Transport Process In Topological Superconductor/Quantum Dot Hybrided Systems Modulated By Majorana Bound States

Majorana fermions,a special particle in high-energy physics that is its own antiparticle and therefore has no energy or charge,and have been extensively studied both theoretically and experimentally in recent years.It has been proved that the Majorana fermions can be realized in the form of a bound state at the edge or topological defect of the unconventional superconductor to form a quasi-particle,i.e.,Majorana bound states(MBSs).These states are topologically protected and follows the non-abelian rather than the usual fermi-dirac statistical rule,so it is an ideal choice for the preparation of quantum bits in topologically fault-tolerant quantum computing.The MBSs also have a significant influence on the electron transport process in mesoscopic systems and has been studied in the field of fabricating efficient and energy-saving quantum devices.We investigated the topological superconducting nanowires-quantum dots Majorana bound state modulation in the mixed system of transport process Majorana in this study.The characteristics of topology Majorana spectral function for two quantum dots coupled to the ordinary metal probe scanning tunneling microscopy(STM)and superconducting nanowires were discussed,taking into account the adjustable energy of a quantum dots,coulomb interaction,quantum dots and electrode coupling strength and different electronic spin direction in different Fermi level;A model of interaction between a single quantum dot connected with a ferromagnet and a scanning tunneling microscope probe and the Majorana bound states at both ends of a topological superconducting nanowire was developed to investigate the effects of Majorana bound states and the spin heat accumulation in a ferromagnet on the spin flow.The general properties of the Majorana bound state and the advance in the research,the background of the topological superconducting nanowire-quantum dot hybrid system and the main effects in the system were described in chapter 1.Then,Green's function calculation method was introduced in chapter 2,and the effect of the two quantum dots coupled to the common metal scanning tunneling microscope probe and topological superconducting nanowire system on the spectrum function was studied in detail.The effects of single quantum dots coupled to ferromagnets,common metal scanning tunneling microscope probes and Majorana bound states atboth ends of topological superconducting nanowires on differential conductance and spin flow were studied by motion equation method in chapter 3.In the double quantum dot structure,we found that the value of the zero-energy spectral function remains to be 1 when an interaction exists between the quantum dot and Majorana,indicating that the signal of the Majorana bound state enters the quantum dot,which is also the reason for the zero-bias conductance peak.It was also found that the signal entering the Majorana bound state of the quantum dot is independent of the coupling strength between the quantum dot and the scanning tunneling microscope probe,the difference of energy levels in the two quantum dots,and the relative coupling strength between the two quantum dots and the Majorana bound state,revealing that the Majorana bound state will be very stable once it enters the quantum dot.This kind of Majorana bound state in space separation is one of the key problems to be solved in topological fault-tolerant quantum computation.In a single quantum dot system,we found that a state where the charge flow is zero and the spin flow is not zero,that is,pure spin flow,can be generated by changing the coupling strength and the accumulation of spin heat between the quantum dot and the Majorana bound state.Under the condition of certain Coulomb interaction in a quantum dot,the direction of pure spin flow can be changed by modulating the quantum dot energy level and the coupling strength between quantum dot and the Majorana bound state.This may be used in spintronics devices.