First-Principles Calculations Of The Rashba Spin-Orbit Coupling In Two-Dimensional Semiconductors

The spintronics based on spin offers significant advantages such as low power consumption,non-volatility,high processing speed,and high integration,and thus holds great promise for information storage and processing.In particular,semiconductor spintronics has significant advantages in the integrated manufacture of spin devices.Spin-orbit coupling provides a purely electrical means of manipulating electronic spin.Among these,the Rashba spin-orbit coupling,arising from structural inversion symmetry breaking,can be tuned by an external electric field,and can be designed as novel spin electronic devices.In addition,two-dimensional(2D)materials are more favorable for miniaturization of devices compared with three-dimensional(3D)materials.In this doctoral dissertation,we present a comprehensive study of the Rashba effect in 2D semiconductor materials using first-principles calculation softwares,including the manipulating methods,materials with large Rashba constants and strong electric field response,and the coupling of the Rashba effect with ferroelectricity,as follows:We investigate the mechanism of the Rashba effect in Janus transition metal dichalcogenides(TMDs)by manipulating the Rashba effect through charge doping.Unlike non-polar TMDs,Janus TMDs(chemical formula MXY,M=Mo/W,X/Y=S/Se/Te,X≠Y)are polar semiconductors with mirror symmetry breaking,and the Rashba effect exhibits in the valence band maximum.We demonstrate a new method for manipulating the Rashba effect in Janus TMDs:charge doping.We find that electron doping can enhance the Rashba effect in Janus TMDs,while hole doping weakens the Rashba effect.Based on the first-principles calculations,we propose an electric triple-layer model,revealing the mechanism of tuning the Rashba effect in Janus TMDs through charge doping,and find that charge transfer plays an important role.This work enriches the understanding of the Rashba effect.Based on our understanding of the mechanism of the Rashba effect,we discover 2D polar perovskite materials with the Rashba effect.2D polar perovskite ABX3(A=Cs/Rb,B=Pb/Sn,X=Cl/Br/I)derived from tetragonal and orthorhombic 3D perovskites possess the Rashba effect in the valence band maximum.Among them,2D orthorhombic RbSnI3 has the largest Rashba constant(α=1.176 eV·?),comparable to that reported 3D Rashba perovskites in theoretical and experimental studies.In addition,some 2D polar perovskites exhibit strong electric field response,particularly 2D tetragonal RbPbI3 and CsPbI3,with electric field response greater than 0.5 e·A2.2D polar perovskites with large Rashba constant and strong electric field response are promising Rashba materials.The spin field-effect transistors based on 2D polar perovskites can have short spin channel lengths(tens of nanometers),which is in favor of the preservation of spin coherence.The common feature of Rashba and ferroelectric materials is their polar structure.The external electric field can flip the ferroelectric polarization direction,suggesting that ferroelectric Rashba materials may achieve flipping of spin texture through the external electric field.Instead of investigating specific properties of a certain 2D semiconductor,we apply a deep understanding of the mechanisms of the Rashba effect and ferroelectricity to perform inverse design based on the C2DB database,and find all 2D ferroelectric Rashba semiconductors.After screening materials from a database based on the enabling design principles,we identified three potential types of structure that simultaneously possess the Rashba effect and ferroelectricity,including A2P2X6 type(space group P31m),ABP2X6 type(space group P3),and AB type(space group P3m1).By performing high-throughput density functional theory calculations of three types of structure and material screening by the optimizing design principles,we find that 14 AB monolayers are promising 2D ferroelectric Rashba semiconductors due to their pure Rashba effect in the conduction band minimum,thinnest 2D Rashba structure,and surmountable energy barriers for ferroelectric polarization.For ferroelectric Rashba semiconductors,the spin texture can be reversed by switching ferroelectric poIarization via nonvolatile electric control.The above research provides a guiding theoretical basis for future spintronics.We summarize the above work at the end of the doctoral dissertation and anticipate further exploration in two directions:one-dimensional SOC,and high-throughput calculations of indirect-to-direct bandgap transition.