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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.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.
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Keywords:
- Bi film /
- topological insulator /
- proximity effect /
- quantum well state
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[1] König M, Wiedmann S, Brne C, Roth A, Buhmann H, Molenkamp L W, Qi X L, Zhang S C 2007 Science 318 766
[2] Bernevig B A, Hughes T L, Zhang S C 2006 Science 314 1757
[3] Kane C L, Mele E J 2005 Phys. Rev. Lett. 95 146802
[4] Moore J E 2010 Nature 464 194
[5] Qi X L, Zhang S C 2010 Phys. Today 63 33
[6] Qi X L, Hughes T L, Zhang S C 2008 Phys. Rev. B 78 195424
[7] Chang C Z, Zhang J S, Feng X, Shen J, Zhang Z C, Guo M H, Li K, Ou Y B, Wei P, Wang L L, Ji Z Q, Feng Y, Ji S H, Chen X, Jia J F, Dai X, Fang Z, Zhang S C, He K, Wang Y Y, Lu L, Ma X C, Xue Q K 2013 Science 340 167
[8] Deutscher G 1971 Solid State Commun. 9 891
[9] Majorana E 1937 Ⅱ Nuovo Cimento 14 171
[10] Fu L, Kane C L 2008 Phys. Rev. Lett. 100 096407
[11] Murakami S 2006 Phys. Rev. Lett. 97 236805
[12] Liu Z, Liu C X, Wu Y S, Duan W H, Liu Feng, Wu J 2011 Phys. Rev. Lett. 107 136805
[13] Xiao S H, Wei D H, Jin X F 2012 Phys. Rev. Lett. 109 166805
[14] Hirahara T, Bihlmayer G, Sakamoto Y, Yamada M, Miyazaki H, Kimura S, Blgel S, Hasegawa S 2011 Phys. Rev. Lett. 107 166801
[15] Yang F, Miao L, Wang Z F, Yao M Y, Zhu F F, Song Y R, Wang M X, Xu J P, Fedorov A V, Sun Z, Zhang G B, Liu C H, Liu F, Qian D, Gao C L, Jia J F 2012 Phys. Rev. Lett. 109 016801
[16] Sun H H, Wang M X, Zhu F F, Wang G Y, Ma H Y, Xu Z A, Liao Q, Lu Y H, Gao C L, Li Y Y, Liu C H, Qian D, Guan D D, Jia J F 2017 Nano Lett. 17 3035
[17] Wada M, Murakami S, Freimuth F, Bihlmayer G 2011 Phys. Rev. B 83 121310
[18] Lu Y H, Xu W H, Zeng M G, Yao G G, Shen L, Yao M, Luo Z Y, Pan F, Wu K 2015 Nano Lett. 15 80
[19] Nagao T, Sadowski J T, Saito M, Yaginuma S, Fujikawa Y, Kogure T, Ohno T, Hasegawa Y, Hasegawa S, Sakurai T 2004 Phys. Rev. Lett. 93 105501
[20] Bian G, Wang X, Miller T, Chiang T C, Kowalczyk P J, Mahapatra O, Brown S A 2014 Phys. Rev. B 90 195409
[21] Yaginuma S, Nagao T, Sadowski J T, Saito M, Nagaoka K, Fujikawa Y, Sakurai T, Nakayama T 2007 Surf. Sci. 601 3593
[22] Hatta S, Ohtsubo Y, Miyamoto S, Okuyama H, Aruga T 2009 Appl. Surf. Sci. 256 1252
[23] Chiang T C 2000 Surf. Sci. Rep. 39 181
[24] Paggel J J, Miller T, Chiang T C 1999 Science 283 1709
[25] Zhang Y F, Jia J F, Han T Z, Tang Z, Shen Q T, Guo Y, Qiu Z Q, Xue Q K 2005 Phys. Rev. Lett. 95 096802
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