Browsing by Author "Tian, Jinpeng"

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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.
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    Spatially indirect intervalley excitons in bilayer WSe2
    (American Physical Society, 2022-01-15) Huang, Zhiheng; Zhao, Yanchong; Bo, Tao; Chu, Yanbang; Tian, Jinpeng; Liu, Le; Yuan, Yalong; Wu, Fanfan; Zhao, Jiaojiao; Xian, Lede; Watanabe, Kenji; Taniguchi, Takashi; Yang, Rong; Shi, Dongxia; Du, Luojun; Sun, Zhipei; Meng, Sheng; Yang, Wei; Zhang, Guangyu; Department of Electronics and Nanoengineering; Zhipei Sun Group; Centre of Excellence in Quantum Technology, QTF; CAS - Institute of Physics; Songshan Lake Materials Laboratory; National Institute for Materials Science
    Spatially indirect excitons with displaced wave functions of electrons and holes play a pivotal role in a large portfolio of fascinating physical phenomena and emerging optoelectronic applications, such as valleytronics, exciton spin Hall effect, excitonic integrated circuit, and high-temperature superfluidity. Here, we uncover three types of spatially indirect excitons (including their phonon replicas) and their quantum-confined Stark effects in hexagonal boron nitride encapsulated bilayer WSe2 by performing electric field-tunable photoluminescence measurements. Because of different out-of-plane electric dipole moments, the energy order between the three types of spatially indirect excitons can be switched by a vertical electric field. Remarkably, we demonstrate, assisted by first-principles calculations, that the observed spatially indirect excitons in bilayer WSe2 are also momentum indirect, involving electrons and holes from Λ and K/Γ valleys in the Brillouin zone, respectively. This is in contrast to the previously reported spatially indirect excitons with electrons and holes localized in the same valley. Furthermore, we find that the spatially indirect intervalley excitons in bilayer WSe2 can exhibit considerable, doping-sensitive circular polarization. The spatially indirect excitons with momentum-dark nature and highly tunable circular polarization may provide a firm basis for the understanding and engineering of technological applications in photonics and optoelectronics.
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    Thermally induced band hybridization in bilayer-bilayer MoS2/WS2heterostructure
    (IOP Publishing Ltd., 2021-05) Zhao, Yanchong; Bo, Tao; Du, Luojun; Tian, Jinpeng; Li, Xiaomei; Watanabe, Kenji; Taniguchi, Takashi; Yang, Rong; Shi, Dongxia; Meng, Sheng; Yang, Wei; Zhang, Guangyu; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; CAS - Institute of Physics; National Institute for Materials Science
    Transition metal dichalcogenides (TMDs), being valley selectively, are an ideal system hosting excitons. Stacking TMDs together to form heterostructure offers an exciting platform to engineer new optical and electronic properties in solid-state systems. However, due to the limited accuracy and repetitiveness of sample preparation, the effects of interlayer coupling on the electronic and excitonic properties have not been systematically investigated. In this report, we study the photoluminescence spectra of bilayer-bilayer MoS2/WS2 heterostructure with a type II band alignment. We demonstrate that thermal annealing can increase interlayer coupling in the van der Waals heterostructures, and after thermally induced band hybridization such heterostructure behaves more like an artificial new solid, rather than just the combination of two individual TMD components. We also carry out experimental and theoretical studies of the electric controllable direct and indirect infrared interlayer excitons in such system. Our study reveals the impact of interlayer coupling on interlayer excitons and will shed light on the understanding and engineering of layer-controlled spin-valley configuration in twisted van der Waals heterostructures.