Person: Liu, Xiaomeng
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Xiaomeng
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Liu, Xiaomeng
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Publication Observation of the Dirac fluid and the breakdown of the Wiedemann-Franz law in graphene(American Association for the Advancement of Science (AAAS), 2016) Crossno, Jesse Dylan; Shi, Jing; Wang, Ke; Liu, Xiaomeng; Harzheim, Achim; Lucas, Andrew James; Sachdev, Subir; Kim, Philip; Taniguchi, T.; Watanabe, K.; Ohki, T. A.; Fong, K. C.Interactions between particles in quantum many-body systems can lead to collective behavior described by hydrodynamics. One such system is the electron-hole plasma in graphene near the charge neutrality point which can form a strongly coupled Dirac fluid. This charge neutral plasma of quasi-relativistic fermions is expected to exhibit a substantial enhancement of the thermal conductivity, due to decoupling of charge and heat currents within hydrodynamics. Employing high sensitivity Johnson noise thermometry, we report the breakdown of the Wiedemann-Franz law in graphene, with a thermal conductivity an order of magnitude larger than the value predicted by Fermi liquid theory. This result is a signature of the Dirac fluid, and constitutes direct evidence of collective motion in a quantum electronic fluid.Publication Quantum Hall drag of exciton condensate in graphene(Springer Nature, 2017) Liu, Xiaomeng; Watanabe, Kenji; Taniguchi, Takashi; Halperin, Bertrand; Kim, PhilipExciton condensate is a Bose-Einstein condensate (BEC) of electron and hole pairs bound by the Coulomb interaction1,2. In an electronic double layer (EDL) under strong magnetic fields, filled Landau states in one layer bind with empty states of the other layer to form exciton condensate3–9. Here we report exciton condensation in bilayer graphene EDL separated by hexagonal boron nitride (hBN). Driving current in one graphene layer generates a near-quantized Hall voltage in the other layer, signifying coherent exciton transport4,6. Owing to the strong Coulomb coupling across the atomically thin dielectric, quantum Hall drag in graphene appears at a temperature ten times higher than previously observed in GaAs EDL. The wide-range tunability of densities and displacement fields enables exploration of a rich phase diagram of BEC across Landau levels with different filling factors and internal quantum degrees of freedom. The observed robust exciton condensation opens up opportunities to investigate various many-body exciton phases.Publication Interlayer Fractional Quantum Hall Effect in a Coupled Graphene Double Layer(Springer Science and Business Media LLC, 2019-06-24) Liu, Xiaomeng; Hao, Zeyu; Watanabe, Kenji; Taniguchi, Takashi; Halperin, Bertrand; Kim, PhilipWhen a strong magnetic field is applied to a two-dimensional (2D) electron system, interactions between the electrons can cause fractional quantum Hall (FQH) effects. Bringing two 2D conductors close to each other, a new set of correlated states can emerge due to interactions between electrons in the same and opposite layers. Here we report interlayer correlated FQH states in a device consisting of two parallel graphene layers separated by a thin insulator. Current flow in one layer generates different quantized Hall signals in the two layers. This result is interpreted by composite fermion (CF) theory with different intralayer and interlayer Chern-Simons gauge-field coupling. We observe FQH states corresponding to integer values of CF Landau level (LL) filling in both layers, as well as "semi-quantized" states, where a full CF LL couples to a continuously varying partially filled CF LL. We also find a quantized state between two coupled half-filled CF LLs and attribute it to an interlayer CF exciton condensate.