Superconductivity,Magnetism And Quantum Criticality In Ce-based Strongly Correlated Materials

Emergent quantum phases and the associated quantum phase transitions are currently hot topics in condensed matter physics. In strongly correlated electron systems, the interplay of d-and f-electrons have yielded rich phenomena, including magnetism, heavy fermions (HF), superconductivity (SC), quantum phase transitions, Kondo insulator and non-Fermi liquid (NFL). Many Ce-based compounds are typical strongly correlated electron systems, which ground state is usually determined by the competitions of Kondo effect and RKKY interactions. Non-thermal parameters, e.g., doping, pressure and magnetic field, may effectively tune the interplay of d-and f-electrons in these compounds and, therefore, the gound-state properties, leading to a rich phase diagram at zero temperature.In this Dissertation, we have successfully synthesized a few Ce-based materials, namely the iron pnictides and the heavy fermions, and investigated their physical properties by tuning the interplay of d-and f-electrons with chemical substitutions or application of a magnetic field. Special attentions were paid to the magnetism, superconductivity and quantum critical behaviors emerging in the following five series of compounds.(1) The interplay of3d-and4f-electrons in the iron pnictides CeFe1-xCoxAsO and GdFe1-yCoyAsO. Here Ce and Gd represent two rather different cases of4f magnetic moments. Both CeFeAsO and GdFeAsO undergo a structural phase transition accompanied by a spin-density-wave (SDW) transition of Fe3d electrons upon cooling down from room temperature, which is rapidly suppressed by Co/Fe substitution. Superconductivity appears in a narrow doping range:0.05<x<0.2for CeFe1-xCoxAsO and0.05<y<0.25for GdFe1-yCoyAsO, showing a maximum transition temperature of about13.5K for Ce and19K for Gd. In both compounds, the4f electrons form an antiferromagnetic (AFM) order at low temperatures over the entire doping range and the Co3d electrons form a ferromagnetic (FM) order on the Co-rich side, the Curie temperature reaching about TCCo≈75K at x=1and y=1. In CeFe1-xCoxAsO, the Neel temperature TNCe increases upon suppressing the SDW transition of Fe and then remains nearly unchanged with further increasing Co concentration up to x≈0.8(TNCe≈4K). Furthermore, evidence of Co-induced polarization on Ce-moments is observed on the Co-rich side. In GdFe1-yCoyAsO, the two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic behavior in the magnetic susceptibility on the Co-rich side. For0.7≤y<1, the system undergoes a possible magnetic reorientation below the Neel temperature of Gd TxGd. These results suggest that the effects of electron hybridizations and magnetic exchange coupling among the3d-4f electrons lead to a rich phase diagram in the rare-earth iron pnictides.(2) The effects of chemical pressure in CeCoAs1-xPxO, CeCoAsO is a homologue of the iron-based high temperature superconductors, in which the Co3d electrons are ferromagnetically ordered below TCCo≈75K and the Ce4f electrons are antiferromagnetically ordered below TNCe≈6.5K. Upon substituting As with P, the FM order of Co3d electrons is relatively robust, with a nearly unchanged Curie temperature in the whole doping range. Similarly, the Neel temperature of Ce-AFM order hardly changes with the P-content for x<0.9, but the transition is strongly polarized by the Co ferromagnetism. However, for x≥0.9, the Ce-AFM order vanishes and HF behaviors develop at low temperatures. No SC is observed in this series compounds down to2K. The P/As substitution, acting as chemical pressure, substantially enhances the hybridizations between Ce4f and Co3d electrons, showing NFL behavior near x=0.9where the Ce-AFM order is suppressed. These observations suggest the existence of a magnetic quantum critical point (QCP) associated with Ce4f electrons near x=0.9in CeCoAs1-xPxO.(3) Tunable magnetic orders in the CePd2As2-xPx system. CePd2As2-xPx crystallizes in the ThCr2Si2-type tetragonal structure. CePd2As2exhibits a moderate specific heat coefficient of γ=88mJ/molK2, and undergoes an AFM transition at TN≈15K. Upon substituting As with P, TN is nearly unchanged up to x≈0.6, while an FM transition develops below TN for x≈0.4. The Curie temperature TC increases with increasing x and eventually merges with the AFM transition at x≈0.6. With further increasing x, the system follows typical FM behaviors and its TC monotonically increases, reaching TC≈28K in CePd2P2. Moreover, a metamagnetic (MM) transition is observed in the As-rich samples, but vanishes for x≥0.4. More interestingly, we fund that the Ni/Pd substitution can suppress the FM order in CePd2P2, leading NFL behaviors and HF state on the Ni-rich compounds. Such a tunable magnetic ground state may provide an opportunity to explore the possible quantum critical behaviors in CePd2As2-xPx.(4) Antiferromagnetic order and field-induced NFL behavior in CePd3As2. CePd3As2, crystallizing in a monoclinic structure with two different Ce sites, undergoes an AFM transition at TN≈2.2K. Above TN, it shows a moderate specific heat coefficient γ=63.5mJ/molK2. Application of a magnetic field slowly shifts TN to lower temperatures and weakens the anomalies at TN in the electrical resistibity and the specific heat. The magnetic transition remains visible around1.7K in the specific heat at μ0H=9T. Inside the AFM state, the Ce-moments likely undergo a series of spin-orientation (SR) transitions with increasing magnetic field. Furthermore, NFL behaviors are observed at low temperatures in a field range of2T≥μ0H≤6T, which might be associated with the SR transition of Ce-moments.(5) Superconductivity on the magnetic instability in CeIrIns. The pairing mechanism of the heavy fermion superconductor CeIrIns has been highly controversial. In order to elucidate this issue, we systemtatically substituted Ir/Pt, In/Hg, In/Sn, and studied their physical properties. Measurements of the specific heat and electrical resistivity demonstrate that hole doping via Hg/In substitution gives rise to an AFM ground state, but substitutions of In by Sn or Ir by Pt (electron doping) favor a paramagnetic FL state. A cone-like NFL region is observed near CeIrIns, showing a diverging effective mass on the slightly Hg-doped side. The obtained temperature-doping phase diagram suggests that CeIrIns is in proximity to an AFM QCP, and HF SC in this compound is mediated by magnetic quantum fluctuations rather than by valence fluctuations.