Study Of Spintronic And Valleytronic Transport In Heavy Group-? Monolayers

Since the sucssesful fabrication of monolayer graphene in 2004,a serials of two-dimensional materilas such as boron nitride,silicene and Mo S₂ have attracted much attention in their fabrications and studies,and thus provide many possibilities for designing and exploring new types of quantum devices with low dissipation.This thesis focuses on the heavy group-IV monolayers,such as silicene,germanene and stanene,and explores the realization of the design of quantum components and novel physical effects associated with the electron spin and valley degrees of freedom,with the application of the research method based on theoretical derivations assisted by numerical calculations.In the starting two Chapters,we mainly introduce the fabrication,structure and fundamental porperties of several typical two-dimensional materials,and some basic concepts and theorectical methods relevant to electronic transport.In Chapter 3,we show our study of the spin diode effect based on the concept of valley.In the ferromagnetic/antiferromagnetic junctions based on heavy group-IV monolayers such as silicene,germanene and stanene,a bipolar-unipolar switch of spin diode effect was realized by applying the driving of longitudinal bias and modulating the interlayer electric field.When the electric field intensity is zero,only electrons from one valley contribute to electric current,with one spin only moving forward and the opposite spin only moving backward,thus corresponding to a bipolar spin diode.When the electric field intensity increaset,the active valley can be switched to the other valley,and only electrons of one spin orientation are involved in transport and only move forward,corresponding to a unipolar spin diode.By reversing the direction of the electric field intensity,the bipolar or unipolar regime of the spin diode does not change as a result of the symmetry in transport.The signals produced by the spin diode are strong enough to be observed even when the magnetic exchange fields are on the order of 1 me V.In Chapter 4,we present the realization of the novel spin Seebck effect and valley Seebeck effect.The conventional spin Seebeck effect concerns only about the electron spin and charge transport.In silicene with ferromagnetic exchange field,we found the longitudinal spin Seebeck effect,i.e.the electrons with opposite spins counterpropogating,was realized by introducing temperature difference,as expected.In a device composed of two ferromagnetic electrodes and the central electrically-controllable gate region,it was discovered that the electrons from two different valleys counterpropagate,correspongding to the unconventional valley Seebeck effect.By further tuning the gate voltage,the switch between valley Seebeck effect and spin Seebeck effect was realized.In Chapter 5,we show our research of the new types of valley-locked spin Seebeck effect.Under the condition of the out-of-plane asymmetric exchange field,this valley-locked spin Seebeck effect was realized,i.e.the electrons for the oppositing spins counterpropogating only come from one valley,however,the other valley is not conducting due to the presence of energy gap.Futher calculations demonstrate that such an effect could be broken by high temperature,weakened as the interlayer electric field increases,but enhanced by increasing the temperature difference between source and drain.This effect is also robust against the perturbations such as Fermi energy and magnetization magnetitude.These results were suggested to be applied in designing and exploring of spin-valley logic electronic devices.Finally,we give the conlusion and prospect as well as the acknowledgements.