| Since the discovery of quantum Hall effect in the two-dimensional electron gas in the 1980s,it has been recognized that the topology of band structure in the solid materials plays an important role in the classification of matter and the understanding of physical properties.Then many new concepts such as topological insulator,Dirac semimetal,Weyl semimetal and topological superconductor have been proposed,and a series of exotic physical properties have been discovered,such as large linear magnetoresistance,chiral anomaly,quantum spin Hall effect and nonzero Berry phase.As a new class of topological materials,with a monoclinic structure(space group C2/m,No.12)the transition metal dipnictides XPn2(X=Nb,Ta;Pn=P,As,Sb)have attracted wide attention due to its extremely large magnetoresistance(XMR),abnormal negative magnetoresistance,anisotropy and other interesting properties.In this paper,we choose the VP2 and VAs2 compounds with the same monoclinic structure,and successfully grew their single crystals,the measurements of their structure,composition,magnetization,longitudinal resistivity,and Hall resistivity were systematically carried out.Meanwhile,we also calculated and analyzed their electronic bands and Fermi surfaces.The results are shown in the following:1.For the VP2 single crystal,we observed a minimum resistivity around 50 K,which may be due to the Kondo effect,caused by magnetic impurities,confirmed by the magnetic susceptibility measurement.The magnetoresistance(9 T)reach 170%at the temperature with a minimum resistivity,and still 17%at the room temperature(300 K,9 T).It was found that the magnetoresistance consists of two parts:in the lower field(<2 T),it mainly origins from the circular motion of the carrier in the magnetic field(conventional magnetoresistance):MR ?(?),and its contribution is only 8%.At higher field(>2 T),the magnetoresistance exhibits a linear field dependence(? B),and does not show any signature of saturation up to 9 T,which may be due to the special electronic structure of VP2,i.e.the existance of a node-line in the momentum space,evidenced by the band structure calculation.2.As for VAs2 compound,our first principle calculations predict it to be a type-? nodal-line semimetal.The susceptibility data indicate that out VAs2 crystals are a paramagnet with small amount V4+impurities(0.38%).The resistivity exhibits a minimum around 11K,which may be due to the Kondo effect caused by V+(s=1/2)magnetic ions,but its resistivity data can also be fitted by a weak localization effect.The magnetoresistance in VAs2 crystals has the similar behaviors observed in the most semimetals with an extremely large magnetoresistance(XMR):under a magnetic field,with decreasing temperature,the resistivity begins to decrease,then increases and finally reaches a saturation value;the field dependence of magnetoresistance can be fitted by the Kohler's law with a power exponent is 1.76,nearly a quadratic relation,which may be due to the complete compensation of both electron and hole carries,evidenced by the Hall resistivity measurements,such as,the electron(hole)concentration is 1.37×2021 cm-3,and the mobility is of 2.38×103 cm2V-1s-1(2.64× 103 cm2V-1s-1)respectively.In our studies,both VP2 and VAs2 compounds exhibit the nodal-line semimetal behaviors,evidenced by our band calculations and MR measurements.Meanwhile,their crystals always contain the V4+(3d1,1/2)impurities,indicating both VP2 and VAs2 are an interesting material system for studying the Kondo effect and the electron-electron correlation effect in the topological materials. |
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