephson junctions. In particular, we find that superconducting and dissipative currents coexist within the same region of graphene. We show that the presence of dissipative currents primarily results in electron heating and estimate the associated temperature rise. We find that the electrons in encapsulated graphene are efficiently cooled through the electron-phonon coupling.
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    Zero Crossing Steps and Anomalous Shapiro Maps in Graphene Josephson Junctions.
    (Nano letters, 2020-10) Larson, Trevyn FQ; Zhao, Lingfei; Arnault, Ethan G; Wei, Ming-Tso; Seredinski, Andrew; Li, Henming; Watanabe, Kenji; Taniguchi, Takashi; Amet, François; Finkelstein, Gleb
    The AC Josephson effect manifests itself in the form of "Shapiro steps" of quantized voltage in Josephson junctions subject to radiofrequency (RF) radiation. This effect presents an early example of a driven-dissipative quantum phenomenon and is presently utilized in primary voltage standards. Shapiro steps have also become one of the standard tools to probe junctions made in a variety of novel materials. Here we study Shapiro steps in a widely tunable graphene-based Josephson junction in which the high-frequency dynamics is determined by the on-chip environment. We investigate the variety of patterns that can be obtained in this well-understood system depending on the carrier density, temperature, RF frequency, and magnetic field. Although the patterns of Shapiro steps can change drastically when just one parameter is varied, the overall trends can be understood and the behaviors straightforwardly simulated, showing some key differences from the conventional RCSJ model. The resulting understanding may help interpret similar measurements in more complex materials.