STM Study On Superconductivity Of Topological Crystalline Insulator Sn_1-xPb_xTe(001)

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 length?is 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(?>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.