Tip-induced Superconductivity On Topological Materials

As an important branch of condensed matter physics,superconductivity has attracted continuously extensive attention of researchers due to its unique physical properties and potential practical values,since the discovery in 1911.From single element,alloy to complex compounds with multiple elements,from conventional superconductivity to unconventional superconductivity,the researches of superconductivity have made a series of important achievements,whereas also confronted with many opportunities and challenges.In recent years,the topological nontrivial material has attracted considerable attention in material science and condensed matter physics,due to their unique electronic properties and promising applications in quantum computing.It has a novel state with nonzero topological invariant and there always exists exotic surface states on the boundary between bulk and vacuum.As the extension of topological properties in superconducting materials,the concept of topological superconductivity comes into being.The topological superconductor possesses a superconducting bulk state with gap,while there exists Majorana bound state without gap on the surface or boundary.Majorana quasiparticle can be potentially used to realize highly fault-tolerant quantum computing due to its non-Abelian statistical properties,resulting in the significantly improvement in computing efficiency.This enormous application prospect has attracted close attention from researchers.It is an effective way to achieve topological superconductivity by introducing superconductivity in non-superconducting topological materials.For instance,superconductivity has been observed in various topological materials by applying high pressure,charge carrier doping,or the proximity effect up to now.As a characteristic technique to detect superconducting order parameters by Andreev reflection,point-contact plays an important role in the field of superconductivity research.In recent years,several research groups home and abroad have detected superconducting signals directly on the surface of various non-superconducting topological semimetals by point-contact,providing a possible way to integrate superconductivity and topological properties more easily,which has drawn enormous attention.In this paper,we attempt to induce superconductivity on several topological materials by point-contact,and perform systematical study on the induced superconductivity,providing useful clues in understanding this phenomenon.The main contents are as follows:The first part is the research foundations of this thesis,mainly composed of the first two chapters.In the first chapter,we briefly review the basic concepts and research overview of superconductivity,emphatically introduce the relevant theories of superconductivity,then briefly introduce the spin-singlet pairing and spin-triplet pairing,and finally give a brief overview in topological materials.In the second chapter,we mainly introduce the theoretical and experimental basis of point-contact.According to the difference of the barrier height at the interface between normal metals and superconductors,the transports of electrons at the interface are affected by the single-particle tunneling and Andreev reflection,thus producing specific differential conductance spectra.The information of superconducting gap and pairing symmetry could be obtained by calculating the conductance spectra with the Blonder-Tinkham-Klapwijk(BTK)model.The second part is the experiments of tip-induced superconductivity(TISC)on topological semi-metal materials by point contact,mainly including several transition metal dipnictides and grey arsenic.In the third chapter,we systematically present the tip-induced superconductivity on topological materials TaAs2 and NbAs2.TaAs2 and NbAs2 are members of the MPn2 family with weak topological invariants of Z2=[0;111].We report the TISC achieved on both non-superconducting topological materials using the point-contact technique.The superconducting transition temperatures of TaAs2 and NbAs2 are nearly 2.3-7.9 K and 2.1-5.0 K,respectively.For NbAs2,the Hc2(0)of the pressure-induced superconductivity at 16 GPa is almost one order of magnitude lower than the TISC observed here,indicating very different mechanisms between them,and ruling out the possibility of tip pressure induced superconductivity in point-contact experiments.Superconductivity mainly comes from the interface coupling between metal tips and samples.Though the weak Z2 number leads to strongly crystal-orientation-dependent surface states in MPn2 family,we observed a universal relationship between the zero-temperature upper critical field Hc2(0)and the critical temperature Tc for different crystal orientations of TaAs2 and NbAs2.Combined with first-principles calculations,the findings here could identify a bulk band,instead of the much-speculated surface bands,playing the dominant role in the induced superconductivity in this family.These results predict a great possibility to realize superconductivity with similar properties in other members of the MPn2 family possessing similar band structures.Therefore,in the fourth chapter,we performed point-contact experiments on the NbSb2,another member of the MPn2 family,and successfully achieved TISC with transition temperatures of nearly 2.2-6.1 K.Its properties are consistent with those obtained on TaAs2 and NbAs2,indicating the tip-induced superconductivity may be closely related to their topological nature and share a common mechanism.Further analyses suggest that the bulk band should play the dominant role in such local superconductivity most likely through interface coupling.In addition,the compatibility between the induced superconductivity on MPn2 family and tips’ ferromagnetism give an evidence for its unconventional nature.These results provide further clues to elucidate the mechanism of the tip-induced superconductivity observed in topological materials.In the fifth chapter,we systematically introduce the anisotropic interfacial superconductivity between topological material grey arsenic and normal metal tip.Up to now,tip-induced superconductivity has been observed in a variety of non-superconducting topological semimetals.However,there is some non-intrinsic factors need to be eliminated.For example,the materials previously reported to realize tip-induced superconductivity mostly contain superconducting elements such as Cd,Pb,Ta,Nb,Zr or Sn,or exist superconducting isomers.Therefore,it is significant to realize tip-induced superconductivity in topological materials that no way to contain superconducting impurities.As a topological material composed by single element,no superconducting materials are involved in the preparation of grey arsenic(grey As).And its isomers are not superconducting under normal pressure.Therefore,grey arsenic provides a suitable platform to investigate TISC.We have performed the point-contact experiments on grey As and successfully induced superconductivity up to 9 K.The determined temperature dependencies of superconducting gap Δ(T)distribute randomly around the Bardeen-Cooper-Schrieffer(BCS)law,deviating from the BCS law.A negative correlation between the superconducting gap Δ and the effective barrier height Z was obtained by tuning junction resistance in suit,validating the contact state or coupling strength of interface as an important ingredient in the observed tip-induced superconductivity.In addition,the upper critical field of the detected superconductivity has obvious anisotropy,when the magnetic field along perpendicular or parallel directions of the ab plane.When the magnetic field is perpendicular to the ab plane,the Hc2(0)vs.Tc can be described by a quadratic law.When the magnetic field is parallel to the ab plane,Hc2(0)distribute randomly with some points much higher than the vertical data.The phenomenon can be understood from the anisotropy of the projected interfacial Fermi surface of the bulk band of grey As.These phenomena further imply the importance of the bulk band in the tip-induced superconductivity and provide more important clues for understanding the tip-induced superconductivity.The third part as well as the sixth chapter,summary and look forward to the whole paper.