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SnTe类拓扑晶体绝缘体发生超导转变后, 在其单个磁通涡旋中能够形成受晶体对称性保护的多重Majorana零能模. 这种奇特的性质能够降低多个Majorana零能模之间相互作用的难度. 最近多重Majorana零能模存在的实验证据已在SnTe/Pb超导异质结单个磁通涡旋中被观测到. SnTe是一种非常p型的半导体材料, 如何调控其电子性质, 在分辨和操控Majorana零能模方面具有重要研究意义. 本文利用分子束外延技术在Si (111)衬底生长的Pb (111)薄膜上制备了Sn1–xPbxTe薄膜, 并且通过扫描隧道显微镜研究了薄膜边缘、畴界以及位错对其电子态的影响. 扫描隧道显微镜的微分电导谱显示, 在薄膜边缘、畴界以及位错附近, Sn1–xPbxTe电子态相对于费米能级的位置能够发生显著改变, 载流子类型能从p型转变到n型. 在远离这些缺陷的区域, Pb含量对Sn1–xPbxTe的费米能级的影响不显著, 但是过多的Pb含量会抑制磁通中零能峰的形成. 该研究将为基于SnTe类材料的拓扑超导器件的设计提供新的思路.SnTe-type topological crystalline insulators (TCIs) possess multiple Dirac-like topological surface states under the mirror-symmetry protection. Superconducting SnTe-type TCIs are predicted to form multiple Majorana zero modes (MZMs) in a single magnetic vortex. For the spatially isolated MZMs, there is only one MZM in a single vortex at surface. However, experimental demonstration of coupling the two isolated MZMs by changing wire length or intervortex distance is very challenging. For the multiple MZMs, two or more MZMs can coexist together in a single vortex. Thus, the novel property is expected to significantly reduce the difficulty in producing hybridization between MZMs. Recently, the experimental evidence for multiple MZMs has been observed in a single vortex of the superconducting SnTe/Pb heterostructure. However, SnTe is a heavily p-type semiconductor which is very difficult to induce the p-type to n-type transition via doping or alloying. The study on the Fermi-level tuning of SnTe-type TCIs is important for detecting and manipulating multiple MZMs. In this work, we report the influence of defects, such as film edge, grain boundary and dislocation, on the electronic property of Sn1–xPbxTe/Pb. The Sn1–xPbxTe films are prepared on the Pb (111) films grown on the Si (111) substrate by the molecular beam epitaxial technology. The structural and electronic properties of the Sn1–xPbxTe films are detected in situ by using low-temperature scanning tunneling microscopy and spectroscopy. The differential conductance tunneling spectra show that the minima of dI/dV spectra taken in the areas near the film edge, the grain boundary and the dislocation of Sn1–xPbxTe grown on Pb can be significantly changed to the energy very close to the Fermi level or even about -0.2 eV below the Fermi level, whereas the minima of dI/dV spectra taken in the areas far away from the defects are always at about 0.2 eV above the Fermi level. It indicates that these quasi one-dimensional defects, rather than Pb alloying, play an important role in modifying electronic property of the Sn1–xPbxTe/Pb heterostructure. Moreover, the Pb alloying will suppress the formation of zero-energy peak in the vortex. These results are expected to develop the method of the Fermi-level tuning for the SnTe-type topological superconducting devices that do not require doping or alloying.
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Keywords:
- topological crystalline insulator /
- defect /
- superconducting heterostructure /
- scanning tunneling microscope
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图 1 Sn1–xPbxTe/Pb的薄膜边缘附近的STM和STS测量 (a)在190 ℃的Pb膜上沉积SnTe得到的平整薄膜的STM图, 扫描尺寸450 nm × 450 nm, 样品偏压Vset = 2 V, 隧穿电流Iset = 0.1 nA; (b)在图(a)中红点处测的dI/dV谱, 样品偏压Vset = 0.35 V, 隧穿电流Iset = 0.1 nA; (c)薄膜边缘附近的STM图, 扫描尺寸450 nm × 450 nm, 样品偏压Vset = 2 V, 隧穿电流Iset = 0.1 nA; (d)沿图(c)中绿色线条处的高度轮廓图, 沟壑的深度约9 nm; (e) 在图(c)中黑点处测的dI/dV谱, 样品偏压Vset = 0.20 V, 隧穿电流Iset = 0.1 nA; (f)在图(c)中蓝点处测的dI/dV谱, 样品偏压Vset = 0.35 V, 隧穿电流Iset = 0.1 nA; (g)在图(c)黑点处和(h)蓝点处的原子分辨STM图, 扫描尺寸10 nm × 10 nm, 样品偏压Vset = 0.8 V, 隧穿电流Iset = 0.1 nA, 在图(g)和(h)中Pb含量分别为23.2%和25.1%
Fig. 1. STM and STS measurements near the edge of Sn1–xPbxTe film grown on Pb: (a) STM image (450 nm × 450 nm) of the flat film obtained by deposition of SnTe on Pb film at 190 ℃ (Vset = 2 V, Iset = 0.1 nA); (b) dI/dV spectrum taken at the red dot in Fig. (a) (Vset = 0.35 V, Iset = 0.1 nA); (c) STM image (450 nm × 450 nm) near the film edge (Vset = 2 V, Iset = 0.1 nA); (d) line profile taken along the green line in (c), the depth of the trench is about 9 nm; (e) dI/dV spectra taken at the black dot in (c)(Vset = 0.20 V, Iset = 0.1 nA); (f) dI/dV spectra taken at the blue dot in (c)(Vset = 0.35 V, Iset = 0.1 nA); (g), (h) atomically resolved STM images (10 nm × 10 nm) taken at the black and blue dots in (c), respectively. Vset = 0.8 V, Iset = 0.1 nA, the Pb content of (g) and (h) are 23.2% and 25.1%.
图 2 Sn1–xPbxTe/Pb的畴界附近的STM和STS测量 (a)在Pb膜上先室温沉积20 nm厚的SnTe然后在180 ℃继续沉积20 nm厚的SnTe得到的样品的STM图, 扫描尺寸200 nm × 200 nm, 样品偏压Vset = 2 V, 隧穿电流Iset = 0.1 nA; (b)—(e)分别为在图(a)中A1—A4 方框处的原子分辨STM图, 扫描尺寸10 nm × 10 nm, 样品偏压Vset = 0.8 V, 隧穿电流Iset = 0.1 nA, 在图(b)—(e)中Pb含量分别为25.3%, 16.6%, 32.7%, 7.9%; (f)图(c)中D1—D3 点处测的dI/dV谱, 对于dI/dV谱D1, 样品偏压Vset = 0.40 V, 隧穿电流Iset = 0.1 nA, 对于dI/dV谱D2, 样品偏压Vset = 0.20 V, 隧穿电流Iset = 0.1 nA, 对于dI/dV谱D3, 样品偏压Vset = 0.25 V, 隧穿电流Iset = 0.1 nA
Fig. 2. STM and STS measurements near the grain boundary of Sn1–xPbxTe film grown on Pb: (a) STM image (200 nm × 200 nm) of the sample obtained by deposition of 20 nm thick SnTe on Pb film at room temperature and then further deposition of 20 nm thick SnTe at 180 ℃ (Vset = 2 V, Iset = 0.1 nA); (b)–(e) atomically resolved STM images (10 nm × 10 nm) taken at the squares A1—A4 in (a), respectively, Vset = 0.8 V, Iset = 0.1 nA, the Pb content of (b)–(e) are 25.3%, 16.6%, 32.7% and 7.9%; (f) dI/dV spectra taken at the dots D1—D3 in (a), respectively, for the dI/dV spectrum D1, Vset = 0.40 V, Iset = 0.1 nA, for the dI/dV spectrum D2, Vset = 0.20 V, Iset = 0.1 nA, for the dI/dV spectrum D3, Vset = 0.25 V, Iset = 0.1 nA.
图 3 Sn1–xPbxTe/Pb的位错附近的STM和STS测量 (a)在Pb膜上室温沉积40 nm厚的SnTe然后在100 ℃退火3 h得到的样品的STM图, 扫描尺寸200 nm × 200 nm, 样品偏压Vset = 2 V, 隧穿电流Iset = 0.1 nA; (b) 在图(a)中红色方框处的原子分辨STM图, 扫描尺寸10 nm × 10 nm, 样品偏压Vset = 0.8 V, 隧穿电流Iset = 0.1 nA, Pb含量为20.2%; (c)在图(a)中红色方框和蓝色箭头处测的dI/dV谱, 对于红色dI/dV谱, 样品偏压Vset = 0.40 V, 隧穿电流Iset = 0.1 nA, 对于蓝色dI/dV谱, 样品偏压Vset = 0.10 V, 隧穿电流Iset = 0.1 nA; (d) 在Pb膜上室温沉积40 nm厚的SnTe得到的样品的STM图, 扫描尺寸200 nm × 200 nm, 样品偏压Vset = 2 V, 隧穿电流Iset = 0.1 nA; (e)在图(d)中红色方框处的原子分辨STM图, 扫描尺寸10 nm × 10 nm, 样品偏压Vset = 0.7 V, 隧穿电流Iset = 0.1 nA; (f) 在图(d)中红色方框处测的dI/dV谱, 样品偏压Vset = 0.58 V, 隧穿电流Iset = 0.1 nA.
Fig. 3. STM and STS measurements near the dislocation of Sn1–xPbxTe film grown on Pb: (a) STM image (200 nm × 200 nm) of the sample obtained by deposition of 40 nm thick SnTe on Pb film at room temperature and followed by annealing at 100 ℃ for 3 h (Vset = 2 V, Iset = 0.1 nA); (b) atomically resolved STM image (10 nm × 10 nm) taken at the red square in (a), Vset = 0.8 V, Iset = 0.1 nA, the Pb content is 20.2%; (c) dI/dV spectra taken at the red square and blue arrow in (a), respectively, for the red dI/dV spectrum, Vset = 0.40 V, Iset = 0.1 nA, for the blue dI/dV spectrum, Vset = 0.10 V, Iset = 0.1 nA; (d) STM image (200 nm × 200 nm) of the sample obtained by deposition of 40 nm thick SnTe on Pb film at room temperature (Vset = 2 V, Iset = 0.1 nA); (e) atomically resolved STM image (10 nm × 10 nm) taken at the red square in (d), Vset = 0.7 V, Iset = 0.1 nA; (f) dI/dV spectrum taken at the red square in (d), Vset = 0.58 V, Iset = 0.1 nA.
图 4 合金元素Pb对磁通束缚态的影响 (a)在190 ℃的Pb膜上沉积SnTe得到的样品表面的原子分辨STM图, 扫描尺寸10 nm × 10 nm, 样品偏压Vset = 0.8 V, 隧穿电流Iset = 0.1 nA; (b) 该平整薄膜的大范围STM图, 扫描尺寸450 nm × 450 nm, 样品偏压Vset = 2 V, 隧穿电流Iset = 0.1 nA; (c)相应的零偏压dI/dV映射图, 磁场强度B = 0.02 T, 扫描尺寸450 nm × 450 nm, 样品偏压Vset = 6 mV, 隧穿电流Iset = 0.1 nA; (d)沿图(c)中绿色线条处测的一系列dI/dV谱, 样品偏压Vset = 3 mV, 隧穿电流Iset = 0.1 nA; (e)在180 ℃的Pb膜上共沉积SnTe和PbTe得到的样品表面的原子分辨STM图, 扫描尺寸10 nm × 10 nm, 样品偏压Vset = 0.7 V, 隧穿电流Iset = 0.1 nA. Pb含量为19.2%; (f)该平整薄膜的大范围STM图, 扫描尺寸450 nm × 450 nm, 样品偏压Vset = 2 V, 隧穿电流Iset = 0.1 nA; (g)相应的零偏压dI/dV映射图, 磁场强度B = 0.04 T, 扫描尺寸450 nm × 450 nm, 样品偏压Vset = 6 mV, 隧穿电流Iset = 0.1 nA; (h)沿图(g)中绿色线条处测的一系列dI/dV谱, 样品偏压Vset = 3 mV, 隧穿电流Iset = 0.1 nA
Fig. 4. Effect of alloying element Pb on vortex bound states: (a) Atomically resolved STM image (10 nm × 10 nm) taken on the surface of the sample obtained by deposition of SnTe on Pb film at 190 ℃ (Vset = 0.8 V, Iset = 0.1 nA); (b) large-scale STM image (450 nm × 450 nm) of the flat film (Vset = 2 V, Iset = 0.1 nA); (c) corresponding zero-bias dI/dV map under magnetic field of B = 0.02 T (450 nm × 450 nm, Vset = 6 mV, Iset = 0.1 nA); (d) series of dI/dV spectra taken alone the green line in (c) (Vset = 3 mV, Iset = 0.1 nA); (e) atomically resolved STM image (10 nm × 10 nm) taken on the surface of the sample obtained by co-deposition of SnTe and PbTe on Pb film at 180 ℃ (Vset = 0.7 V, Iset = 0.1 nA), the Pb content is 19.2%; (f) large-scale STM image (450 nm × 450 nm) of the flat film (Vset = 2 V, Iset = 0.1 nA); (g) corresponding zero-bias dI/dV map under magnetic field of B = 0.04 T (450 nm × 450 nm, Vset = 6 mV, Iset = 0.1 nA); (h)series of dI/dV spectra taken alone the green line in (g) (Vset = 3 mV, Iset = 0.1 nA).
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