Browsing by Author "Shen, Cheng"

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    Interlayer exciton complexes in bilayer MoS2
    (American Physical Society, 2022-01-15) Zhao, Yanchong; Du, Luojun; Yang, Shiqi; Tian, Jinpeng; Li, Xiaomei; Shen, Cheng; Tang, Jian; Chu, Yanbang; Watanabe, Kenji; Taniguchi, Takashi; Yang, Rong; Shi, Dongxia; Sun, Zhipei; Ye, Yu; Yang, Wei; Zhang, Guangyu; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; CAS - Institute of Physics; Peking University; National Institute for Materials Science
    Atomically thin transition metal dichalcogenides can show a rich variety of bound exciton complex states, such as trions, biexcitons, Fermi polarons, and phonon replicas, because of the reduced dielectric screening and enhanced Coulomb interaction. To date, studies have mainly focused on the complexes of intralayer excitons, while the electrically tunable interlayer exciton (IX) complexes remain elusive. Here, we report the observation of IX complexes with large out-of-plane electric dipole, strong emission intensity, and giant valley responses in bilayer MoS2, through on-resonance photoluminescence spectroscopy. In sharp contrast to the small, positive circular dichroism of intralayer excitons, the circular polarization of IX complexes in bilayer MoS2 can basically reach the theoretical limit (100%) but is negative. Such highly unusual light-valley responses of IX complexes in bilayer MoS2 demonstrate the strongly suppressed valley depolarization and spin-preserving scattering of electrons during the formation. Remarkably, by breaking the time-reversal symmetry with an out-of-plane magnetic field, a record level of spontaneous valley polarization (7.7%/Tesla) is identified for IX complexes in bilayer MoS2. The giant valley polarization of IX complexes in bilayer MoS2, together with the feasibility of electrical/optical/magnetic control, provides a firm basis for the development of next-generation electronic and optoelectronic applications with valley functionalities.
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    Observation of logarithmic Kohn anomaly in monolayer graphene
    (American Physical Society, 2020-10-23) Zhao, Yanchong; Du, Luojun; Yang, Wei; Shen, Cheng; Tang, Jian; Li, Xiaomei; Chu, Yanbang; Tian, Jinpeng; Watanabe, Kenji; Taniguchi, Takashi; Yang, Rong; Shi, Dongxia; Sun, Zhipei; Zhang, Guangyu; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; Chinese Academy of Sciences; National Institute for Materials Science
    Electron-phonon coupling in monolayer graphene breaks the adiabatic Born-Oppenheimer approximation and could lead to exotic logarithmic Kohn anomaly, manifested as logarithmic singularity in optical-phonon energy. However, unraveling unambiguously the fascinating logarithmic Kohn anomaly in monolayer graphene remains challenging due to the large carrier inhomogeneity originating from the unique massless Dirac-like band dispersion and the underneath substrate doping effect. Here we demonstrate a clear signature of intriguing logarithmic Kohn anomaly in monolayer graphene with ultralow carrier inhomogeneity via h-BN encapsulation. Significantly, the magnitude of anomalous phonon softening at 25 K shows an enhancement factor of 2 as compared to that previously observed in bilayer graphene at 12 K, even though bilayer graphene with nearly parabolic band dispersion is more immune to charged impurities. The uncovered unusual logarithmic Kohn anomaly in monolayer graphene can provide a firm basis for the understanding of various peculiar physics and may shed light on the nature of superconductivity in magic-angle graphene superlattices.