| Superconducting materials have excellent properties and have great prospects in the field of basic research and application.In recent years,heterojunctions composed of superconducting materials,topological materials,nanomaterials and so on have become one of the hot spots in scientific research.The size of the superconductor heterojunctions is in the order of micrometer or even nanometer scale,and there are abundant physical phenomena in them,such as the superconducting proximity effect and Andreev reflec-tion.Combined with magnetism,topology,etc.,new electronic states will appear.Espe-cially the topological superconducting state which has non-Abelian statistics is a strong candidate for quantum computing,and the possible existence of Majorana fermions is also a popular research direction in condensed matter physics.Superconductor hetero-junctions have become an important branch of modern physics.When the material size decreases to the nanometer scale,the quantum size effect will trigger novel phenomena different from bulk materials.The physical properties of nanoscale superconductors and their interactions with other materials have become an emerging research field,nanosu-perconductivity science.We fabricated three types of superconductor heterojunctions using micro-nanofabrication techniques:superconducting Josephson junctions,point contact junctions of superconductor with Weyl semimetals,and nanoscale supercon-ducting semiconductor heterojunctions.The transport properties of the three types of superconducting heterojunctions such as zero-bias conductivity peaks in superconduct-ing tunneling spectra（differential conductance curves）,electron spin polarizability,tun-neling spectrum line type inversion,etc.,were investigated.The possible microphysical mechanism explanations of these unusual experimental phenomena such as the evolu-tion law of the superconducting coherence peaks（valleys）,conductance peaks（valleys）in the tunneling spectrum and the abnormal magnetoresistance effect,etc.,were pro-posed to understand their mechanism,and preliminary computational characterization of the results was carried out by using classical theory.This paper mainly includes the following content:（1）Benefiting from the progress of modern micro-nano processing technology,combined with thin film and single crystal growth technology,we have explored our own sample preparation process on the basis of summarizing the experience of prede-cessors,and we have successfully obtained three types of superconductor heterostruc-tures:Nb/Al-AlOx/Nb structure superconducting Josephson junction,magnetic Weyl semimetal Co3Sn2S2 point contact junction with superconducting Nb film and Nb/Si superconductor semiconductor heterojunctions at the nanoscale.（2）The paper firstly studies the superconducting Josephson junction constructed of Nb/Al-AlOx/Nb（hereinafter briefly referred to as Nb junction）.In order to ensure the quality of the Nb junction,we first optimized the experimental parameters and ob-tained an Nb film with a superconducting critical temperature close to that of the bulk material and a small surface fluctuation.Then the quality of the Nb junction was con-firmed by the scanning electron microscope（SEM）topography and R-T curve,but there was residual resistance when the Nb junction was in the superconducting state.The Ⅰ-Ⅴ curve of the Nb junction has no hysteresis,the nonlinearity is not obvious,and it is in the over-damping region.We obtain three types of differential conductance spectra of Nb junctions:the typical Giavere tunneling spectrum of conventional super-conductors,the Andreev enhanced conductance tunneling spectrum and the latter one where the superconducting energy gap is much lower than the corresponding value of the critical temperature.There are also special phenomena such as satellite peaks,zero-bias conductivity peaks,and superconducting coherence peak-to-valley transitions,etc.We try to explain the origins of Andreev conductance enhancement,zero bias conduc-tance peak,satellite peaks,and line shape changes using the microscopic mechanism of local hot spots model,size effects for point-contact transport classification,and super-conducting proximity effects,but the quantitative results of Nb junctions still need to go further.There is also plenty of room for optimization in the preparation process.（3）The thesis is followed by the study of transport properties of the point contact junction between the magnetic Weyl semimetal Co3Sn2S2 and the superconducting Nb film.First,the high quality Weyl semimetal Co3Sn2S2 was confirmed by X-Ray Diffrac-tion（XRD）and electron energy loss spectrum（EDS）.The transport measurement of the point contact junction focuses on the appearance of the zero-bias conductivity peak in the tunneling spectrum,but unfortunately this zero-bias conductivity peak is not caused by topological superconducting state.It is most likely due to Andreev reflections or lo-cal hot spots effect,and the exploration of topological superconducting states requires further research.The fine structures such as satellite peaks are initially understood uti-lizing the superconducting proximity effect.We fitted the superconducting energy gap and critical current of the point-contact junction using BCS theory,and obtained the electron spin polarizability（which is an important parameter for spin device applica-tions）of the Co3Sn2S2 single crystal from the point contact tunneling spectrum,but the result of the electron spin polarizability is small,which may be caused by the low quality of the point contact junction.We demonstrate that the point contact junction is the most direct method of measuring the electron spin polarizability of the material.The scheme to realize topological superconducting state through the superconducting proximity effect is feasible,but further efforts are needed.（4）The final part of the thesis is the research on the abnormal magnetoresistance in the Nb/Si heterojunction composed of superconductors and semiconductors at the nanoscale.When the ultrathin Nb film（8 nm）is in the superconducting state,a novel giant-magnetoresistance-like magnetoresistance effect occurs in the Nb/Si heterojunc-tion:there is a peak in the resistance of the heterojunction under zero external magnetic field;as the magnetic field increases,the junction resistance drops rapidly,eventually reaching saturation（essentially no change with magnetic field）,the resistance change rate is as high as 85%.First,we ruled out the contribution of the ultra-thin Nb film to the anomalous magnetoresistance.The subsequent variation of the abnormal magnetoresis-tance with temperature indicates that the abnormal magnetoresistance is caused by the Nb/Si junction and is closely related to superconductivity.Accordingly,two possible qualitative microscopic explanations of Giaver electron tunneling and Andreev reflec-tion based on the superconductor-barrier layer-metal SIN junction which is formed at the interface of the Nb/Si heterojunctions owing to the existence of the semiconduc-tor Schottky barrier,surface oxidation or others are proposed,but lacks a quantitative description.The inflection point behaviors in the R-T and R-H curves of Nb/Si het-erojunctions are most likely related to the columnar crystal structure of the ultrathin Nb films.Similar heterojunctions of superconductors and semiconductors have potential applications in integrated circuits and superconducting quantum computing. |
