Scanning tunneling microscope studies of 2D superconductor and 3D intrinsic topological insulator
Electrons show unusual and interesting behaviors both in low dimensions and on material surfaces, distinct from what they display in bulk materials. These intriguing properties have been studied in order to understand their origins. One area where this can be seen is in the case of superconductivity, where superconducting phase fluctuation in a thin superconductor is supposed to substantially suppress the superconductivity of the material as the film thickness decreases. To test this, we prepared epitaxially grown and globally flat lead (Pb) films; here, the thinnest film was 1.4 nm thick. Four different length scale measurements, ranging from the nm to the mm scale, gave consistent superconducting transition temperatures. Our results proved that the film of 1.4 nm still has strong superconducting phase stiffness; namely, the superfluid phase is rigid even in 1.4 nm thin superconductor film. Moreover, the parallel critical magnetic field is remarkably strong so that superconductivity is still observed in Zeeman fields, exceeding the Pauli limit. In addition, the surface of 3D topological insulator has a novel quantum state induced by strong spin-orbit interaction. A number of material studies were conducted to find a surface dominated conduction topological insulator that has a large energy gap and a single Dirac cone. Moreover, it is necessary for the material to be stable against aging unlike most 3D topological insulators, such as Bi₂Se₃. Here, Bi₂Te₂Se and BiSbTeSe₂ were studied in terms of their structures, electronic properties, and aging effects on them. Scanning tunneling microscopy analysis attested that Bi₂Te₂Se is an order alloy, which has a slight randomness of 15 %, whereas BiSbTeSe₂ is a random alloy. Scanning tunneling spectroscopy on BiSbTeSe₂ confirmed that the Dirac point tends to stay around the Fermi level under the strong band structure change, induced by random structure. The most surprising observation was that BiSbTeSe₂ showed remarkable stability despite the rich composition of selenium (Se). Even after aging for seven days, the Fermi level and the Dirac point remained at almost the same level in bulk band gap. Both observations are very important for applications to utilize the exotic topological surface state.