Single Crystal Growth And Transport Properties Of Several Bi(Sb)-Based Topological Materials

In the past two decades,topological material has become an important research direction in condensed matter physics.Different types of topological materials correspond to different topological states based on their unique electronic band structures,and exhibit exotic physical phenomena,such as weak（anti）localization（W（A）L）,negative magnetoresistance,extremely large magnetoresistance（XMR）,Shubnikov-de Hass（Sd H）oscillation,nontrivial Berry phase,ultrahigh carrier mobility and so on.Therefore,exploring new topological materials and studying the transport properties of topological materials with novel electronic states have become a hotspot in condensed matter physics.At present,topological superconductor,as an important platform for Majorana fermion research,has received extensive interest,but lack of suitable materials.Type-Ⅱnodal line semimetals（NLSMs）,as a newly proposed topological semimetal,which is supposed to exhibit different magnetic,optical,and transport properties.However,there are still many blanks to be filled for the basic physical properties of type-ⅡNLSMs.The main contents of this paper are as follows:First of all,we grew Ba₅In₄Bi₅ single crystals with noncentrosymmetric structure by the flux method.A superconducting transition at 4.1 K was observed for the first time.The magnetization,resistivity,and specific heat measurements reveal that Ba₅In₄Bi₅ is a type-II moderately coupled superconductor.And the specific heat capacity data can be fitted by the Bardeen-Cooper-Schrieffer theory based on the s-wave superconductor.Importantly,the electronic band structure of Ba₅In₄Bi₅ with spin-orbit coupling suggests it is a noncentrosymmetric nodal chain semimetal featuring nodal chain loops at the high-symmetry points R and X.The presence of intrinsic superconductivity in a possible topologically nontrivial state makes Ba₅In₄Bi₅a new material platform to study the possible topological superconductivity.Secondly,we systematically studied the magnetoresistance and anisotropic magnetoelectric transport properties of Ca Bi₂single crystal,which possesses both superconductivity and topologically nontrivial band structure.A magnetic-field-induced upturn behavior with a plateau in resistivity and XMR at low temperature were observed.The possible reason for the magnetic field and temperature dependence of resistivity and XMR at low temperature were discussed by adopting the Kohler’s scaling law,which can be understood by the compensation effect confirmed by the Hall effect measurement.In addition,the gourd-shaped out-plane angle-dependent magnetoresistance indicates that the Fermi surface has a two-fold symmetry and anisotropy.Through the analysis ofρ（H）curves obtained at a fixed temperature with various angles by using the magnetic-field scaling factor,it was found that the mass anisotropy of Fermi surface is 2.64 at 3 K and has no obvious change withing 3 K-10 K,which reflects the anisotropy of Fermi surface at this temperature.Our results deepen the understanding of Ca Bi₂ and provides an important material platform for further exploring the relationship among topology,superconductivity,and XMR.Thirdly,the magnetoelectric transport properties of the potential type-ⅡNLSM Mg₃Bi₂ were systematically studied.A strong anisotropy is revealed in the transverse magnetoresistance at 2 K,which is 5.9%for H⊥ab plane and about 115%for H//ab plane under a magnetic field of 5 T when I//ab plane.The anisotropy was further confirmed by the angular magnetoresistance.In addition,a cusp-like magnetoresistance was observed at low temperature under low magnetic field perpendicular to（0kl）（k=integer,l=2k）plane with I//（0kl）,which implies the presence of WAL in Mg₃Bi₂ single crystal.We found that this behavior may come from the contribution of the two-dimensional surface state through the analyses of the Hikami-Larkin-Nagaoka model and angle-dependent magnetoresistance.Our findings provide a deeper understanding of the physical properties of Mg₃Bi₂ and establish a solid foundation for further research on the exotic features of type-II NLSM candidates.Finally,we grew Sr Zn₂Sb₂ single crystals by the flux method using Sb as flux,which crystallizes in a Ca Al₂Si₂-type structure,having the nonsymmorphic space group P-3m1.An obvious quantum oscillation was observed under the magnetic field of 14 T at 2 K.Analysis of the Sd H oscillation reveals two frequencies of 16 T（?pocket）and 37 T（?pocket）,and the?pocket has a nontrivial Berry phase.Hall resistivity measurement reveals that the dominant carrier is hole,and the corresponding carrier concentration is 1.75?10¹⁹ cm^-3 at 2 K.Combined with first-principles calculations,it is suggested that Sr Zn₂Sb₂ may be a topological semimetal.This study provides an idea for finding nontrivial topological properties in Ca Al₂Si₂-type compounds and understanding the origin of topology.