Investigation On Spin Transport And Spin Pump Current In Silicene

For the last several decades,quantum mechanics continuous to develop conceptual under-standing of the system more profound.Exactly,it has been recognized that even if there is no potential difference in the quantum system,it is possible to generate a nonzero charge current or equilibrium current as long as the parameters of the quantum system change periodically over with time.This is the quantum pump effect.Due to the discovery of the quantum pump effect,the quantum pump has some similarities with other fascinating phenomena such as the per-sistent currents and Superconductivity.The quantum pumping explores the fundamental issues regarding the role of different symmetries in transport.One of the ultimate goals in this research is to find a quantized charge pump that in a pumping periodicity,an integer number of charges are pumped out to flow through the whole quantum system.It is urged that the quantized pump can revolutionize electrical metrology by enabling the ampere to be redefined in terms of elementary charge of an electron.Certainly,the topological charge pumping can be understood as a dynamical analogue of the integer quantum Hall effect.In this thesis,we proposed a quantized pump model based on the traditional pump protocol in the silicene system and showed that two time-dependent staggered potentials with a phase lag as pumping parameters can result in a quantized charge pump effect.The key point is that the pumping potentials introduced can open an energy gap of a massless Dirac electrons of silicene.Since the staggered potentials are very difficult to be operated in graphene,and the massless Dirac electrons of graphene are ubiquitous in the edge or surface of a topological insulator.Firstly,we have investigated the pumped spin currents in a silicene-based pump device.The pumping potentials are from the perpendicular electric fields that results in the staggered potentials in silicene and the magnetization is assumed to be induced by the magnetic proximity effect.We considered mainly an analytic derivation for the low-energy continuum model in this thesis.According to the adiabatic pumping theory,the pumped currents flowing in the devices were found and the currents were quantized.Furthermore,the pure spin currents and fully spin-polarized currents could also be obtained.These findings are very useful in the spintronics field.Secondly,we studied a workable scheme for generating a bulk valley pump current in a silicene based-device by cyclic modulation of time-dependent strain and staggered potentials.Since silicene is a 2D system and possesses a remarkable flexibility,the time-dependent strain would be availed by using mircomechanics method.Because the strain-induced opposite vector potentials at K and K~?valleys generating a?phase shift between the pumping signals of strain in the two valleys,it is possible to acquire a pure valley pump current flowing out the system.However,we showed that it was crucially relying on some parameters of the system such as the pump phase,Fermi energy,and even the device size.Finally,the physics origins underlying the pump quantization in the above two different pumping model are the same:the pumping potential can make the pumping region enter into a topological state,the possible topological interface state between two pumping regions will transport charge dissipationlessly from one electrode to the other electrode.When different degrees of freedom of electrons are involved in the pumping event and they make different contributions like the dynamic phases of electrons or the opposite additive phase in pumping process,the corresponding degrees associated with electron charge will be pumped out like the spin and valley mentioned above.Moreover,the pump degree are quantized.Our study may make a new way to generate a quantized current with specific degree of freedom of electrons and such pump quantization is protected by topology of the system.