Auger-type light emission from scanning tunneling microscopy junction on Ag(100)

Lu Shin-Ming^1*, Chang Ho-Hsiang¹, Chan Wen-Yuan², Su Wei-Bin¹

¹Institute of Physics, Academia Sinica, Taipei, Taiwan
²Department of Electronic Engineering, Minghsin University of Science and Technology, Hsinchu, Taiwan

* Presenter:Lu Shin-Ming, email:simonluyang@gmail.com

We employ scanning tunneling microscopy (STM) combined with an optical spectrometer to investigate the energy distribution of light emitted from the STM vacuum junction on Ag(100) surface. When the STM bias was set to 5.21V, corresponding to the lowest-order field emission resonance (LOFER) energy, the optical spectra revealed not only the conventional radiative decay of surface plasmons (RDOSP) but also a distinct peak at approximately 3 eV. Notably, this peak persisted even as the bias was reduced to 4.97 V, 4.75 V, and 4.00 V, indicating that electron transitions between quantized states occur in the STM vacuum junction. To explain this bias-insensitive peak, we propose a model based on electron excitation and transition processes. For the bias voltage at the LOFER energy, two-electron tunneling through exchange interaction [1,2] enables an Auger-type process: one electron in the LOFER state emits photons via RDOSP, exciting another electron to a higher-energy state, which subsequently transitions to LOFER state and emits a 3-eV photon. For bias voltages below 5.21 V, one-electron tunneling dominates. RDOSP-generated photons excite an electron in the tip, which then tunnels into a quantized state in the vacuum and transitions to a low-energy state, emitting a photon. This is another Auger- type process: the excitation of electrons occurs in the tip, different from those in the vacuum when the bias was set to LOFER. Using the WKB approximation [3], we calculate the quantized state energy before and after tunneling, considering the applied potential and its superposition with the sharpness-dependent image potential. The energy difference between these states is close to 3 eV across different bias voltages, in agreement with experimental observations.
[1] Nanoscale Adv. 2, 5848–5856 (2020).
[2] Phys. Rev. B 105, 195411 (2022).
[3] New J. Phys. 20, 043014 (2018).

Keywords: Auger effect, field emission resonances, bias-insensitive, two-electron and one-electron tunneling, WKB approximation