Heterostructures in Yugroup

1. 12月26日北京交通大学环境工程实验室发生一起严重的实验室安全事故，造成三名学生死亡。12月26日下午，北京市教委召开紧急会议，要求各高校在元旦前开展安全隐患排查，明确整改措施和整改时间表，于12月30日中午之前报市教委，元旦后市教委和市安监局将组织抽查。经请示学校主管领导，学校决定元旦前在全校范围内开展危险化学品专项检查工作。要求各院（系、所、中心）按照“全覆盖、零盲区”的总体要求进行自查，深入开展安全隐患全面排查；实验室与设备管部工作人员联合相关单位工作人员将进行重点抽查。12月26日北京交通大学环境工程实验室发生一起严重的实验室安全事故，造成三名学生死亡。12月26日下午，北京市教委召开紧急会议，要求各高校在元旦前开展安全隐患排查，明确整改措施和整改时间表，于12月30日中午之前报市教委，元旦后市教委和市安监局将组织抽查。经请示学校主管领导，学校决定元旦前在全校范围内开展危险化学品专项检查工作。要求各院（系、所、中心）按照“全覆盖、零盲区”的总体要求进行自查，深入开展安全隐患全面排查；实验室与设备管部工作人员联合相关单位工作人员将进行重点抽查。 请各团队实验室按照以下要求做好自查并迎接上级检查，请各团队负责人切实安排，各团队实验室管理员具体抓好日常自查和迎接检查：     一、         检查方式 本次检查采取自查与巡查相结合的方式。 二、         自查及检查重点 1、              危险化学品规范操作流程 2、              危险化学品台账管理 3、              易制毒、易制爆化学品的使用及储存 4、              气瓶的采购、使用及储存 5、              剧毒化学品的使用 6、              危险废弃物的处置 请各团队实验室，各位老师，吸取北京交通大学的教训，高度重视危险化学品安全隐患专项检查工作，对发现的各种安全隐患和管理漏洞限期整改、解决。

2. Heterostructures in Yugroup

3. Heterostructures in Yugroup Vertical p-n tunneling junctions Twist angle physics Electron-phonon interaction Interlayer mechanical interaction Interlayer excitons MoSe2 WSe2

4. Interlayer valley excitons in heterobilayer of TMDC December 29, 2018

5. Indirect exciton in coupled quantum wells Spatially separated exciton

6. Coherent Solid Exciton: 10-1 me, 1 mK-1 K • New types of quasiparticles point to Bose-Einstein condensation, called coherent solid. • Undergo a phase transition to spontaneous coherence. • Analogous to superconductivity • Emits coherent radiation. Nature, 483, 585, (2012).

7. Excitons in bilayer electron system An electron-electron bilayer system in a strong magnetic field is equivalent to an electron-hole bilayer. Nature, 432, (2004).

8. Corbino Coulomb Drag Electrostatic gating of the 2DESs in the annulus allow to tune d/l from 2.35 to 1.49 at vT=1. l is the magnetic length d is the separation between two parallel 2DESs. • Negligible drag current when layers are independent • Drag and drive current equal at small V. “Perfect” Coulomb Drag • Significant drag only at vT=1. Nature, 488, 482 (2012).

9. Valley index for the single-layer TMDC However, the valley degree of freedom is subject to picosecond timescale depolarization in single-layer TMDCs, stemming from strong electron–hole exchange interactions. Nature Comm. 3, 887 (2012).

10. Band offsets and heterobilayer of TMDCs Ionization energy difference WSe2 MoS2 The reduced overlap of the electron and hole wavefunctions causes dramatic changes in the exciton properties. APL 102, 012111 (2013). The conduction minimum and valance band maximum sit in two different layers

11. Experimental demonstrations of interlayer excitons Layer-by-layer stacking Reported long decay times (~ 100 ns to several µs) The extended population lifetime for the interlayer excitonreflecs its reduced oscillator strength, partially resulting from the spatial separation of the electron and hole. CVD Nature Comm. 6, 6242 (2015).

12. Interlayer excitons in the momentum eigenstate basis Kinetic momentum Q=k+k’ Light cones Q0=τK-τ’K’ The radiative recombination of interlayer excitons with zero kinetic energy is forbidden by momentum conservation. PRL 115, 187002 (2015).

13. Twist angle dependent interlayer excitons A bright interlayer exciton requires both finite interlayer hopping and kinetic energy. ACS Nano 11, 4041 (2017).

14. The formation of the interlayer exciton (i). Interband optical absoption in either monolayer (ii). Rapid interlayer charge transfer (iii). Subsequent formation of the interlayer excitons Nano Lett. 14, 3869 (2014) Nano Lett. 17, 3591 (2017) 2D Mater. 4, 025112 (2017)

15. Valley polarization of the interlayer excitons It demonstrates that the spin of charge carriers is robust against the interlayer charge transfer, which implies the possibility for optical injection valley-polarized carriers. Science 351, 688 (2016)

16. Ultralong valley lifetime of interlayer excitons These valley-polarized holes exhibit a population decay lifetime of more than 1 μs and a depolarization lifetime (that is, intervalley scattering lifetime) of more than 40 μs at 10 K, which is orders of magnitude larger than previously reported values. Sci. Adv. 3, e1700518 (2017)

17. Symmetry dictated selection rules When the rotation operator acts on the wave function, we have: where the rotation operator is: Therefore, we can get the angular momentum at z direction of the quantum state. For a single-layer TMDC, at the BZ corners , the Bloch functions are the eigenfunctions of operations, The quantum number m depends on the band index n as well the choice of the rotation center. The symmetry requires that a circularly polarized photon can excite an electron from nth band to n’th band only when Chem. Soc. Rev.44, 2643 (2015)

18. Selection rules of interlayer excitons Similarly, the K-point interband optical transition from v to c’ bands of different layers couples with circularly polarized photon only when From the table, on concludes that, for in-plane light polarization, a K-valley interlayer exciton in () heterobilayer couples with , while is optically dark. PRL 115, 187002 (2015)

19. Moireexcitons The vdW interaction depends sensitively on interlayer translation, resulting in a lateral modulation of the interlayer distance across a single moire supercell. Sci. Adv. 3, e1601459 (2017)

20. Experimental observation of Moireexcitons (1) arXiv 1807.03771, will appear in Nature Physics

21. Experimental observation of Moireexcitons (2) The moiré potential can mix exciton states with momenta, leading to additional absorption peaks from the K-point states of higher-energy minibands. arXiv 1812.09815

22. Experimental observation of Moireexcitons (2) The dramatic change in the exciton dispersion in momentum space implies that the exciton center-of-mass wavefunction is also strongly modified in real space.

23. Experimental observation of Moireexcitons (3) The evolution of the broad interlayer PL peak into several narrow lines at low power suggests that the interlayer excitons are trapped in confinement potentials. arXiv 1809.04562 will appear in Nature

24. Conclusions: • With the ultralong population lifetime and valley depolarization time, the valley pseudospin of interlayer excitons can be as an information carrier in optoelectronic devcies. • The controlled flow of excitons can connect electronic signal processing and optical communication in optoelectronic device. • The potential minima of the hetorobilayermoiresuperlattice can trap low-energy interlayer excitons, effectively forming a highly ordered and uniform quantum dot array with valley optical selection rules. • The enormous binding energy of interlayer exciton in TMDC heterobilayers offers unique opportunities to study excitonic condensation phenomena at relatively high temperature. • Thank you for your attention.