Quantum Coherent Transport Study Based On Bi₂O₂Se Micro-Nano Devices

The development of quantum computing-based information technology is expected to trigger a new quantum technological revolution.The construction of topologically protected qubits based on non-abelian statistics of Majorana zero energy modes（MZMs）is expected to solve the decoherence problem and realize fault-tolerant topological quan-tum computing.The experimental observation of MZMs is the first and key step,and one-and two-dimensional systems with strong spin-orbit coupling（SOC）are one of the platforms to realize MZMs.Among them,strong SOCs play an important role in achieving effective p-wave superconductivity.This has motivated researchers to search for suitable systems with strong SOCs to study in the hope of realizing experimental observations of MZMs.A new semiconductor material with high mobility and strong SOC,Bi₂O₂Se,has entered our research field.Bi₂O₂Se has high mobility,excellent electronic properties and environmental stability,which makes Bi₂O₂Se one of the good platforms for build-ing topological quantum devices.In this paper,we investigate the quantum transport phenomena based in strong SOC Bi₂O₂Se micro and nano devices,focusing on the quantum phenomena related to superconductivity,SOC and Zeeman effect,in order to lay foundation for the realization of MZMs.The first part of the paper presents an experimental study of the anomalous Joseph-son effect based on Bi₂O₂Se nanoflakes.Studying the interaction of spin-orbit coupling and Zeeman effect in Josephson junctions is a key step to realize MZMs.The interac-tion of spin-orbit coupling and Zeeman effect causes the 0-transition and anomalous Josephson effect in the system.Therefore,based on superconducting quantum inter-ference devices（SQUID）on Bi₂O₂Se nanoplates,we applied an in-plane parallel field to reflect the anomalous phase shift by tracing the shift of the interference pattern.In our experiments,we observed the fundamental phase shift and found that the unstable behavior caused by the para-magnetism of the superconducting electrode near the zero magnetic field also leads to the phase shift,which can strongly interfere with the real anomalous phase shift.However previous studies did not pay enough attention to this issue.In the Bi₂O₂Se nanoplates,the gate tunability is limited and we did not achieve modulattion of the anomalous phase shift by gate tuning.In the second part of the paper,the diamond-shaped evolution of the superconduct-ing interference pattern in Nb Ti N weak-link Josephson junctions based on Bi₂O₂Se nanoplates is presented.Firstly,introducing in-plane Zeeman effect into Josephson junctions is of fundamental significance in exploring a variety of interesting physical phenomena,such as Zeeman-driven 0-transition,planar topological superconduc-tivity,etc.However,the orbital effects introduced by the tiny differential geometries（known as rippled structures）in realistic system can lead to characteristic phase interfer-ence phenomena.In our study,upon application of in-plane field,the superconducting interference pattern based on Nb Ti N weak link Josephson junctions exhibits a charac-teristic evolution pattern,which manifests as a diamond block diagram.We explore the orbital effect of rippled structure and further take into account of non-uniform current density distributions pertaining in our junctions.We found the rippled effect in com-bination with specific current density distribution can give rise to the similar evolution pattern as revealed in the experimental case.Our results reveal the characteristic inter-ference behavior of the combination of orbital effects with specific current distributions in the real system,which has important implications for understanding the geometric interference phenomena in Josephson junctions,and also has important references for experimental studies of the Zeeman-driven 0-transition.In the third part of the paper,we extended the 2D system to the 1D nanowires and a systematic quantum transport study were performed.Bi₂O₂Se nanowires syn-thesized by chemical vapor deposition method present a high field-effect mobility up to～1.34×10⁴cm²V^-1s^-1and a gate-tunable SOC（spin relaxation length ranging from～100 nm to～250 nm）.The nanowire exhibited quasi-ballistic transport in the low back-gate voltage((1₂))regime where conductance plateaus were observed.When further increasing the electron density by increasing(1₂),we entered the phase coherent regime and weak antilocalization（WAL）was observed.The spin relaxation length extracted from the WAL was found to be gate tunable and reaching a stronger spin-obit coupling（SOC）than the two-dimensional counterpart（nanoplates）.We attribute the strong SOC and the gate tunability to the presence of a surface accumulation layer which induces a strong inversion asymmetry on the surface.Such scenario was supported by the obser-vation of two Shubnikov-de Haas（Sd H）oscillation frequencies that correspond to two types of carriers,one on the surface,and the other in the bulk.Therefore,high-quality Bi₂O₂Se nanowires present high mobility,strong SOC,and gate tunability,making it a prospective material in future electronics and spintronics,and a potential platform for realizing helical states and further for topological quantum computation.Lastly,the summary and prospect of the paper are presented.In order to better understand and utilize the excellent properties of Bi₂O₂Se,we still need to continue to carry out theoretical and experimental studies,optimize the synthesis methods and improve the device design,etc.Looking forward to the realization of high-performance,scalable quantum computing systems in the near future.