Quantum Transport Studies On Hybrid Superconductor-semiconductor Nanowire Devices

In condensed matter physics,Majorana fermions act as quasiparticles and are known as Majorana zero modes(MZM).They are expected to be applied in topological quantum computation and therefore have attracted wide attention.Theoretically,it is proposed that an artificial topological superconductor can be induced by combining a semiconductor nanowire with strong spin-orbit coupling and a superconductor,and MZMs appear at the ends of the topological superconducting nanowire.Zero-bias conductance peak(ZBCP)can serve as one of the signatures for the existence of MZM.However,in most of the current experiments,ZBCP can be explained by other trivial mechanisms,such as reflectionless tunneling,Kondo effect,Josephson supercurrent,weak antilocalization effect,trivial Andreev bound state(ABS),quasi-Majorana and disorder,which makes it a considerable challenge to prove the existence of MZM unambiguously.In order to further explore MZM and its related physical phenomena,the superconductor-semiconductor nanowire hybrid devices based on InAsSb nanowires were studied in depth.Compared with the widely studied In As and In Sb nanowires,the theory predicts that InAsSb nanowires have stronger spin-orbit coupling and larger g factor,and have great potential in the study of MZM.The first work presented in this dissertation is the fabrication of InAsSb-Al nanowire hybrid devices and the investigation of their transport properties at low temperatures.By analyzing the experimental results,we consider that a quantum dot is formed at the junction area between the normal metal and the superconducting nanowire,and this quantum dot is coupled with the ABS in the nanowire.Different coupling strengths correspond to different gate-voltage diagrams.When the quantum dot is weakly coupled with the ABS in the nanowire,the gate-voltage diagram shows a pair of parallel lines.In this case,the magnetic field evolution of the ABSs shows a very stable ZBCP,which may be quasi-Majorana states in the inhomogeneous nanowires.When the quantum dot is intermediately coupled with the ABS in the nanowire,the diagram shows a double arc,and the anti-crossing region can be formed at the center position.In the intermediate coupling case,ZBCP forms later than weak coupling,which we think is caused by the“leakage” of ABS in the nanowire to the quantum dot.When the quantum dots are strongly coupled with the ABS in the nanowires,the phase diagram is similar to the single quantum dot phase diagram with one arch.This work enriches the gate-voltage diagram of quantum dot-ABS coupling and increases the understanding of ZBCP in nanowire hybrid systems.In addition to the widely studied ZBCPs,there are many other interesting physical phenomena in superconductor-semiconductor nanowire hybrid devices.The second work of this dissertation is the study of negative differential conductance(NDC)in superconductor-semiconductor nanowire hybrid devices.NDC outside the superconducting gap is observed in two different geometric structures of normal metal-superconducting nanowire-normal metal(N-SNW-N)device and normal metalsuperconducting nanowire-superconductor(N-SNW-S)device based on InAsSb nanowires,which can be tuned by the gate voltage and magnetic field.As the gate voltage increases,NDC turns into a positive differential conductance dip and moves to a higher bias voltage.When the magnetic field increases,NDC disappears as the superconducting gap closes.The BTK-supercurrent model we proposed gives a good explanation for NDC,and our study helps to understand the physical phenomena in semiconductor-superconductor hybrid devices more deeply and provides references for related research.In summary,the hybrid superconductor-semiconductor devices based on InAsSb nanowires were constructed,and the quantum transport phenomena were systematically studied and analyzed in this dissertation.Understanding ZBCPs and the novel physical properties in devices deeply,enhancing our knowledge and comprehension of transport characteristics in hybrid superconductor-semiconductor nanowire systems will contribute to advancing the progress of research related to MZM.