The Searching And Physicial Properties Study Of New 3D Topological Insulator

The research of this thesis mainly including two aspects: large and anisotropic linearmagnetoresistance of Cd3As2 crystals, anisotropic XMR and quantum oscillations studies ofweak topological insulator Ta As2.In this paper, single-crystals of Cd3As2 were grown with the method of vapor transitionmethod. Two different kinds of single crystals were obtained, one is granule-like, and the other isplate. Both of them have the structure of body-centered tetragonal structure with I41/acd(No.142) symmetry, but the direction is different. The granule-like one is [1 1 2] direction, while theplate one is [1 0 0] direction. Temperature dependent specific heat(Cp(T)) shows no structuretransition for the temperature range of 2 K to 300 K. From Debye model, we get the electronicspecific heat coefficient of Cd3As2: g ≈ 5.33 m J mol-1 K-2. And Debye temperature TD=122.86 K.The diamagnetic behavior is confirmed by the M(H) curve, and no clear hysteresis loops arevisible in M(H) curves at both temperatures of 2K and 300 K. The temperature dependence ofresistivity ρ(T) is quite similar in the two case, he residual resistivity ratio RRR(= r(300 K)/r(2K)) are quite small, which means there maybe some vacuum in the single crystals, and Cd3As2issemi-metallic rather than metallic. Under the condition of 2K and 14 T, both crystals show largelinear MR at whole the temperature range of 2 K-300 K. As the temperature is increasing, theMR decreased. This might be related to the anisotropic linear dispersion relationship. ClearShubnikov-de Hass quantum oscillations could also be observed at low temperature and at highfield. The phenomenon gives support to study the behavior of the electron near Fermi surface.TaAs2 single crystals could be grown by vapor transition method or self-flux method. Thecrystal is characterized by a monoclinic structure with the space group of C2/m(No. 12), quitedifferent from the reported XMR material WTe2. With the help of Powder-X program, weobtained the crystal parameter. Through transport measurements, we found the behavior of theresistivity shows metallic. And the RRR is about 1000, which means the quality of the singlecrystal is rather high. In the whole temperature range, the resistivity(or MR) shows a field-induced metal-insulator transition behavior. MR is almost proportional to the square of magneticfield at different temperature. The MR could be as large as ten thousand times at the condition of2 K and 14 T. Otherwise, clear Shubnikov-de Hass quantum oscillations could also be observed atlow temperature and at high field. Form the Ong-Sagon relation, we can fitting the quantumoscillations data of different temperature at certain magnetic field. The results shows that theeffective mass is quite small and there are several electron pockets and hole pockets in theBrillouin zone(BZ). The first-principles calculations considering spin-orbit coupling indicatedthat there are several electron pockets and hole pockets, in agreement with the experimentalobservations. The angle-dependence MR show slightly anisotropic MR. Through hallmeasurement, we found there are two different types of carriers under low temperature. With thetwo band model fitting, the density and Hall mobility of the two different kinds of carriers wereobtained, the large parabolic MR origin form the compensate hole and electron, which is quitefamiliar in the case of Nb Sb2. Under higher field of 60 T, the MR did not saturate but shows thetrack of saturating. From the fitting of the low temperature heat capacity, The Sommerfeldcoefficient of Ta As2 is =1.1m J mol-1K-2, which means the density of the electron at the Fermisurface is quite low, the relation of the electron is weak. The low temperature M-H curves athigh field shows clear d Hv A oscillations which are related to orbital Landau diamagnetism. Thisinformation will be helpful to understand the structure of Fermi surface of Ta As2.