The Study Of Orbit Ordering、Spin Ordering And Charge Ordering In Low Dimensional System

The study of low dimensional strongly correlated systems has been one of the most interesting fields of condensed matter physics. High temperature superconductors, giant magnetoresistance, topological insulator, spin density wave and charge density wave are representatives of these systems. A shared characteristic in these systems is the phenomena of various ordering. There are orbital ordering, spin ordering, charge ordering and even superconductor ordering. In many cases, different orderings compete and coexist with each other. Thus, the study of low dimensional ordering would enhance the understanding of coupling interactions of electrons in correlated systems. The main work of this dissertation is based on orbital and spin ordering spinel compounds ATi2O4(A=Mg, Ca) and quasi-ID charge density wave compound Rb0.3MoO3, the dissentation is formed by the following aspects:1. High quality polycrystalline of MgTi2O4 were synthesized by spark plasma sintering. High quality and large-size single crystals of CaTi2O4 and Rb0.3MoO3 were prepared by the molten salt flux method and electrolytic reduction method. According to X-ray diffraction, XPS and TEM, all the samples were characterized and determined to be good quality.2. The temperature dependence of resistivity in MgTi2O4 was measured. MgTi2O4 in cubic phase is a semiconductor with energy gapΔ=0.24 eV, and for the tetragonal phase, MgTi2O4 isn’t a normal band insulator. Compared with charge density wave (CDW), strong nonlinear electric response involved with orbit density wave (ODW) of MgTi2O4 is observed. The characteristics of coupling interactions of ODW and the electric response under external electric field are given. The temperature dependence of ODW’s distribution is also confirmed.3. Specific heat of MgTi2O4 under both zero and non-zero fields have been measured, we firstly provide the electronic specific heat coefficientγ=8.806 mJ/molK2, Debye temperature (?)D=348.94 K and Fermi energyεF=0.803 eV. A two-level Schottky anomaly was found and determined by the low temperature specific heat, and largeγindicates that MgTi2O4 is a strong-correlated electron system. In transition area, the standard scaling relationship between magnetic susceptibility and specific heat was observed and the features of second order transition was also exhibited. The critical region was determined to be 5 K and the specific heat behavior of MgTi2O4 could be described by the 3D XY model with critical exponentα=-0.00775; The phase transformation is also consistent with the Clapyron equation; The primary results have indicated that the transition of MgTi2O4 near 260 K is a complex progress, and its internal mechanism is expected in the following study.4. The magnetic properties of CaTi2O4 was studied in detail. CaTi2O4 is proved to be a quasi-1D antiferromagnetic system with a negative Curie-Weiss temperature, and the unpaired Ti3+ions are less than 1%. According to the anisotropic magnetic properties and the anomalous bump around 40 K, the existence of Heisenberg antiferromagnetic (HAF) chain and its direction were confirmed. Magnetic CaTi2O4 is demonstrated to be a 1D strong and uniform S=1/2 HAF gapless chain compound, exchange interaction was first given by J=-95.94K. According to the analysis of Brillouin function with mean field correction, the low dimensional structures in CaTi2O4 were proved to be more stable at lower temperature.5. The electronic structure and transport properties of W-doped rubidium blue bronzes Rbo.3Mo1-xWxO3 were investigated. The results showed that B-site doping could cause an enormous impact on the system, the doping of the W ions destroyed the low dimensional structure, depressed the Peierls transition, and led to big change on the temperature dependence of TEP, which is mainly ascribed to the introduction of the holes near the valance band and the big change on the topology of the Fermi surface due to the W doping.