Studies On The Band Structure,Topological Property And Magnetoresistance In Topologically Trivial And Non-Trivial Materials

Extremely large magnetoresistance(XMR)at low temperature has recently been reported for a large number of both topologically trivial and nontrivial materials.Different mechanisms have been proposed to explain the observed magnetotransport properties,yet without arriving to defini-tive conclusions or portraying a global picture.In our work,we investigate the magnetoresistance(MR)of nodal-line semimetals:MoO2 and VAs2,ordinary metal ReO3 and topologically trivial semimetal SiP2 by combining the band structures and Fermi surfaces calculated from first princi-ples with the Boltzmann transport theory.The MR of the above compounds are explained perfectly by the theory.The detailed results are as follows:(1)The band structure and Fermi surface calculation of monoclinic MoO2 shows that without considering spin-orbit coupling,the closed nodal rings and nodal lines are formed near the high symmetry point in momentum space.The emergency of Weyl phase in MoO2 may be due to the breaking of crystal symmetry resulting from,for example,the presence of oxygen vacancies in the crystal or SOC.in the first Brillouin zone,the volume of hole pockets and electron pockets are the same,which means MoO2 is a charge-carrier compensated semimetal,This result is confirmed by hall resistivity and MR.(2)The band structure and Fermi surface calculation of monoclinic VAs2 shows that there are two kinds of nodal lines in the first Brillouin zone when SOC is neglected:the helix lines passing through the plane of Z High symmetry point and two closed rings near L high symmetry point,so the VAs2 is a nodal-line semimetal.Our group measured its resistivity,Hall resistivity,MR.Similar to many topologically trivial and nontrivial semimetals,VAs2 also exhibits extremely large MR and charge-carrier compensation.It is interesting that the Kondo effect caused by magnetic impurity V4+(S=1/2)is also observed in VAs2.(3)as an "ordinary" metal ReO3,its physical properties have been widely studied.After growing ReO3 single crystals successfully,we are surprised to find that similar to many semimetals,it also presents extremely large MR and strong anisotropy of resistance.When the magnetic field is along different crystal directions,the power relations between the MR and the magnetic field are different.We calculated the band structure and Fermi surface in detail.The results show that the bands near Fermi level all come from the d orbits of Re.In the first Brillouin zone,there are three electron pockets centered on the ? high symmetry point:closed pockets ?,?.But ? pocket is more complicated:it is formed by three intersecting cylinders along<100>crystallographic direction families.Because of this unique and simple configuration of Fermi surface,when the magnetic field is parallel to c axis,the projected Fermi surface perpendicular to the magnetic field is transformed into electron and hole pockets,and the compensation of these two kinds of carriers is realized.When the angle between magnetic and c axis is 15°,the open orbit of the electron is formed,which explains the MR perfectly.(4)SiP2 is a topologically trivial semimetal with a cubic pyrite-type structure.The anisotropy of its MR is observed systematically.As H is applied in the a axis,a nonsaturating(31.2 T)linear field dependence of MR occurs.While H is applied in the[101]direction,MR(5.88 × 104%at 1.8 K,31.2 T)exhibits a nonsaturating quadratic H dependence.We calculate the electronic structure and Fermi surface,then we performed numerical simulations based on the Boltzmann transport theory within the relaxation time approximation.It is found that the perfect compensation between the electron and hole charge carriers is altered upon applying magnetic field oriented along the a axis,and the incomplete compensation induces the departure of resistivity from the ideal parabolic to nearly linear scaling.When the magnetic field is along the[101]direction,due to the existence of open orbit,the nonsaturating quadratic field dependence MR occurs.At the same time,we also found SiP2 is a topologically trivial semimetal,which is confirmed by the quantum oscillation of dHvA.The above results show that the unsaturating XMR of both topologically trivial/nontrivial semimetals and ordinary metals mainly originates from charge-carrier compensation and open-orbit mechanisms,and the power exponent of the MR depends on the topology of Fermi surface.This paper deepens the understanding of XmR.