Plane-wave-based electron tunneling through Au nanojunctions: Numerical calculations

Electron tunneling across a nanojunction is an important topic relevant to scanning tunnel microscope imaging, nanoconductance measurements, and nanoelectronic devices. To understand such tunneling phenomena, one needs to comprehend the electron-state coupling between the metal electrode and the vacuum, the dependence of such coupling on the shape of the electrode tip, and the dependence of the tunneling currents on the electrode-electrode distance. Due to the experimental difficulty to determine the exact atomic structure of the electrode tip, theoretical simulation can play an important role on such studies. This requires high-fidelity quantum-transport calculations for the tunneling system. However, most of the current quantum-transport calculations are performed using atom-centered localized basis sets, which cannot adequately describe the wave function in the vacuum region. In this work, we present tunneling-conductance calculations obtained using the transport calculation method introduced by Wang [Phys. Rev. B 72, 045417 (2005)] and Garcia-Lekue and Wang [Phys. Rev. B 74, 245404 (2006)]. Since this method employs a plane-wave basis set, it provides variational description for the electron wave functions in all real space. We will present results for the tunneling-current dependence on the electrode-electrode distance, the electrode wave functions in the vacuum region depending on the electrode shape, and electron state couplings between the vacuum and the electrode.