Magnetism And Kondo Effect In Ce-based Pnictides

Magnetism and Kondo effect are the two leading actors on the stage of strongly correlated electronic systems. The classical Doniach’s picture tells us that both of them rely on the Kon-do coupling between local moment and conduction electrons, but meanwhile they compete with each other. In realistic materials, this competition yields various phase diagrams and exotic phys-ical phenomena. Within the framework of Iron based superconductors, the present dissertation is mainly focused on the rich interplay between magnetism and Kondo effect in the Ce-contained pnictides. In such layered pnictides, the hybridization between rare earth element (e.g., Ce) and transition metal layers provides a natural Kondo coupling, while the strength of this Kondo cou-pling depends on the particular crystalline structure and variety of transition metals, which in turn results in a number of interesting physical properties studied in this thesis.This dissertation mainly covers three families of Ce-contained pnictides:(1), The P/As doping effect in CeFeAsO1-yFy, the two quantum critical points (QCP’s) in the phase diagram, and the interplay between superconductivity (SC) and d-f interaction;(2), The enhanced correlation effect in the antiferromagnet CeNiAsO, and the QCP phenomenon driven both by hydrostatic pressure and chemical pressure;(3), Magnetism and crystalline electric field (CEF) effect in the ThCr2Si2-type CeNi2As2. The main conclusions are highlighted as following:(1), In the CeFeAsO system, we first obtained the whole phase diagram of CeFeAs1-xPxO (0≤x≤1). We found that the P/As doping, on the one hand suppresses the antiferromagnetic (AFM) transition related to Fe2+, and leads to the first QCP xc1(?)0.4, while on the other hand, this chemical pressure also drives the AFM-FM transition of Ce3+at xc3-0.37, and meanwhile, in the case of higher P doping concentration, we discovered a second QCP xc2(?)0.92corresponding to the disappearance of Ce-FM ordering, accompanied with heavy fermion behavior and peculiar non-fermion liquid behavior. However, we did not observe SC down to2K. Later on, we continued the P/As doping in CeFeAsO0.95F0.05.CeFeAsO0.95F0.05is an antiferromagnet on the verge to SC, and SC re-entrance induced by Ce-AFM was observed. Since the Fe-AFM has already been greatly reduced by5%F-doping, we got xc1<<0.4, while xc3moves to0.45. The separation of xcl and xc3leads to a broad SC region,0≤x≤0.53that covers the AFM-FM transition (namely xc3) related to Ce3+. We found that in the region of Ce-AFM, SC and Ce-4f magnetism coexist harmonically, while in the region of Ce-FM, they compete strongly with each other. The Ce-FM region persists to CeFePO0.95F0.05(x=1), driving xc2out of the phase diagram. We interpreted these observations via the different roles of F-doping in CeFeAsO and CeFePO.(2), For the family of CeNiAsO, we first systematically studied its physical properties, and found that Ce3+moments undergoes two successive AFM transitions at TN1=9.3K and TN2=7.3K, respectively. Meanwhile, the strong hybridization between Ce-4f and Ni-3d electrons results in an enhanced electronic Sommerfeld coefficient γ0=203mJ/(mol·K2) and a relatively high Kon-do temperature TK～15K. Later on, we studied the pressure effect of CeNiAsO, both physical pressure and chemical pressure induced by P/As doping. we found that both physical chemical pressure and chemical pressure succeed in tuning the electronic structure of CeNiAsO, suppress-ing the AFM ordering of Ce3+continuously and leading to the QCP’s, pc=6.5kbar and xc=0.4, respectively. Further measurements based on magnetization and Hall effect suggested that the observed QCP is accompanied with a localisation-delocalisation transition of Ce-4f electron, in-dicative of the Kondo destruction QCP. Such local quantum criticality nature is in agreement with the LDA+DMFT calculation on CeNiAsO and CeNiPO.(3), In the section of CeNi2As2, we firstly synthesized millimeter-sized single crystalline ThCr2Si2-type CeNi2As2by NaAs flux method, and systematically investigated its physical prop-erties. First, we found that different to CaBe2Ge2-type CeNi2As2, ThCr2Si2-type CeNi2As2is a highly anisotropic uniaxial antiferromagnet, which forms long range AFM ordering below TN=4.8K with c-axis being the magnetic easy axis. Meanwhile, we observed a spin-flop transition under B‖c. Second, a prominent CEF effect was observed and confirmed to play a key role in determining the electronic, magnetic and thermodynamic properties of CeNi2As2. We calculated the CEF effect in CeNi2As2, obtaining the crystalline field splitting of Ce3+(j=5/2) in CEF, Δ1=325K and Δ2=520K. Third, a giant frustration parameter f=|θW|/TN was obtained. Combined with the assumed magnetic structure in CeNi2As2, this frustration reminds us of a weak structural distortion, which was experimentally supported by the anisotropic in-plane resistivities and Low Temperature X-Ray Diffraction measurement. In this material, the Kondo effect is rela-tively weak. We compared the magnetism and Kondo effect in CeFeAs(P)O, CeNiAs(P)O and CeNi2As2, from which some general conclusions were drawn.Finally, I also briefly reported several other researches during the Ph. D period, including:(1), Magnetically driven structural transition in LnFeAsO (Ln=La,Sm,Gd and Tb), studied by Low Temperature X-Ray Diffraction;(2), Hydrostatic pressure effect in the topological insulator Bi2Te2Se;(3), Spin-glass state in the relativistic Mott insulator Li2RhO3.