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Lee, Seung Hwan

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Seung Hwan

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Lee, Seung Hwan

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    High-Energy Quasiparticle Injection into Mesoscopic Superconductors
    (Springer Science and Business Media LLC, 2021-01-18) Alegria, Loren D.; Bøttcher, Charlotte G. L.; Saydjari, Andrew K.; Pierce, Andrew; Lee, Seung Hwan; Harvey, Shannon; Vool, Uri; Yacoby, Amir
    At nonzero temperatures, superconductors contain excitations known as Bogoliubov quasiparticles. The mesoscopic dynamics of quasiparticles inform the design of quantum information processors, among other devices. Knowledge of these dynamics stems from experiments in which quasiparticles are injected in a controlled fashion, typically at energies comparable to the pairing energy \cite{Levine1968,Smith1975,Ullom2000,Barends2008,Patel2017}. Here we perform tunnel spectroscopy of a mesoscopic superconductor under high electric field. We observe quasiparticle injection due to field-emitted electrons with $\mathbf{10^6}$ times the pairing energy, an unexplored regime of quasiparticle dynamics. Upon application of a gate voltage, the quasiparticle injection decreases the critical current and, at sufficiently high electric field, the field-emission current (< 0.1 nA) switches the mesoscopic superconductor into the normal state, consistent with earlier results \cite{DeSimoni2018}. We expect that high-energy injection will be useful for developing quasiparticle-tolerant quantum information processors, will allow rapid control of resonator quality factors, and will enable the design of electric-field-controlled superconducting devices with new functionality.
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    Fractional Chern Insulators in Magic-Angle Twisted Bilayer Graphene
    (Springer Science and Business Media LLC, 2021-12-15) Xie, Yonglong; Pierce, Andrew; Park, Jeong Min; Parker, Daniel E.; Khalaf, Eslam; Ledwith, Patrick; Cao, Yuan; Lee, Seung Hwan; Chen, Shaowen; Forrester, Patrick R.; Watanabe, Kenji; Taniguchi, Takashi; Vishwanath, Ashvin; Jarillo-Herrero, Pablo; Yacoby, Amir
    AbstractFractional Chern insulators (FCIs) are lattice analogues of fractional quantum Hall states that may provide a new avenue towards manipulating non-Abelian excitations. Early theoretical studies1–7 have predicted their existence in systems with flat Chern bands and highlighted the critical role of a particular quantum geometry. However, FCI states have been observed only in Bernal-stacked bilayer graphene (BLG) aligned with hexagonal boron nitride (hBN)8, in which a very large magnetic field is responsible for the existence of the Chern bands, precluding the realization of FCIs at zero field. By contrast, magic-angle twisted BLG9–12 supports flat Chern bands at zero magnetic field13–17, and therefore offers a promising route towards stabilizing zero-field FCIs. Here we report the observation of eight FCI states at low magnetic field in magic-angle twisted BLG enabled by high-resolution local compressibility measurements. The first of these states emerge at 5 T, and their appearance is accompanied by the simultaneous disappearance of nearby topologically trivial charge density wave states. We demonstrate that, unlike the case of the BLG/hBN platform, the principal role of the weak magnetic field is merely to redistribute the Berry curvature of the native Chern bands and thereby realize a quantum geometry favourable for the emergence of FCIs. Our findings strongly suggest that FCIs may be realized at zero magnetic field and pave the way for the exploration and manipulation of anyonic excitations in flat moiré Chern bands.