Magnetism And Superconductivity In Bi Based Rare Earth Compounds

The physical mechanism of unconventional superconductivity and the relationship between unconventional superconductivity and magnetism are the frontiers of condensed matter physics.Rare earth compounds often exhibit rich physical properties associated with the relatively local-ized 4f electrons,including magnetic order,superconductivity,Kondo effect and heavy fermion liquid.Furthermore,the occurrence of superconductivity in magnetic rare earth compounds makes the ground state more complex.The research on the relationship between superconductivity and magnetic order will promote the deeper understanding of the physical mechanism of unconven-tional superconductivity.This dissertation focuses on three Bi-based rare earth compounds:Ce2O2Bi,Er2O2Bi and GdPdBi.Their crystal structures and physical properties were systematically studied,and the following innovations were achieved.(1)Kondo effect in the compound Ce2O2Bi with anti-ThCr2Si2-type structure.The Ce2O2Bi compound crystallizes in the anti-ThCr2Si2-type structure consisting of a sepa-rated Bi square net layer between the CeO layers.In the high temperature regime between 200-300 K,the compound shows the usual metallic behavior and 4f electrons of Ce3+ ions are local-ized.In the low temperature regime between 15-30 K,the resistivity increases with a logarithmic temperature dependence,accompanied with the negative magnetoresistance and a low carrier den-sity.Upon further decreasing temperature,the resistivity drops rapidly below 6.2 K,signaling an antiferromagnetic ordering transition within the Kondo phase.The susceptibility and specific heat also display a transition around the same temperature,accompanied with a reduced magnetic en-tropy in the ordered state.A large Sommerfeld coefficient of 336 mJ/mol K2 is obtained by fitting the specific heat data and no superconductivity is observed down to 0.3 K.All these observations suggest that Ce2O2Bi is a rare low carrier density Kondo lattice compound whose interlayer hy-bridization between 4f electrons of Ce3+ ions and conduction 6p electrons of Bi2-ions plays a crucial role.(2)Coexistence of superconductivity and antiferromagnetic order in the compound Ce2O2Bi.The superconducting transition temperature Tc of 1.23 K and antiferromagnetic transition temperature TN of 3 K are observed in the sample with the best nominal composition.The su-perconducting upper critical field Hc2(0)and electron-phonon coupling constant in Ce2O2Bi are similar to those in the previously reported non-magnetic superconductor Y2O2Bi with the same structure,indicating that the origin of superconductivity in Ce2O2Bi may be as the same as that in Y2O2Bi.The first-principle calculations show that the Fermi surface of Ce2O2Bi is mainly com-posed of the Bi-6p orbitals both in the paramagnetic and antiferromagnetic state,implying minor effect of the 4f electrons on the Fermi surface.Besides,upon increasing the oxygen incorporation in Ce2O2Bi,Tc increases from 1 to 1.23 K and TN decreases slightly from 3 to 2.96 K,revealing that superconductivity and antiferromagnetic order may compete with each other.The Hall ef-fect measurements indicate that hole-type carrier density indeed increases with increasing oxygen content,which may account for the variations of Tc and TN with different oxygen content.(3)Magnetoresistance,Hall effect and planar Hall effect in GdPdBi.It has been proposed that there exist possible Weyl points in the analogical compound GdPt-Bi,and it is supported by the following three main experimental evidences:a negative longitudinal magnetoresistance,anomalous Hall effect(AHE)and planar Hall effect(PHE).The remarkable d-ifference between GdPdBi and GdPtBi is that GdPtBi has an energy band inversion while GdPdBi does not.Therefore,there should be no Weyl points in GdPdBi in principle and then it is chosen for comparison study to distinguish the different effects on transport properties by either magnetism or Weyl points in GdPtBi.Though the negative magnetoresistance is indeed observed in GdPdBi,the negative magnetoresistance changes little when the angle between magnetic field and current is varied from 0 to 90 degree.The angle dependence of the magnetoresistance in GdPdBi is not con-sistent with the magnetoresistance model induced by chirality anomaly.An anomalous Hall angle tan?(>0.1)has also been found in GdPdBi.However,the anomalous Hall angel does not show the sharp peak as in GdPtBi.The planar Hall effect of GdPdBi is far smaller than that in GdPtBi.The comparison study between GdPdBi and GdPtBi can provide us a new view to understand the origins of various experimental phenomena(including magnetoresistance,anomalous Hall effect and planar Hall effect)due to either band topology or magnetism in GdPtBi.Finally,a summary of this dissertation and a brief outlook for further study are provided in the last chapter.