Spin-dependent Transport Properties in Heavy-metal AdatomsAdsorbed on Graphene Nanoribbons

• Main Authors: Shun-Jhou Jhan, 詹舜州
• Other Authors: Ching-Ray Chang
• Format: Others
• Language: en_US
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/8s5qtn

碩士 === 國立臺灣大學 === 物理學研究所 === 107 === Graphene, a two-dimensional material which has become active since first created by Scotch tape method at 2004. Due to some advantageous characteristics in spintronic such as the long spin-relaxation time at room-temperature, carrier high mobility, and easy to fabricate, the study of the spin properties of graphene has turned into a significant topic at the last decade. However, the intrinsic spin-orbital coupling (SOC) in graphene is too weak to observe Quantum Spin Hall effect, a specific property which theoretical expected and also known as 2D Topological Insulator. Furthermore, the weak intrinsic SOC is also difficult to manipulate. Therefore, enhance intrinsic SOC or induce large extrinsic SOC in graphene, has become a new direction to investigate the spin transmission in graphene. Adsorbing heavy-metal adatoms such as Thallium (Tl), Indium (In), Gold (Au) randomly on graphene can increase a large SOC, and a robust quantum spin hall (QSH) state in Thallium(TI) adatom Graphene was predicted by an ab-initio article in 2011. Nevertheless, we can’t see QSH state in Gold (Au) adatoms graphene. It may due to the Rashba effect which is induced by the mirror symmetry-breaking at Z-plane is dominant than the intrinsic SOC and destroy QSH state in gold adatoms graphene. In this thesis, we use the Tight Binding model and Non-equilibrium Green’s Function (NEGF) approach to simulate the electron transport properties in graphene nanoribbons (GNR) which decorated with Au and TI adatoms. The aim of this study is to investigate the difference between Au and TI adatoms GNR. The distinguishable SOC and Rashba effect in both cases make a huge impact and change the spin-dependent transport properties such as spin relaxation, charge distribution, and electrical conductivity. By comparing those differences, it may help us to clarify the influences of SOC and Rashba effect in graphene. %Most importantly, adsorbing atom on graphene may be a promising direction for manipulate graphene Our results supports that the Rashba effect which is induced in gold adatom graphene nanoribbons is harmful for the QSH phase, and we also use the NEGF method to examine the quantized conductance and chiral edge state in Tl adatom graphene nanoribbons, the both prosperity under time-reversal symmetry is the evidence of the QSH phase in Tl adatom graphene nanoribbons. Moreover, the faster Spin-relaxation in gold adatom graphene is also demonstrated in our simulation.