超导电性是一种宏观量子现象，具有两个方面的基本特征：零电阻效应和迈斯纳效应，其在电力、交通、能源和医疗方面具有广泛的应用。超导一直以来都是凝聚态物理研究的重点，特别是铜氧化物高温超导体和铁基超导体的发现对超导的机理研究提出了挑战。对于铜氧化物超导体，人们普遍认为超导发生在CuO₂层，电子通过交换反铁磁涨落形成库珀对，配对对称性是d波，超导序参量在动量空间会发生相位变化且存在能隙结点。最近，清华大学薛其坤课题组利用氩离子轰击，分子束外延和扫描隧道显微镜技术研究发现：铜氧化物材料不同原子层的本征电子态结构完全不同，裸露的CuO₂面表现为无结点的超导能隙，这对d波配对提出了挑战。铁基超导体具有和铜氧化物材料相似的层状结构和相图，它的发现被赋予了解决高温超导问题的希望。然而铁基超导体是多带体系，性质更加复杂。重电子掺杂FeSe材料因为转变温度很高和独特的费米面结构吸引了人们的广泛关注。深入研究其电子态结构以及载流子掺杂相图，对进一步理解铁基超导的机理，提高超导转变温度具有着极为重要的意义。另外，寻找更高转变温度的超导材料也一直是超导领域的重要内容。除铜氧化物和铁基超导体外还有很多其他类型的超导材料，如金属合金超导体、重费米子超导体、有机超导体、钛基磷族氧化物超导体。然而他们的转变温度都没有超过40K。最近陈晓嘉课题组在有机材料三联苯中发现了120K超导的迹象，激起了人们的广泛研究，但是其超导电性仍没有得到证实。为了澄清或者深入研究以上问题，我们对铜氧化物， (Li〓Fe〓OH)FeSe和三联苯等材料进行了扫描隧道显微学研究。主要结果如下： 1.Bi〓Sr〓CaCu〓O〓和YBa〓Cu〓O〓的超导能隙结构研究 我们利用截面STM测量了Bi2212以及YBCO样品(100)面和(110)面的超导能隙结构。我们发现它们的超导能隙沿着晶体c方向具有很强的空间不均匀性。 Bi2212(100)／YBCO(100)面具有U型和V型两种特征扫描隧道谱。U型能隙在费米能量附近完全没有态密度，沿c方向逐渐演化成V型能隙。我们在YBCO(110)面只观测到U型能隙，没有观测到零偏压电导峰。U型扫描隧道谱说明铜氧化物超导体没有能隙结点，不能用d波配对进行拟合。我们的实验结果对研究铜氧化物的配对对称性具有重要意义。 2.(Li〓Fe〓OH)FeSe的Lifshitz相变和反常绝缘相研究 我们采用表面K沉积的方法进一步电子掺杂(Li〓Fe〓OH)FeSe体系，并进行原位STM和角分辨光电子能谱(ARPES)研究。我们发现Γ点未占据的导带可以逐渐被调节到费米面，发生Lifshitz相变，使得(Li〓Fe〓OH)FeSe费米面的Γ点和M点同时具有电子口袋。随着更多电子掺杂，Γ点电子口袋逐渐打开低能能隙，能隙大小只有约3-5meV，这说明Γ点和M点电子口袋的带间散射并不增强超导。进一步电子掺杂后，(Li〓Fe〓OH)FeSe费米能量处的电子态密度逐渐被耗尽，进入绝缘相，这表明材料中的电子关联效应很强。该工作为理解过掺杂区FeSe体系的超导和电子态演化提供了实验依据。 3.Na₂Ti₂Sb₂O晶格畸变和电荷密度波研究 Na₂Ti₂Sb₂O是钛基磷族氧化物超导体的母体，其在T〓=115K时发生相变，该相变的起源仍存在争论。我们利用STM细致研究了Na₂Ti₂Sb₂O的表面形貌和电子态行为。我们观测到Na表面在T〓下发生2×2晶格畸变，调制波矢q和费米面的嵌套波矢非常吻合，这意味着晶格畸变来自于公度的电荷密度波调制。此外，扫描隧道谱也验证了电荷密度波能隙的打开。电荷密度波的出现意味着Na₂Ti₂Sb₂O的电声子相互作用很强，可能是引起钛基磷族氧化物超导的原因。 4.三联苯单层膜中奇异能隙的STM研究 我们在Au(111)表面成功的生长了高质量的三联苯单层膜，并详细的研究了单层膜的表面形貌和电子态结构随着K掺杂的演化。由于K的分布不均匀，三联苯薄膜发生很强的相分离。当K掺杂1.45ML时，我们观测到了奇异的有序相和11meV的低能能隙，该能隙关于费米能量对称，大概在50K闭合。在11T磁场下，我们没有观测到磁通涡旋态的存在，且该能隙没有受到任何压制，这表明该能隙并非起源于超导电性。能隙的起源需要更多理论和实验的研究。我们的结果对澄清三联苯体材料是否超导具有重要的参考意义。 关键词：扫描隧道显微镜 铜氧化物超导体 超导能隙结构 (Li〓Fe〓OH)FeSe 费米面结构 绝缘相 Na₂Ti₂Sb₂O 电荷密度波 晶格畸变 三联苯 分子束外延生长磁通涡旋态 中图分类号：O469

As a macroscopic quantum phenomenon, superconductivity has two typical feathers: zero-resistance and Meissner effect. It has been widely used in electricity, transportation, energy and health-care fields. Superconductivity has been one of the most important research topic in condensed matter physics. The discovery of cuprates and iron-based superconductors gives great challenge in understanding the mechanism of high-Tc superconductors. People generally agree that superconductivity in cuprates resides in CuO₂ plane and Cooper pairs are glued by anti-ferromagnetic fluctuation. The pairing symmetry of cuprates is d wave with sign change and nodes of the pairing parameter in momentum space. Recently, Xue’s group made significant study on the electronic states of cuprates combined the Ar⁺ sputtering, molecular beam epitaxy (MBE) and scanning tunneling microscopy (STM). They concluded that the electronic states of each atomic layers are quite different and the CuO₂ plane shows a nodeless superconducting gap, which conflicts the d-wave pairing symmetry. Iron-based superconductors (IBSs), which possess layered structure and phase diagram similar to cuprates, raises hopes for understanding high-Tc superconductivity. However, IBSs are more complicated due to the multi-orbital nature and the pairing symmetry is still under debate. Heavily electron-doped FeSe system have attracted much attention because its high-Tc and unique Fermi surface topology. Thus, a comprehensive study of the electronic states and phase diagram of electron-doped FeSe is crucial for understanding its superconducting mechanism and arising its Tc. Besides, exploration of materials with higher superconducting Tc is of vital importance in superconductivity research. Except for cuprates and IBSs, the transition temperatures of most the superconductors (metal and alloy, heavy fermion materials, organic and Ti-based superconductors etc.) are all no more than 40K. Recently, signature of superconductivity above 120K was observed in potassium-doped p-terphenyl. However, the superconductivity has not been confirmed until now, because the lack of resistance measurement. To clarify or make further research on the above questions, we performed STM measurement on cuprates, (Li〓Fe〓OH)FeSe and p-terphenyl film. The main results are as follows: 1.Superconducting gap structure of Bi〓Sr〓CaCu〓O〓 and YBa〓Cu〓O〓 We carried out systematic cross-sectional STM study on optimally doped Bi〓Sr〓CaCu〓O〓 (Bi2212) crystal and YBa〓Cu〓O〓 (YBCO) thin film. We reveal that the superconducting gap structure of Bi2212 and YBCO along their c axis are spatially inhomogeneous and both U-shaped and V-shaped tunneling spectra are observed. The U-shaped spectra, which has flat zero-conduction bottom near E〓, gradually evolves into the V-shaped gap spectra along c axis. An U-shaped gap without any zero-bias peak is observed when tunneling into the YBCO(110) surface.. The U-shaped gap indicates that the superconducting gap of cuprates is nodeless, which cannot be explained by the d wave pairing symmetry. Our data provide a new dimension of information for depicting a more comprehensive picture of cuprate superconductors. 2.Lifshitz transition and insulating phase in over-doped (Li〓Fe〓OH)FeSe We studied the electronic structure and superconductivity of (Li₀.₈Fe₀.₂OH)FeSe in the deeply electron-doped regime with surface K-dosing by using STM and ARPES. We find that the unoccupied band around Γ points can be can be continuously tuned to cross E〓 and contribute a new electron pocket around Γ, resulting in a Lifshitz transition. With more K dosing, a possible superconducting gap with a small size (～5 meV) opens on the new Γ electron pocket. This indicating that the scattering between Γ electron pocket and M electron pocket does not promote superconductivity. Upon further K dosing, the density of states (DOS) around E〓 gradually depletes and the system eventually evolves into an insulating state due to the strong correlation effect. Our findings provide new clues to understand superconductivity versus Fermi surface topology and the correlation effect in FeSe-based superconductors 3.Lattice distortion and charge density wave in Na₂Ti₂Sb₂O Na₂Ti₂Sb₂O is a parent compound of titanium-based oxypnictide superconductors and possesses a phase transition around T〓=115K, whose origin is still under debate. We performed comprehensive STM study on the surface morphology and electronic states ofNa₂Ti₂Sb₂O. We observed a lattice distortion on Na-terminated surface bellow T〓, which displays a 2×2 modulation. The modulation vectors match well with the Fermi surface nesting, indicating that the lattice distortion is driven by commensurate charge density wave (CDW). Besides, the dI/dV spectra also evidences the opening of the CDW gap. Our results suggest the electron-phonon interaction is strong in Na₂Ti₂Sb₂O and could be responsible for the superconductivity in titanium-based oxypnictides. 4.Novel low-energy gap in K-dosed p-terphenyl monolayer film Combined with STM, MBE and in-situ surface potassium (K)-dosing technique, we grew high-quality p-terphenyl monolayer film on Au (111) and studied the evolution of its electronic states and morphologies with electron doping. The p-terphenyl film occurs strong phase transition because of the inhomogeneous distribution of K atoms. With 1.45ML K-dosing, we observed a novel ordered phase, on which a low-energy gap of 11meV opens. The gap is symmetric with E〓 and closes up at about 50K. In particular, no evidence of vortex states or suppression of the gap is observed under 11T, eliminating the superconductivity and spin related origin. The origin of the gap and the configuration of the ordered phase demand more theoretical calculation and experimental measurement. Our work is of vital importance to clarify the possible superconductivity in K-doped p-terphenyl bulk materials. Keywords: Scanning tunneling microscopy (STM), Cuprates, Superconducting gap structure, (Li〓Fe〓OH)FeSe, Ferimi surface, superconductivity, insulating phase, Na₂Ti₂Sb₂O, Charge density wave(CDW), Lattice distortion, p-terphenyl, molecular beam epitaxy(MBE), Vortex lattice CLC: 0469