Studies of work function, electron-phonon coupling constant, and superconductivity transition temperature of Pb thin films on Ge(111)

Abstract

We measured work function of Pb thin films/Ge(111) from 1 to 13 ML. The work function shows a Bilayer oscillation with thin film thickness. Besides, the higher highest occupied quantum well state (HOQWS), the higher work function. A turning point of the bilayer oscillation was found near 11ML. Compare the work function with thermal stability of thin films, we found that the phase difference between work function and thermal stability is 1.62, which is close to the phase difference, 1/4 of periodicity, 1.57 predicted by free electron model. Another topic is the microscopic four-point probes (micro 4PP) conductivity measurement on Pb thin films. We measured the superconductivity transition temperature (Tc) of 3 ML, 4 ML, and 8 ML is 1.60 K, 3.65 K, and 5.21 K. We also measured the electron phonon coupling (EPC) of 6 ML and 8 ML Pb thin film by ARPES. The EPC of 6 ML and 8 ML QWS of Pb thin film is 1.09 and 1.27. From above results, we found that both the Tc and EPC of the Pb films are increasing with film thickness. It is concordant with many results found in similar systems. Finally, we made a comparison and discussion of superconductivity transition temperature by micro 4PP measurement and by EPC prediction.