| Heavy fermion compound are an ideal system to study quantum phase transition and quantum control. The ground state of heavy fermion system is determined by the competition of Kondo effect which tends to form nonmagnetically ordered state and RKKY interaction which leads to long range magnetic order. The ground states of heavy femion system, due to the small energy scale, could be effectivly tuned by non-thermal parameters, such as pressure, magnetic field and chemical doping, etc.While the applied control parameters tune the strength of Kondo interaction comparable to that of RKKY interaction, the system enteres into the quantum critical region where the spin fluctuations take over. Meanwhile many anomalous ground states would appear there, such as unconventional superconductivity, spin liquid,non-Fermi liquid, etc.In this thesis,we have synthesized quasi-one materials R2Ni12As7(R=Ce,Yb),EuNi5As3 and studied their physical properties. Furthermore, we have put much effort to explore the possible quantum phase transition related to magnetic/valence instability in this system, by means of chemical substitution, pressure and magnetic field.(1) Phase-diagram of Ferromagnetic(FM) heavy fermion compound Ce2Ni12As7.We have successfully grown the single crystal and polycrystalline Ce2Ni12As7 and performed a comprehensive research on the physical properties. Ce2Ni12As7 is a ferromagnetic heavy fermion compound with the Curie temperatures TM1=1.42K,TM2=0.51K. The ground states of polycrystalline Ce2Ni12As7 show sample dependence behavior, indicating Ce2Ni12As7 itself near FM quantum phase transiton region at ambient conditions. When the unit volume of synthesized polycrystalline sample exceeds the critical value 297.646A3,Ce2Ni12As7 displays more stable FM ground state. Under pressure, FM state of Ce2Ni12As7 may undergo a first-order quantum phase transition and meanwhile we could observe pronounced non-Fermi liquid c/T?logT, ???T5/3 at the critical pressure pc.(2) Crossover from mixed valence to a heavy fermion state in Yb2Ni12(As,P)7. We have successfully synthesized high quality Yb2Ni12(As,P)7 crystals,and studied the physical properties and electron structure by measuring the electrical resistivity,magnetic susceptibility ,specific heat and X-ray spectroscopy. From our results,Yb2Ni12As7 is a typical mixed valence compound with Yb valence being +2.65. XRD and Yb valence measurements on Yb2Ni12As7 indicate there is might a first-order volume collapse transition at 1GPa. Upon applying pressure or P/As substitution, Yb valence increases towards +3 Yb2Ni12P7 is a heavy fermion compound with larger electron specific heat coefficient ? =0.174J/Yb-mol K2 and trivial Yb valence. There demonstrates a crossover from a mixed valence state in Yb2Ni12As7 to a heavy-fermion paramagnetic state in Yb2Ni12P7. Further application of pressure to Yb2Ni12P7 suppresses the paramagnetic Fermi liquid temperature TFL, suggesting the presence of a quantum critical point under pressure. Thus any magnetic QCP in this system is well separated from strong valence fluctuations.(3) Temperature-field phase diagram of Localized antiferromagnetism(AFM)compound EuNi5As3. We have successfully obtained single crystalline EuNi5As3 by using a flux method. EuNi5As3 undergoes two close AFM transitions at TN1=7.2K and TN2=6.4K. Both two transitions are suppressed to lower temperature upon applying a field. After two transitions are suppressed to zero temperature,we haven't observed non-Fermi liquid behavior,indicating the absence of quantum criticality in EuNi5As3 PFY-XAS measurements reveal that the Eu is strongly divalent with weak temperature dependence, indicating a lack of evidence for heavy-fermion behavior. |
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