Single Crystal Growth And Physical Properties Of Ternary Topological Semimetal Materials

Due to the unique energy band structure,topologically protected Fermi surface states,non-dissipative electron transport and other novel quantum properties,topological materials have attracted researchers’huge attention since the first discovery.According to the degeneracy and distribution of energy band intersections in Brillouin region,topological semimetal materials can be divided into Dirac,Weyl and Nodal-line semimetal materials.Its rich physical properties,such as ultra-high carrier mobility,non-dissipation and spontaneous spin-polarization transport,show great application potential in the fields of new electronic devices,quantum computing and quantum communication,which continues to set off a research upsurge to explore new topological states.Nowadays,the research of topological materials has entered a new stage.The emergence of high-throughput theoretical calculation has rapidly promoted the development of experimental field,but single crystal growth restricts the rapid verification.Based on the previous crystal growth experience,this paper clarifies the applicable methods of various topological materials and refines the growth parameters,so as to realize the rapid and high-quality preparation of crystals.In addition,we focus on a high-throughput single crystal growth and design scheme which is quite suitable for transition metal chalcogenides with high melting points.The research results obtained are as follows.1)Taking topological semimetal Nb_1-xTa_xSb₂（NTS）as an example,based on chemical vapor transport（CVT）,a high-throughput single crystal growth and design scheme suitable for high melting point transition metal chalcogenides is proposed to accelerate the synthesis of crystals and improve the efficiency of the research.That is,a series of NTS single crystals with Ta doping level ranging from 0 to 1 were grown simultaneously in a sealed quartz tube by CVT method.Firstly,the high quality of crystals is demonstrated by means of XRD,EDS and TEM,and then studied the electronic transport properties of the family systematically.The Nb_1-xTa_xSb₂ series showed a similar phenomenon to the parent compound Nb Sb₂/Ta Sb₂,such as the magnetic-field-induced resistivity upturn in the low temperature region and extremely large MR.Slight Ta doping significantly inhibited MR and resistivity upturn,which MR reaches the minimum value of only 78.4%at T=3K and B=9T and the resistivity upturn behavior disappears completely for the x=0.5 crystal whose Nb/Ta molar ratio is 1.With further increase in Ta doping level,MR and resistivity upturn behaviors resume.Moreover,kohler’s rule not only explains MR results,but also implies the similar lattice,electron structure and scattering mechanism in the NTS family combining with the scattering parameters results.An overall analysis of the Hall effect,magnetotransport,and DFT calculation results imply that the degree of Ta/Nb atomic disorder and electron-hole compensation could account for the Ta-doping-induced huge changes in the magnetotransport behaviors.Our results may encourage more researches in these families of single crystals and provide a high throughput method for growing quasi-continuously stoichiometric crystals of certain quantum material systems,especially transition metal chalcogenides with high melting point.2)Following the previous work,we report the transport behaviors of high-quality Ta_0.7Nb_0.3Sb₂ single crystals and the observation of clear signs of weak anti-localization effect（WAL）in the temperature region below 50 K,resulting from the strong spin-orbital coupling in 3D bulk.A large MR of 120%can be obtained under a magnetic field of 1T at T=2K,this large low-field magnetoresistance feature makes it become a potential material candidate for applications in high-sensitivity magnetic sensors.Hall measurements and two-band model fitting reveal high carrier mobility(>1000 cm²V^-1s^-1 in 2-300K region),off-compensation electron/hole ratio of～8:1 and predominant electron-electron scattering at low temperatures.Due to the angular dependence of the WAL effect and the fermiology of the Ta_0.7Nb_0.3Sb₂ crystals,interesting magnetic-field-induced changes of the symmetry of the anisotropic magnetoresistance（MR）from two-fold（<0.6T）to four-fold（0.8-1.5T）and finally to two-fold（>3T）is observed,which is attributed to the evolution of the mechanism from the low-field WAL dominated MR to WAL and fermiology co-dominated MR and finally to high-field fermiology dominated MR.All these indications suggest that Ta_0.7Nb_0.3Sb₂ should be a strong candidate for topological semimetal,and these magneto-transport properties may attract more theoretical and experimental exploration of the（Ta,Nb）Sb₂ family and make the Ta_0.7Nb_0.3Sb₂ a potential candidate for applications of high-sensitivity electronic devices.3)The CVT method was extended to more transition metal chalcogenides with high melting point and successfully prepared centimeter-grade high-quality ZrGeSe single crystals.Here,the electrical transport evidence of the topological nontrivial state in ZrGeSe was systematically given for the first time.Resistivity plateaus of temperature dependent resistivity curves both in the presence and absence of magnetic fields as well as large,non-saturating magnetoresistance in low-temperature region were observed.The Berry phase analysis based on the temperature and angle dependent Sd H oscillation data shows that the carriers contributing to the quantum oscillation in ZrGeSe come from the topological non-trivial energy band.Furthermore,first principles calculations demonstrate that ZrGeSe possesses Dirac bands and normal bands near Fermi surface,and Dirac fermions dominate the electronic transport behaviors,which results in the observed magnetotransport phenomena.These results demonstrate that ZrGeSe is a topological nodal-line semimetal,which provides another ideal platform for further theoretical and experimental research.Thanks to the optimized CVT method,we have obtained a large number of high-quality ZrGeSe single crystals,and studied the phonon excitations in topological nodal-line semimetal ZrGeSe by means of inelastic neutron scattering measurements for the first time.Both acoustic phonon and optical phonons dispersions have been observed during the experiment,which are similiar to the phonon spectra obtained by theoretical simulation.4)We successfully grew single crystals of Si-and Ge-square-net compounds of NbSiSb and NbGeSb whose excellent crystalline quality are verified using single-crystal X-ray diffractionq-2qscans,rocking curves,scanning and transmission electron microscopies.Since these two compounds share major crystallographic similarity with the topological nodal-line semimetals of Zr Si S family,we employ density functional theory（DFT）calculations and magnetotransport measurements to demonstrate their band structures as well as the electron scattering mechanisms.DFT calculations show that the two materials have typical semimetal energy band characteristics,in which Nb’s 4d orbitals play the major role in the density of states near the Fermi level,and the linear-like dispersion bands can be found on（38）Y and（38）Z directions.Besides the fermiology shows strong anisotropy from the crystallographic c-axis to the ab-plane and weak anisotropy within the ab plane,which is consistent with the strong anisotropic magnetotransport behaviors.Following the Kohler’s scaling rule we prove that similar interband and intraband electron-phonon scattering mechanisms work in both the NbSiSb and NbGeSb compounds.The study of electronic transport mechanism in the presence of external magnetic field renders deep insight into topological behavior together with it’s Fermi surface.NbSiSb and NbGeSb may have topological non-trivial state and are also strong candidate for topological materials,and the high similarity of crystallography and strong difference in band structures between the present single crystals and that of Zr Si S family provides the possibility to tune the band structure via element doping.