Research Of Non-local Quantum Transport In Multi-terminal Junction Devices Based On Topological Insulator

Compared with traditional computers,quantum computers,which rely on new physics and algorithm and are capable to process information much faster,may prevail in the future.Among different approaches of quantum computing,topological quantum computing has the advantage that it is fault-tolerant,intrinsically immuning from environmental perturbation and thus having a long coherence time.This advantage makes topological quantum computing stand out among the different approaches.The quasi-particles for realizing topological quantum computing are Majorana zero modes,which obey non-Abelian statistics.Searching for Majorana zero modes is a key step to achieve topological quantum computing.This dissertation first introduces the research background and significance of the subject,past research progress of non-local electron transport,motivation and experimental protocols.Many research groups in the world have carried out two kinds of experiment methods to prove the existence of Majorana zero modes,either detecting the zero-bias-conductance-peak or searching for 4π-period current-phase relation in topologically non-trivial Josephson junction.According to the existing results,there is still no definite evidence to prove the existence of Majorana zero modes.In this dissertation,we planed to use a different experiment approach,non-local quantum transport via Majorana bound states,to search for Majorana zero modes,hoping to get some progresses on topological quantum computing as well as some insight on entangled topological quantum states.Three-terminal mesoscopic devices composed of normal metal-superconductor-normal metal(N-S-N)structure provide a platform for studying non-local electron quantum transport in solid-state systems.Non-local signals mainly derive from three kinds of physical processes.One is Cooper pair splitting(CPS)and its reverse process cross Andreev reflection(CAR).The second is electron elastic co-tunneling(EC).The thrid is electron teleportation(ET).Experimentally,CPS and EC have been realized in graphene systems,carbon nanotube systems,semiconductor nanowires with spin-orbit coupling and quantum dots systems and so on.CAR has been detected in quantum Hall edge states,Josephson junction based on semiconductor flakes,and so on.Electron teleportation for non-local transport was proposed by Liang Fu in 2010,which is used to design a phase coherent quantum transport system involving Majoranas and modulated by flux for proving the existence of Majorana zero modes.Base on these backgrounds,we plan to use superconducting quantum interference device(SQUID)to modulate non-local quantum transport,for searching for Majorana zero modes.The second part of the dissertation is about nanowires growth,characterization,transferring onto substrate,device fabrication,and measurement configuration at ultra-low temperatures.We used gold-catalyzed vapor-liquid-solid deposition method to grow binary and ternary topological insulator nanowires,nanoribbons,and nanosheet materials,and used the energy dispersive spectroscopy for preliminary material characterization.Then,we used microfabrication techniques to make devices after choosing suitable nanowires and transferring onto substrate.Finally,we carried out measurements in a dilution refrigerator.The third part of the dissertation presents the results of anomalous enhancement in critical supercurrent,multiple Andreev reflection and half-integer Shapiro steps in the topological insulator nanowires-based Josephson junction.We have found out a reasonable explanation for these phenomena.The fourth part of the dissertation first presents the results of non-local transport measurement on topological insulator nanowires-based N-S-N devices.In the traditional three-terminal N-S-N devices,non-local signals mainly derive from Cooper pair splitting and elastic co-tunneling.The sign of the non-local differential conductance is independent of the direction of bias current.In our devices,however,we observed that non-local differential resistance is an odd function of bias current,namely the signal is rectified.This is consistent with the prediction of a theory proposed by T.(?).Rosdahl,et al.in 2018,which demonstrates that Andreev rectifier can be achieved at the topological phase transition,and negative non-local differential conductance is caused by CAR.Then,the dissertation also presents the results of non-local transport measurement on topological insulator nanowires-based superconductor-radio frequency SQUID-superconductor(S-rf SQUID-S)devices.We observed that the non-local differential resistance oscillates periodically with magnetic flux.After considering the loop magnetic flux and the supercurrent distribution in Josephson single junction,we were able to theoretically explain and simulate the experimental data.It turns out that negative non-local differential resistance is caused by cross Andreev reflection.We discussed and gave a reasonable physical picture for the phenomenon.Lastly,we give the summary and prospect of this dissertation.