Manipulation Of The Physical Properties By Chemical Doping And Superconductivity In Topological Semimetals

The effect of topological properties of materials on their physical properties has expanded understanding of the laws of the physical world.Through the study of topological physical properties in insulators,semiconductors and metals(semimetals),a variety of topological materials with different electronic band structure characteristics have been discovered,such as Dirac semimetals and Weyl semimetals.The transport properties of these topological materials are sensitive to the topological bands near the Fermi level.Most topological materials have their own topological band crossings(Dirac/Weyl points)either above the Fermi level or below the Fermi level,which have negligible effect on their intrinsic physical properties.Therefore,it is necessary to further regulate and study the electronic band structure and physical properties of topological materials by applying pressure,magnetic field,stress or chemical doping,and at the same time help us to understand the underlying physical mechanism more deeply.In addition to the study of Dirac/Weyl fermions,the search for experimental evidence for the existence of Majorana fermions has always been one of the frontier topics in physics,which has great potential for application in the field of fault-tolerant quantum computing.Topological superconductors are closely related to Majorana fermions,while intrinsic three-dimensional topological superconductors are still in the exploratory stage.Materials with both superconductivity and topological bands(or surface states)are candidate systems for topological superconductors.Finding and studying such materials will help advance the understanding of topological superconductivity.In this dissertation,we have grown a series of single crystal samples of topological semimetals,and measured their magnetoelectric transport and thermodynamic properties based on the correct characterization of their crystal structures and elemental compositions,meanwhile,combined with theoretical calculations and angle-resolved photoelectron spectroscopy to systematically study the regulation of chemical doping on the band structure of topological semimetals and the effect of their physical properties;we investigate the novel physical properties in topological semimetals with both nontrivial topological band structure and superconductivity.This dissertation mainly includes the following four parts:Chapter one:Research on the topological properties of energy bands in nodalline semimetal ZrSiSeIn this chapter,we combined the calculation of the electronic band structure and analyzed the electronic band structure characteristics and topological properties of the topological nodal line semimetal ZrSiSe through the electrical transport results under the magnetic field.We measured the magnetoresistance behavior of ZrSiSe under different magnetic fields and temperatures.We found that ZrSiSe obeys Kohler’s law.Comparing the electronic band structure and physical images of ZrSiS,the extremely large magnetoresistance and Kohler plot mainly comes from the carriers compensation effect and the contribution of the trivial band near the Fermi level in ZrSiSe.In order to obtain the information of the Fermi surface in ZrSiSe,we measured the quantum oscillation data at different angles and observed the anisotropic Berry phase effect.Combined with the theoretical calculation of the electronic band structure and the Fermi surface,it is mainly due to the substitution of S by Se element,which leads to the enhanced spin-orbit coupling and the coupling between trivial and topological bands near the Fermi level.Chapter two:Chemical doping induced Fermi surface topological phase transitions in ZrSiSe1-xTexIn this chapter,we described the topological phase transitions of Fermi surfaces induced by tuning the ratio of Se/Te in the topological nodal-line semimetal ZrSiSe1xTex.According to the analysis of the experimental results of electrical transport properties and magnetic properties,we found that the carrier concentration and Fermi surface of the material change drastically at the special chemical potentials,showing the behavior of topological phase transition:a 21/2 order Lifshitz transition occurs when 0.20≤x≤0.33 is revealed by the change of anisotropic magnetoresistance and carrier density.Another 31/2 order electron topological transition is revealed by a large diamagnetic anomaly of susceptibility at x～0.80.In combination with the calculated electronic band structure and Fermi surface in ZrSiSe and ZrSiTe,we found that spinorbit coupling,charge transfer and shifts of chemical potential play vital roles in the evolution of Fermi surfaces in this topological nodal-line semimetal system.Chapter three:Research on the evolution of electronic band structures and superconductivity in(LaxSr1-x)yZn1-z2Sb2This chapter introduces the topological semimetal(LaxSr1-x)yZn1-zSb2 by systematically tuning the La content to modulate its electronic band structure.A lifshitz transition was identified by anisotropic electrical transport and carrier density when La content x～0.5,which is associated with the simultaneous emergence of superconductivity.The non-trivial properties in the superconducting samples are demonstrated by quantum oscillation analysis under the high magnetic field.The van Hove singularity is further confirmed in calculated electronic band structure by a saddle point near the Fermi level and a peak in density of states.Both van Hove singularity and Dirac bands are found to be dominated by the Sb square-net layers,indicating that this 112-type structural system with both relativistic fermions and superconducting carriers is a candidate material for the study of topological superconductivity.Chapter four:Strong coupling superconductivity in topological semimetal SrSn3This chapter introduces the research on superconductivity and topological properties of SrSn3 single crystal samples.Theoretical calculations predict a topologically nontrivial electronic band structure in SrSn3,possibly a topological insulator and semimetal,while its superconducting transition temperature Tc is about 5.5 K.Here,we systematically measure its resistivity,susceptibility and specific heat data,the analyses show that SrSn3 is a strongly coupled type-Ⅱ superconductor.In addition,we observed an extremely high ratio of surface superconducting critical magnetic field to bulk superconducting critical magnetic field(Hc3/Hc2 up to 18.4),we consider that this phenomenon may be related to its topological property.Next,we demonstrate the existence of relativistic fermions in this material based on the analysis of the dHvA oscillation and the calculated electronic band structure,suggesting the possibility of topologically nontrivial superconductivity in SrSn3.