Browsing by Author "Ke, CT"
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Item Open Access Anomalous periodicity of magnetic interference patterns in encapsulated graphene Josephson junctions(Physical Review Research) Ke, CT; Draelos, AW; Seredinski, A; Wei, MT; Li, H; Hernandez-Rivera, M; Watanabe, K; Taniguchi, T; Yamamoto, M; Tarucha, S; Bomze, Y; Borzenets, IV; Amet, F; Finkelstein, GItem Open Access Ballistic Graphene Josephson Junctions from the Short to the Long Junction Regimes.(Physical review letters, 2016-12-02) Borzenets, IV; Amet, F; Ke, CT; Draelos, AW; Wei, MT; Seredinski, A; Watanabe, K; Taniguchi, T; Bomze, Y; Yamamoto, M; Tarucha, S; Finkelstein, GWe investigate the critical current I_{C} of ballistic Josephson junctions made of encapsulated graphene-boron-nitride heterostructures. We observe a crossover from the short to the long junction regimes as the length of the device increases. In long ballistic junctions, I_{C} is found to scale as ∝exp(-k_{B}T/δE). The extracted energies δE are independent of the carrier density and proportional to the level spacing of the ballistic cavity. As T→0 the critical current of a long (or short) junction saturates at a level determined by the product of δE (or Δ) and the number of the junction's transversal modes.Item Open Access Chiral quasiparticle tunneling between quantum Hall edges in proximity with a superconductor(Physical Review B, 2019-09-10) Wei, MT; Draelos, AW; Seredinski, A; Ke, CT; Li, H; Mehta, Y; Watanabe, K; Taniguchi, T; Yamamoto, M; Tarucha, S; Finkelstein, G; Amet, F; Borzenets, IV© 2019 American Physical Society. We study a two-terminal graphene Josephson junction with contacts shaped to form a narrow constriction, less than 100nm in length. The contacts are made from type-II superconducting contacts and able to withstand magnetic fields high enough to reach the quantum Hall regime in graphene. In this regime, the device conductance is determined by edge states, plus the contribution from the constricted region. In particular, the constriction area can support supercurrents up to fields of ∼2.5T. Additionally, enhanced conductance is observed through a wide range of magnetic fields and gate voltages. This additional conductance and the appearance of supercurrent is attributed to the tunneling between counterpropagating quantum Hall edge states along opposite superconducting contacts.Item Open Access Doping-driven structural phase transition and loss of superconductivity in MxFe1-xSe delta (M=Mn, Cu)(2010) Huang, TW; Chen, TK; Yeh, KW; Ke, CT; Chen, CL; Huang, YL; Hsu, FC; Wu, MK; Wu, PM; Avdeev, M; Studer, AJIn this paper, we report the results of detailed studies on Mn and Cu substitution to Fe site of beta-FeSe, namely, MnxFe(1-x)Se(1-delta) and CuxFe1-xSe1-delta (delta equals to 0.03-0.05 based on our neutron-diffraction refinements). The results show that with only 10 at. % Cu doping the compound becomes a Mott insulator. Detailed temperature-dependent structural analyses of these Mn- and Cu-substituted compounds show that the structural transition, which is associated with the changes in the building block FeSe4 tetrahedron, is essential to the occurrence of superconductivity in beta-FeSe.Item Open Access Nonstoichiometry of LixCu2O2+delta single crystal and its relation to magnetic ordering(2010) Yeh, KW; Huang, TW; Ke, CT; Wu, PM; Zhao, L; Chao, WH; Lee, YC; Chen, CL; Wu, MKWe report the first systematic study on the nonstoichiometry of Li/Cu ionic ratios x and oxygen defect concentrations delta in LixCu2O2+delta solid solution. Nonstoichiometric crystals with intentional Li-deficient (x=0.97) or Li-excessive (x=1.06) concentrations were prepared by slow-cooling and floating-zone methods. By applying different oxygen pressures during growth, different magnitudes of oxygen defects were obtained, revealing varied magnetic and electrical properties. Our results strongly suggest that Li/Cu and oxygen nonstoichiometry in these crystals are responsible for the varied temperatures of helimagnetic ordering. (C) 2010 American Institute of Physics. [doi:10.1063/1.3483244]Item Open Access Subkelvin lateral thermal transport in diffusive graphene(Physical Review B, 2019-03-29) Draelos, AW; Silverman, A; Eniwaye, B; Arnault, EG; Ke, CT; Wei, MT; Vlassiouk, I; Borzenets, IV; Amet, F; Finkelstein, G© 2019 American Physical Society. In this work, we report on hot carrier diffusion in graphene across large enough length scales that the carriers are not thermalized across the crystal. The carriers are injected into graphene at one site and their thermal transport is studied as a function of applied power and distance from the heating source, up to tens of micrometers away. Superconducting contacts prevent out-diffusion of hot carriers to isolate the electron-phonon coupling as the sole channel for thermal relaxation. As local thermometers, we use the amplitude of the universal conductance fluctuations, which varies monotonically as a function of temperature. By measuring the electron temperature simultaneously along the length we observe a thermal gradient which results from the competition between electron-phonon cooling and lateral heat flow.Item Open Access Supercurrent in Graphene Josephson Junctions with Narrow Trenches in the Quantum Hall Regime(MRS Advances, 2018-01-01) Seredinski, A; Draelos, A; Wei, MT; Ke, CT; Fleming, T; Mehta, Y; Mancil, E; Li, H; Taniguchi, T; Watanabe, K; Tarucha, S; Yamamoto, M; Borzenets, IV; Amet, F; Finkelstein, G© 2018 Materials Research Society. Coupling superconductors to quantum Hall edge states is the subject of intense investigation as part of the ongoing search for non-abelian excitations. Our group has previously observed supercurrents of hundreds of picoamperes in graphene Josephson junctions in the quantum Hall regime. One of the explanations of this phenomenon involves the coupling of an electron edge state on one side of the junction to a hole edge state on the opposite side. In our previous samples, these states are separated by several microns. Here, a narrow trench perpendicular to the contacts creates counterpropagating quantum Hall edge channels tens of nanometres from each other. Transport measurements demonstrate a change in the low-field Fraunhofer interference pattern for trench devices and show a supercurrent in both trench and reference junctions in the quantum Hall regime. The trench junctions show no enhancement of quantum Hall supercurrent and an unexpected supercurrent periodicity with applied field, suggesting the need for further optimization of device parameters.Item Open Access Supercurrent in the quantum Hall regime.(Science (New York, N.Y.), 2016-05) Amet, F; Ke, CT; Borzenets, IV; Wang, J; Watanabe, K; Taniguchi, T; Deacon, RS; Yamamoto, M; Bomze, Y; Tarucha, S; Finkelstein, GA promising route for creating topological states and excitations is to combine superconductivity and the quantum Hall (QH) effect. Despite this potential, signatures of superconductivity in the QH regime remain scarce, and a superconducting current through a QH weak link has been challenging to observe. We demonstrate the existence of a distinct supercurrent mechanism in encapsulated graphene samples contacted by superconducting electrodes, in magnetic fields as high as 2 tesla. The observation of a supercurrent in the QH regime marks an important step in the quest for exotic topological excitations, such as Majorana fermions and parafermions, which may find applications in fault-tolerant quantum computing.Item Open Access Universal Nonequilibrium I-V Curve at an Interacting Impurity Quantum Critical Point(arXiv, 2016-09) Zhang, G; Chung, C-H; Ke, CT; Lin, C-Y; Mebrahtu, H; Smirnov, AI; Finkelstein, G; Baranger, HUThe nonlinear I-V curve at an interacting quantum critical point (QCP) is typically out of reach theoretically. Here, however, we provide an analytical calculation of the I-V curve at a QCP under nonequilibrium conditions and, furthermore, present experimental results to which the theory is compared. The system is a quantum dot coupled to resistive leads: a spinless resonant level interacting with an ohmic electromagnetic environment. A two channel Kondo like QCP occurs when the level is on resonance and symmetrically coupled to the leads. Though similar to a resonant level in a Luttinger liquid, a key difference enables us to obtain the current at finite temperature and bias: because there are modes that do not initially couple to the environment, an analysis in terms of weak backscattering of non-interacting fermions coupled to a modified environment is possible. Drawing on dynamical Coulomb blockade theory, we then obtain an analytical expression for the nonlinear I-V curve. The agreement between our theoretical and experimental results is remarkable.