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Quantum Phase Transition in Germanene and Stanene Bilayer: From Normal Metal to Topological Insulator Chengxi Huang, Jian Zhou, Haiping Wu, Kaiming Deng, Puru Jena, and Erjun Kan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00651 • Publication Date (Web): 05 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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The Journal of Physical Chemistry Letters

Quantum Phase Transition in Germanene and Stanene Bilayer: From Normal Metal to Topological Insulator Chengxi Huang†, ‡, Jian Zhou‡, Haiping Wu†, Kaiming Deng†,*, Puru Jena‡,*, Erjun Kan†,* †

Department of Applied Physics and Key Laboratory of Soft Chemistry and Functional

Materials (Ministry of Education), Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, P. R. China. ‡

Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284,

United States Corresponding Author * P. J. ([email protected]), K. D ([email protected]) and E. K. ([email protected])

ACS Paragon Plus Environment

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ABSTRACT

Two-dimensional (2D) topological insulators (TIs) which exhibit quantum spin Hall (QSH) effects, are a new class of materials with conducting edge and insulating bulk. The conducting edge bands are spin-polarized, free of back scattering, and protected by time-reversal symmetry (TRS) with potential for high efficiency applications in spintronics. Based on first-principles calculations, we show that under external pressure recently synthesized stanene and germanene buckled bilayers can automatically convert into a new dynamically stable phase with flat honeycomb meshes. In contrast to the active surfaces of buckled bilayer of stanene or germanene, the above new phase is chemically inert. Furthermore, we demonstrate that these flat bilayers are 2D TIs with sizable topologically nontrivial band gaps of ~ 0.1 eV, which makes it viable for room temperature applications. Our results suggest some new design principles for searching stable large-gap 2D TIs.

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Unlike normal insulators (NIs), topological insulators (TIs) belong to a new class of quantum materials because of its gapless boundary states, although these materials are insulating in the bulk.1-3 In two-dimensional (2D) TIs spin polarized electrons move along opposite directions on their edges which are protected by the time-reversal symmetry (TRS). As long as the TRS is not broken, any backscattering caused by nonmagnetic impurities or defects is forbidden. This implies that the edge transportation is dissipationless,4 which meets the demands of spintronics and quantum computing applications.5-6 In three-dimensional (3D) TIs, on the other hand, the surface states along non-180° directions are not protected against scattering, thus making them less desirable for dissipationless spin transport along a specific direction. Since the original theoretical investigation of quantum spin Hall (QSH) effect in graphene7,8 and the experimental observation of quantum wells9-11 (e.g. HgTe), there has been a constant search for new 2D TIs. Due to the weak spin-orbit coupling (SOC) in graphene, the band gap opened by the SOC is too small (~10-3 meV),7 which makes it very difficult to be detected in experiment. And for HgTe, the QSH effect can only be observed at very low temperature (8%). These flat bilayers can be easily achieved from the vdW stacked buckled bilayers by strain engineering, followed by metal-to-TI and metal-to-NI-to-TI transitions. The topological characteristic is confirmed by the calculation of Z2 topological invariant and the spin Hall conductance around the Fermi energy. Our studies demonstrate a promising way to achieve room-temperature 2D TIs and tunable QSH states by structural phase transitions. Notes The authors declare no competing financial interests. Acknowledgements This work was supported by the NSFC (21203096, 11374160, 51522206), by NSF of Jiangsu Province (BK20130031), by PAPD, by New Century Excellent Talents in University (NCET-120628), by the Innovation Program of Jiangsu Province (KYLX15_0406)，and the Fundamental Research Funds for the Central Universities (No.30915011203). We also acknowledge the support from the Shanghai Supercomputer Centre. PJ acknowledges support by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award # DE-FG02-96ER45579. Chengxi Huang acknowledges the China Scholarship Council (CSC) for sponsoring his visit to Virginia Commonwealth University (VCU) where this work was conducted.


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Supporting Information Available: Atomic coordinates for flat bilayers, stability under strain and finite temperature, band structure calculated by HSE06 functional, band structure of nanoribbon and projected band structure with an substrate for FB-Sn.

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Quantum Phase Transition in Germanene and ... - ACS Publications

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