First-principles Investigation On Rashba Spin-Orbit Coupling In Two-Dimensional Transition Metal Dichalcogenide

The conventional microelectronic devices based on the electron charge are restricting the development of semiconductor chip because of the power dissipation,heat dissipation and quantum effects.Thus,the spin electronic devices based on electron spin property emerge as the times require.Novel spintronic devices attract the sustained attention of scientists due to the high speed,low power consumption,small size and so on.The spin-orbit coupling effect(SOC),as an important physical mechanism in spintronic devices,is of great significance in the study of spin modulation and relaxation.Among the SOC,the Rashba SOC in the low dimensional semiconducting materials can be controlled by the electric field and is beneficial to the circuit integration,thus is expected to be the material of the spin electronic devices and has become a research hotspot.The discovery of the graphene has led to the prosperous development of two-dimensional materials.Due to the controllable semiconductor badgap,the two-dimensional transition metal dichalcogenide have great potential applications in the fields of spintronics and valley electronics.In this dissertation,using the first-principles calculation and our developed orbital selective external potential method(OSEP),we study the Rashba effect in two-dimensional transition metal dichalcogenide through the stress manipulation and electric manipulation.The specific contents are as follows:1.Using density functional theory calculations,we investigate the manipulation of Rashba spin-orbit coupling(SOC)in the polar transition metal dichalcogenide(TMD)monolayers MXY(M = Mo,W;X ? Y = S,Se,Te)by setting WSeTe as an example.It is found that the intrinsic out-of-plane electric field due to the mirror symmetry breaking induces the large Rashba spin splitting around the ? point,which,however,can be easily tuned by applying the in-plane biaxial strain.Through a relatively small strain,a large tunability of Rashba SOC can be obtained due to the modified orbital overlap,which can in turn modulate the intrinsic electric field.The orbital selective external potential method further confirms the significance of the orbital overlap in Rashba SOC.In addition,we also explore the influence of the external electric field on Rashba SOC in the WSeTe monolayer,which is less effective than strain.2.We investigate the Rashba spin-orbit coupling of the transition metal dichalcogenide(TMD)monolayers MX2(M = Mo,W;X = S,Se,Te)by applying external electric field.It is found that the anions X play an important role on the manipulation of the external electric field induced Rashba spin-orbit coupling effect.With the increase of the atomic number of X,the degree of the Rashba spin-orbit splitting around the ? point increases more distinctively,and the external electric field can hardly influence the cations because of the coverage by the anions.Thus the strength of the Rashba spin-orbit coupling from large to small is:WTe2>MoTe2>WSe2>MoSe2>WS2>MoS2.Furthermore,the distribution of the spin polarization of the MX2 monolayers at the surface first Brillouin zone along the high symmetry line?-K/K' turn from the vertical direction to the two-dimensional plane under the external electric fields,and the in-plane spin polarization distribution rises with the increase of the external electric field.In addition,the strength of the Rashba spin-orbit coupling varies linearly with the external electric fields.