Quantum Critical Behavior In Heavy Fermion Compounds CeTi_1-xNi_xGe₃ And (Ce_1-xLa_x)₂Ir₃Ge₅

The Quantum phase transition (QPT),which occurs at zero kelvin,is driven by quantum fluctuations, in contrast to the phase transition occurring at finite temperature,which are driven by thermal fluctuations. Although, the QPT occurs at zero temperature,the quantum spin fluctuation is greatly enhanced and dominates the physical properties of the material near the quantum critical point (QCP), at which the long magnetic order just disappears by tuning external parameters, chemical doping, pressure, or magnetic field.Many novel quantum states were discovered near QCP, such as, unconventional superconductivity in both Cuprates and Fe-based compounds, Skyrmion phase in MnSi alloys, and non-Fermi liquid behaviors in heavy fermion compounds, et al.. In this thesis,we choose two heavy fermion systems: CeTiGe3, an anisotropic ferromagnetic (FM)Kondo-lattice system with Curie temperature TC = 14 K; Ce2Ir3Ge5, an antiferromagnetic(AFM) Kondo-lattice system with Neel temperature,TN = 9.5 K. By partial substitution of elements, the long-range magnetic order is suppressed at a certain critical substitution concentration, i.e. QCP. Then we studied systematically the critical behavior of physical properties for both systems.The thesis is divided into 5 Chapters. Chapter 1 presents a review on the magnetic properties of materials, heavy fermion compounds, quantum criticality in iron-based superconductors and heavy fermion compounds. Chapter 2 gives a description for our experimental methods. The main experimental results on CeTi1-xNixGe3 and (Ce1-xLax)2Ir3Ge5 systems were presented in Chapter 3 and Chapter 4,respectively. The summary and some outlooks are provided in the last Chapter. The main results are summarized as follows:1. Ferromagnetic quantum critical behavior in heavy-fermion compounds CeTi1-xNixGe3.The measurements on magnetization (M), resistivity (p) and specific heat (C) were carried out for the FM CeTi1-xNixGe3 (0.0?x?0.45) system. It was found that the Curie temperature, TC, decreases with increasing Ni content, x, and reaches zero kelvin near a critical content xcr = 0.44. A new phase diagram is constructed based on these measurements. Both a non-Fermi liquid (nFL) behavior in ?(T) and a log(T0/T)relationship in C/T in the samples near xcr, demonstrate that strong spin fluctuation emerges in these samples,indicating that they are near a quantum critical point (QCP).Our results indicate that CeTi1-xNixGe3 may provide another platform to study exotic quantum phenomena near ferromagnetic QCP.2. Quantum critical behavior in an anti-ferromagnetic heavy-fermions Kondo lattice system (Cel1-xLax)2Ir3Ge5.The measurements on temperature dependence of magnetic susceptibility ?(T),specific heat C(T) and electrical resistivity ?(T) were carried out for the AFM(Ce1-xLax)2Ir3Ge5 (0.0?x?0.66) system. It was found that the Neel temperature, TN,decreases with increasing La content, x, and reaches to zero kelvin near a critical content,xcr = 0.6. A new phase diagram is constructed based on these measurements. Both a non-Fermi liquid (nFL) behavior in ?(T) and a logT relationship in C(T) were found in the samples near xcr, indicating them to be near an AFM quantum critical point (QCP) with strong spin fluctuation. Our finding indicates that (Ce1-xLax)2Ir3Ge5 may be a new platform to search for unconventional superconductivity.