中国物理学会期刊网
物理学报  2018, Vol.67 Issue (17): 170701  DOI:10.7498/aps.67.20180977
Bi (110)薄膜在NbSe2衬底上的扫描隧道显微镜研究
1. 上海交通大学物理与天文学院, 人工结构及量子调控教育部重点实验室, 上海 200240;2. 人工微结构科学与技术协同创新中心, 南京 210093>
Scanning tunneling microscopy research of Bi(110) thin films grown on NbSe2
1. Key Laboratory of Artificial Structures and Quantum Control, Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China;2. Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China>

摘要

二维拓扑绝缘体因其特殊的能带结构带来的新奇物理性质,成为近年来凝聚态物理的研究热点.尤其是在引入超导电性之后,二维拓扑绝缘体中可能存在马约拉纳费米子(Majorana fermion),因此在量子计算方面具有重大应用前景.在Bi(111)薄膜被证实为二维拓扑绝缘体之后,Bi(110)薄膜引起了广泛关注,然而其拓扑性质还存在争议.本文利用分子束外延技术在室温低生长速率环境下成功制备出了高质量的单晶Bi(110)薄膜.通过扫描隧道显微镜测量发现,薄膜以约8个原子层厚度为分界,从双层生长转变为单层生长模式.结合隧道谱测量发现,在NbSe2衬底上生长的Bi(110) 薄膜因为近邻效应而具有明显的超导性质,但并未显示出拓扑边缘态的存在.此外,对薄膜中特殊的量子阱态现象也进行了讨论.

Abstract

Due to the novel physical properties induced by the strong spin orbit coupling and band inversions in the energy band structure, two-dimensional topological insulator has become a hot research point in the field of condensed matter physics and material science in recent years. Particularly, two-dimensional topological insulator may host exotic Majorana fermionic excitations in its edge state if superconductivity is introduced. Bi thin film with (111) orientation proves to be a two-dimensional topological insulator both in theory and in experiment. However, the topological nature of Bi thin film with (110) orientation has not yet been confirmed. In this study, high quality Bi(110) thin films are successfully prepared on superconductor NbSe2 surfaces, by the molecular beam epitaxial technology at ambient temperature and a low deposition rate (~24℃,~3 min/bilayer). The morphologies and electronic properties of the samples are studied by using scanning tunneling microscopy and spectroscopy. The experimental results reveal that the growth mode changes from bilayer (BL) in BL mode to monolayer (ML) in ML mode. Such transition takes place at a critical height of about 4 BLs. The mechanism of the growth mode transition is believed to be induced by the drastic variation of the surface energies of the thin films with different thickness values. Due to the large coverage of Bi(110) film on the NbSe2 substrate, it is almost impossible to find the exposed areas of NbSe2 substrate surface in practice. Especially on the sample with a large number of layers of Bi thin film, it is hard to directly determine the number of layers for each film. Hence, the critical thickness could be only estimated by controlling the deposition time and growth rate combining with the measurements of stage height of the film. The nearly identical local density of states wherever measured in the interior of a terrace or at the step edges can be discerned from the dI/dV spectra, which is thus hard to corroborate with non-trivial topology in either BL or ML thick Bi(110) film. The superconductivity induced by proximity effect from the superconducting substrate NbSe2 is also observed on the thin films. Through Bardeen-Cooper-Schrieffer type data fitting, the superconducting gap on the Bi thin film is estimated at about 0.5 meV. In addition, the quantum well state, which is often observed in thin films, is also revealed from the Bi(110) thin films, whose characteristic is equal energy spacing between peaks in dI/dV spectra. Noticeably, the spectral shapes of BL and ML are similar, and the local density of states from adjacent film layers displays an approximate πup phase shift.
收稿日期:2018-05-18

基金资助

国家重点基础研究发展计划(批准号:2016YFA0301003,2016YFA0300403)、国家自然科学基金(批准号:11521404,11634009,U1632102,11504230,11674222,11574202,11674226,11574201,U1632272,11655002)和上海市科学技术委员会(批准号:16DZ2260200)资助的课题.
Project supported by the National Basic Research Program of China (Grant Nos. 2016YFA0301003, 2016YFA0300403), the National Natural Science Foundation of China (Grant Nos. 11521404, 11634009, U1632102, 11504230, 11674222, 11574202, 11674226, 11574201, U1632272, 11655002), and the Shanghai Committee of Science and Technology, China (Grant No. 16DZ2260200).

引用本文

[中文]
刘建宇, 孙昊桦, 管丹丹, 李耀义, 王世勇, 刘灿华, 郑浩, 贾金锋. Bi (110)薄膜在NbSe2衬底上的扫描隧道显微镜研究[J]. 物理学报, 2018, 67(17): 170701.
[英文]
Liu Jian-Yu, Sun Hao-Hua, Guan Dan-Dan, Li Yao-Yi, Wang Shi-Yong, Liu Can-Hua, Zheng Hao, Jia Jin-Feng. Scanning tunneling microscopy research of Bi(110) thin films grown on NbSe2[J]. Acta Phys. Sin., 2018, 67(17): 170701.
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