【作者】 杨浩；

【导师】 李耀义； 贾金锋；

【作者基本信息】 上海交通大学 ， 物理学， 2020， 博士

【摘要】 拓扑超导体已成为一个极其热门的研究领域,这是由于拓扑超导体中蕴含着一种奇特的准粒子——Majorana零能模,这种准粒子遵从非Abelian统计规律,理论预言可以把它用于拓扑量子计算,从而极大地降低量子计算出错的概率。然而,天然的拓扑超导体材料非常稀少。在拓扑绝缘体发现后,理论学家预言拓扑超导电性可以在发生超导转变的拓扑绝缘体中实现。其中一种可行的方法是构造拓扑绝缘体与超导体异质结,通过超导近邻效应将超导电性引入到拓扑绝缘体中。理论预言在超导的拓扑绝缘体的磁通涡旋中心存在着Majorana零能模。实验上在Bi₂Te₃/Nb Se₂异质结中通过扫描隧道显微镜观测到了Majorana零能模。最近理论预言在超导的拓扑晶体绝缘体的磁通涡旋中也存在Majorana零能模,而且其特别之处在于多个Majorana零能模可以共存于一个涡旋中。拓扑晶体绝缘体与拓扑绝缘体的拓扑性完全不同,目前人们已在拓扑晶体绝缘体中诱导出超导电性,但是是否具有拓扑超导电性还存在争议。在本论文中,我们用分子束外延法构造了拓扑晶体绝缘体Sn_1-xPb_xTe与传统超导体Pb形成的异质结,通过低温扫描隧道显微谱测量,系统研究了Sn_1-xPb_xTe（001）表面的超导电性。我们在石墨化的SiC衬底上制备出了原子级平整的Sn_1-xPb_xTe-Pb侧向异质结和纵向异质结。变温测量显示,在Sn_1-xPb_xTe中通过超导近邻效应诱导的超导转变温度（T_c）有7.0K,与Pb的超导转变温度7.2K非常接近,而且远高于用In掺杂的Sn_1-xPb_xTe的超导转变温度4.7K。在4.2K下,Sn_1-xPb_xTe超导能隙随距离的变化关系显示这两种异质结的超导近邻效应都非常强,超导相干长度大于200nm。由于超导针尖比正常态金属针尖具有更高的能量分辨率,所以STS谱用超导针尖测量。在4.2K下,Sn_1-xPb_xTe（001）表面的超导能隙呈peak-dip-hump的特征,与传统超导体Pb的超导能隙的U型特征明显不同。在0.38K下,Sn_1-xPb_xTe超导能隙peak-dip-hump的特征变得更加锐利,原先在4.2K下Sn_1-xPb_xTe的一个相干峰实际上由三个能量间距约0.3meV的峰组成。变温实验和数值模拟说明Sn_1-xPb_xTe d I/dV谱相干峰外侧的dip特征应对应于体能隙,而相干峰应为能隙内的in-gap states。准粒子干涉测量进一步证实这些超导能隙内的in-gap states是无能隙的表面态。反常的peak-dip-hump型超导能隙特征,超导能隙内多重的in-gap states,以及在超导能隙内零能处观测到四重对称性的准粒子干涉图案都支持在超导的拓扑晶体绝缘体Sn_1-xPb_xTe中存在拓扑超导电性。我们的工作首次证实拓扑晶体绝缘体的拓扑超导电性能够用超导针尖从态密度上直接分辨出来。另外,Sn_1-xPb_xTe-Pb异质结具有很强的超导近邻效应（相干长度在4.2K大于200nm）,较高的超导转变温度（T_c≈7.0K）,较小的Dirac点能量E_D（-100^-50me V）,这些优点使得该异质结日后很有希望制备成拓扑超导器件用于探测和操控多重Majorana零能模。更多还原

【Abstract】 Topological superconductor（TSC）has become a very hot research field,because it con-tains a novel quasi-particle——Majorana zero mode（MZM）which follows the non-Abelian statistics and has the potential application in fault-tolerant topological quantum computation.However,natural TSCs are rarely found.After the discovery of topological insulators（TIs）,theorists predict that topological superconductivity can be induced in superconducting TIs.A practical method is the preparation of the TI-superconductor heterostructure.The superconduc-tivity will be introduced into TIs through the superconducting proximity effect.The MZM is theoretically predicted to exist at the center of the magnetic vortex and has been observed in the Bi₂Te₃/NbSe₂heterostructure by scanning tunneling microscope（STM）.Recently,it has been predicted that there also exist MZMs in the vortices in the superconducting topological crys-talline insulator（TCI）,for which it’s very special that multiple MZMs can coexist in a single vortex.A TCI is topologically distinct from a TI.The bulk superconductivity of TCIs has been realized,but the experimental evidences for their topological superconductivity are still contro-versial.In this thesis,we fabricate the heterostructures made up of the TCI Sn_1-xPb_xTe and the conventional superconductor Pb using molecular beam epitaxy（MBE）,and systematically detect the superconductivity of Sn_1-xPb_xTe（001）with scanning tunneling spectroscopy（STS）at the low temperature.Atomically flat Sn_1-xPb_xTe-Pb lateral and vertical heterostructures can be prepared on graphitized SiC substrates.Temperature dependent measurements show that the proximity-induced superconducting transition temperature（T_c）of Sn_1-xPb_xTe is 7.0 K,which is very close to the T_c（7.2 K）of Pb and much higher than the maximum T_c（4.7 K）of In doped Sn_1-xPb_xTe.At 4.2K,the spatial evolution of the superconducting gaps taken on Sn_1-xPb_xTe indicates the superconducting proximity effect is so strong that the coherence lengthξis larger than 200 nm.Since the superconducting tip has considerably better energy resolution than the normal metal tip,STS spectra are measured with superconducting tips.The superconducting gap of Sn_1-xPb_xTe displays an unconventional peak-dip-hump gap feature,which is different from the U-shaped one for conventional superconductor Pb.At 0.38 K,the peak-dip-hump gap feature becomes much sharper.Each coherence peak observed at 4.2 K actually contains three peaks separated by 0.3 meV.The temperature dependent experiments and numerical simula-tions indicate that the dip feature outside the coherence peaks should correspond to the bulk superconducting gap,and the coherence peaks should be the in-gap bound states.Quasiparti-cle interference（QPI）measurements further confirm that the in-gap bound states are gapless surface states.Unconventional peak-dip-hump gap features,multiple in-gap states and fourfold symmetric QPI patterns taken at the zero energy in the superconducting gap support the presence of the topological superconductivity in the superconducting TCISn_1-xPb_xTe.Our work demon-strates for the first time that the unique topological superconductivity of a TCI can be directly distinguished in the density of states by the superconducting tip.Moreover,the Sn_1-xPb_xTe-Pb heterostructures have strong proximity effect（ξ>200 nm at 4.2 K）,high T_c（7.0 K）,small Dirac point energy E_D（-100^-50 meV）,which make the heterostructures to be a promising candidate for topological superconducting devices to detect and manipulate multiple MZMs in the future.更多还原