Band renormalization of two-dimensional crystals at two doping limits

Abstract

After the emergence of the two-dimensional (2D) crystals such as graphene, black phosphorus, and transition metal dichalcogenide, they have attracted great attention with their potential to understand fundamental physics. Due to van der Waals interaction between interlayers, 2D crystals can be regarded as one-atom-thick materials which are the counterparts of three-dimensional bulk materials. This low dimensionality enables the wave function of an electron to be confined in 2D where the more effective interaction is expected owing to the increase of wave function overlap. In contrast to bulk crystals, one can control the charge carrier density of 2D crystals which is less sensitive to screening effect, via gating or in situ surface doping. Also, 2D crystals can be a playground owing to their simple atomic and orbital structure not only in theoretical study but also in the experiment. We focused on the interaction between electrons and quasi-particle such as phonon and plasmon in simple 2D materials. It is important to understand these kinds of scatterings because charge carrier behaviors in solid are closely related to these interactions. Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool to examine electron-quasiparticle interactions. ARPES can be used not to measure the band structure of materials but also to acquire the information of interaction. By employing self-energy from single-particle green function, we could expect the strength of the interaction. When considering interaction, the band dispersion is renormalized. One can extract the information such as half-width at half maximum (HWHM) at each energy and the energy difference between bare and dressed band from ARPES data. In this thesis, we employ ARPES technique to analyze and to quantify the strength of electron-quasiparticle interaction in 2D crystal. Firstly, we studied the electron-phonon coupling in Rb-decorated monolayer graphene grown on H- terminated SiC(0001). We made 2 × 2 Rb phase on graphene with parabolic Rb- derived band at Γ point which is surrounded by six folded Dirac bands. Also, we conducted the self-energy analysis both of Rb band and graphene band. Then, we extracted electron-phonon coupling constant and found the in-plane vibrational mode of Rb atoms plays a key role for the enhancement of electron-phonon coupling. Secondly, we used K and Rb as a dopant to perform the study for the evolution of electron-plasmon interaction strength with doping in alkali metal doped monolayer graphene. While alkali metal surface doping, we found that the coupling strength keeps constant. Also, the coupling strength in Rb-doped graphene is higher than that in K-doped graphene.