Quantum Criticality In Ni₃Al Alloy And Magnetic Dichalcogenides

In some strongly correlated electron systems (for example, cuprate superconductor, iron-based superconductor, heavy fermion compound and ruthenate oxide), the magnetic phase transition temperature can be suppressed to 0 K by adjusting some non-thermal control parameters, such as external pressure, magnetic field and element doping. A series of anomalous physical properties has been discovered in the critical region. In contrast to a classical phase transition driving by thermal fluctuation, a quantum phase transition from an order to a disorder state at 0 K occurs as the result of the variation of external parameter, accompanying by strong quantum spin fluctuations which dominates the behavior of electronic systems over a wide range of the phase diagram. So, it is important in condensed matter physics to research the physical properties close to the critical region and to confirm the relationship between non-Fermi liquid behavior in physical properties and quantum phase transition at 0 K. The result of such a study is a key to understand both high Tc superconductivity and the construction of physics in strongly correlated electron system.The content of the dissertation is divided into six chapters and the main results are summarized as follows:The Chapter 1 reviews the progress of the quantum phase transition both in theory and experiments, Meanwhile, the aims and content of the present work were given.In Chapter 2, we study the quantum critical phenomena in polycrystalline alloys Ni3Al1-xGax synthesized by arc-melting method. We investigated systemically their structure, magnetization, electronic resistivity and specific heat. It is found that the ferromagnetic phase transition temperature is depressed gradually with increasing substitution content x, of Ga for Al, and disappears near x= 0.34, where the system changes from an itinerant ferromagnetic state to a paramagnetic state, a quantum phase transition occurs. The magnetic phase diagram for Ni3Al1-xGax system was obtained. Near this quantum critical point, the field dependence of magnetization satisfies a scaling law M∝H1/3, and the temperature dependence of initial magnetic susceptibility in a way of 1/xinitial∝T4/3. We are the first group to observe the "peak" effect in y-H curves, which reflects the energy spectrum information of spin quantum critical fluctuation by analyzing M(H) curves in different temperatures. The temperature dependence of the part in specific heat due to the spin quantum fluctuation at low temperatures is of C/T oc-logT, which is well satisfied with theory. It is suggested that the strength of electron correlation in Ni3Al1-xGax is medium from the measurements of specific heat. And the non-Fermi liquid behavior in the resistivity near quantum critical point was found.In Chapter 3, we studied the antiferromagnetic quantum critical phenomena in the polycrystalline samples of NiS2-xSex(x= 0.96,0.98,1.00,1.05,1.10 and 1.20) which were prepared by a solid state reaction method. And the measurements on their structure, magnetization and resistivity were carried out. It is found that their temperature dependence of susceptibility shows a typical characteristic of a strongly correlated electron system. There are possible two kind of carrier, one is itinerant one, the other is localized one; or, there even exists dynamic electronic phase separation in NiS2-xSex (x>1.0); Similar to high Tc superconductors, the relationship between resistivity and temperature displays a linear behavior in a wider temperature region from 50 to 300 K. For x= 0.98 and 1.00 samples, which is close to the antiferromagnetic critical point, itsρ(T)∝T3/2 at lower temperatures (3-30K), which takes on a non-Fermi-liquid behavior due to the strong quantum spin fluctuation. The x= 1.10 and 1.20 samples, recovers to the Fermi liquid-behavior at lower temperature:ρ(T)∝T2.In Chapter 4, we studied the ferromagnetic quantum critical phenomena in polycrystalline samples of Co(S1-xSex)2 (0.0≤x≤0.16) which were prepared by a solid state reaction method. The measurements on their structure and resistivity were carried out. It is found that the ferromagnetic transition temperature Tc is suppressed by Se doping in 7C～(1-x)1/2 way. The ferromagnetic phase transition goes from the second to the first order. The temperature dependence of resistivity, p(T), shows a Fermi liquid behavior,ρ(T)=ρo+AT2 in Co(S1-xSex)2 (x< 0.08) samples, while non-Fermi liquid behavior of p(T) occurs in the samples near the critical concentration x= 0.11. It is suggested that the phase transition near x= 0.11 is a quantum phase transition and the quantum critical spin fluctuation at zero temperature results in non-Fermi liquid behavior.We investigated the effect of Y substitution for Ce in Ce1-xYxIn3 system. The single crystal Ce1-xYxIn3 samples were prepared by a self-flux method. We studied its structure and magnetic susceptibility systemically. It is found that the antiferromagnetic phase transition temperature is gradually suppressed with increasing substitution content x of Y, and disappears near x= 0.38. We gained the magnetic phase diagram of Ce1-xYxIn3.In the last Chapter, We investigated the typeⅡsuperconductivity in nearly ferromagnetic materials MgCNi3, which was prepared by high pressure method. The superconducting volume ration is almost 100%. We conclude that the flux pinning at lower fields results from the intra-granular structural defects which is different intrinsically in sample prepared by conventional solid state reaction. We also observed the "peak" effect caused by softening of the flux lattice in high magnetic field. On the other hand, it was found that the critical current density in MgCNi3 samples prepared by high pressure is almost the same value as that in NbTi wire. We put forward a practicable preparing method for superconductor MgCNi3 in application in the future.