Local electronic structure of a quantum point contact observed with STM

We report the direct observation of electronic structures of quantum point contacts (QPCs) using a scanning tunneling microscope (STM) spectroscopy at 77 K. The QPCs are formed across boundary gaps in a multidomain Bi film on a Si (111) -β√3×√3-Bi substrate. Since the QPCs are formed in the direction parallel to the substrate, the direct observations of the atomic and the local electronic structures by using STM are possible. The electronic structure of the QPCs is neither sensitive to the external electric field induced by the STM tip nor to the length of the bridging part, and is well explained by electronic confinement in a one-dimensional cylindrical potential well. We discuss the possibility to specify the effective mass of electrons in the QPC.