Quantum Transport Properties In The XPn₂-typed ?X=Ta,Mo;Pn=Sb,As? Semimetal Compounds

Topological insulators and semimetal materials showing the exotic quantum phenomena have attracted great interests in condense matter physics fields.Two abnormal transport properties have been believed to be the key signatures for those discovered topological materials:the saturated resistivity plateau at low temperatures and the negative magnetoresisitance?NMR?under the magnetic fields.Recently,topological semimetals?TSMs?XPn₂?X=Ta,Nb;Pn = P,As,Sb?,which crystallizes in the centro-symmetry monoclinic structure,have been discovered and investigated.Those compounds would like to show the abnormal transport properties like resistivity plateau and negative MR at low temperatures.In this dissertation,our studies mainly focus on two subjects:?1?the study of the anisotropic magnetoresisitance in topological semimetal TaSb₂.?2?Investigating the abnormal transport behaviors in semimetal MoAs₂.Our experimental results are summarized as follow.?1?We found that TaSb₂ shows the extremely large magnetoresistance?MR?without saturation,the metal-insulator-like transition and strongly antisotropic magnetoresisitance at low temperatures.The MR for B ?c is almost twice as large as that of B ?a at low temperatures.The MR as a function of fields at distinct temperatures followed the Kohler's rule as B?c,while it slightly violates as B ? a.The angle-dependent MR exhibits the two-fold rotational symmetry below 70K,consistent with the configuration of the monoclinic crystal structure.The dumbbell-like picture of angle-dependent MR in TaSb₂ suggests a large anisotropic Fermi surface at low temperatures.It finally loses the two-fold symmetry over 70K,implying a possible topological phase transition at around the temperature where Tm is related to a metal-insulator-like transition under magnetic fields.?2?We have grown the bulk MoAs₂ single crystal which crystallizes in a monoclinic structure with C2/m space group.Magnetic Transport measurements show that MoAs₂ displays a metallic behavior at zero field but undergoes a metal-to-semiconductor crossover at low temperatures when the applied magnetic field is over 5 T.A robust resistivity plateau is observed below 18 K and survives up to B = 9 T.A large positive magnetoresistance?MR?,reaching about 2600%at 2 K and 9 T,is observed when the field is normal to the current.The MR becomes negative below 40 K as the field is rotated to the current at high field.It increases with the decrease of temperature,but it is always less than 100%.Hall resistivity shows the non-linear field-dependence below 70 K.The analysis using two-band model indicates a compensated electron-hole carrier density at low temperatures.The breakdown of Kohler's rule,the abnormal drop and the cross point in Hall data all point to a fact that a possible Lifshitz transition has occurred between 30 K and 60 K,likely resulting in the compensated electron-hole density,the large MR as well as the metal-semiconductor transition in MoAs₂.Our results further indicate that the XPn₂ semimetal family has showed rich transport behavior and can provide insight into the mechanism of topological materials.