Studies On The Magnetoresistance And Superconductivity In The Topological Metalloid Compounds

Since the discovery of the topological insulator 15 years ago,it has been recognized that the topology of the electronic structure in solid materials plays an important role in the classifica-tion and physical properties of materials.In addition to the topological insulator,many topological semimetals with various quasiparticles have been proposed and found,such as Dirac,Weyl,Nodal-line and Nodal-chain semimetals.Many properties related to the topology of band structure,such as a large linear unsaturated magnetoresistance,chiral anomaly,non-zero Berry phase and anoma-lous quantum Hall effect,were also found in these semimetals.However,mechanisms of these phenomena are still in debate.For example,there are several explanations for the extremely large magnetoresistance(XMR);whether the negative magnetoresistance in Weyl semimetals originates from the chiral anomaly or the current jetting;how to tune the topological states.Topological su-perconductor,as a counterpart of the topological insulator,has attracted extensive attention due to hosting the Majorana fermions that can be used in the quantum computation.Based on the develop-ments mentioned above,this thesis selects three types of metalloid compounds:Sn1-2x(TlxBix)Te,SiP2 and TlSb,studies on how to obtain topological insulating state,the mechanism of XMR and the superconductivity in topological materials.Many interesting results were obtained as shown in the follows:(1)Although the band calculations show that SnTe is a topological crystalline insulator,the real material exhibits always a heavily hole-doped metal behavior due to the existence of Sn atom vacancy.In order to realize the topological insulate state in SnTe,we successfully grew a series of Sn1-2x(TlxBix)Te(0.05 ≤x≤0.35)single crystals,and measured their longitudinal resistivity,Hall resistance and magnetoresistance.It was found that with the increase of substitution concen-tration x,the transition from a hole carrier dominated metal to a topological insulator(0.27 ≤x ≤0.31),then reaches a electron carrier dominated metal(x=0.32 and 0.35)occurs.The observation of linear magnetoresistance in the x=0.28 crystal gives a evidence to the existence of Dirac sur-face state.We suggest that such a partial substitution modifies the chemical potential of the system and thus increase the Fermi energy(EF).We also tried to tune the topological insulate state into a Weyl semimetal state by applying an external pressure on the x=0.28 crystal.but failed due to the limitation of our experiments,the further investigation is needed to do in the future.In summary.the topological insulator state was obtained in SnTe system by using a partial substitution of Tl1+and Bi3+-for Sn2+,and the surface current arising from the Dirac surface state was obtained(2)In order to reveal the mechanism of XMR observed in many topological trivial/nontrivial semimetals,we have successfully grown SiP2 single crystals,and calculated its electronic structure and Fermi surfaces.The measurements of anisotropy of magnetoresistance,Hall resistance and de Haas-van Alphen(dHvA)oscillation in magnetization and the numeric simulation for the field dependence of MR with the Boltzmann transport theory approach were carried out.It was found that SiP2 is a topological trivial semimetal with an incomplete carriers compensation.The magne-toresistance exhibits strongly anisotropy.When the magnetic field is applied along the a axis,the magnetoresistance is unsaturated up to 31.2 T and exhibits a linear field dependence,which may be due to incomplete compensation of the two carriers.When the magnetic field is along the[101]direction,the unsaturated magnetoresistance reaches 5.88 × 104%(1.8 K)at 31.2 T and exhibits a quadratic field dependence,which may be due to the existence of hole open orbit along the kx direction in the momentum space.The agreement between the simulation and the experimental results shows that the topology and details of Fermi surfaces play an important role in the mag-netoresistance,while the simple two-band model is insufficient to explain the results observed in SiP2.(3)TlSb with a CsCl type cubic structure(space group:Pm3m.No.221)anticipated to be a Dirac semimetal by our calculations of the band structure,Fermi surfaces,surface state and topo-logical indices.At beginning,we tried to grow crystals for studying its topological properties,unfortunately,only obtained its polycrystalline samples.Then we measured its structure,resis-tance,magnetization and specific heat.It was found that TlSb is a type Ⅱ superconductor with the superconducting transition temperature Tc=4.3 K and the upper and lower critical fields Hc2(0)=1.12 T and Hc1(0)=148 Oe,respectively,the Ginzburg-Landau parameter κGL=10.6 and the coherence length ζGL(0)=15.3 nm.The conventional superconductivity emerges in TlSb with the electron-phonon coupling constant λcp～0.78 and the superconducting gap ΔCel/γnTc=1.42 The discovery of s-wave superconductivity in TISb Dirac semimetals provides a possibility for the realization of the Majorana zero energy mode at the edges(such as in the vortex core,near the defects or on the interface of superconducting particles),which is needed to confirm in the future.These results provide a new method and material platform for the realization of topologi-cal insulator,the understanding of the mechanism of extremely large magnetoresistance and the acquisition of Majorana fermions.