Study On Charge Order And Spin Order In Low Dimensional Systems

Low dimensional strongly correlated systems are one of the most important and foreland field of current condensed matter physics. These systems include charge density wave (CDW) and spin density wave, giant or colossal magnetoresistance, and high temperature superconducters etc. A typical feature in these systems is the phenomena of various orders, such as charge order, spin order, orbit order, superconductor order etc. These orders are even considered to coexist but compete in some systems. Undoubtedly, detailed and extensive studies of these orders will make a further understanding on the couple and correlation interaction of electrons in low dimensianl systems. In this thesis, our studies focus on the charge order system and spin order system, including quasi-one-dimensional charge density wave (CDW) conductor blue bronzes A_0.3MoO₃ (A=K, Rb, or Tl), quasi-two-dimensional CDW conductor purple bronze AMo6O17 (A= Na, K, or Tl), and spin-polarized tunneling magnetic Fe₃O₄ granular system. The content of this thesis consists of following aspects:1. We have grown a series of large-sized and high quality single crystals of molybdenum bronzes and their doping samples by molten salt electrolysis of carbonate and MoO₃ . The samples were determined and charactered by XRD, technique of rocking curve, TEM, as well as the electrical transport measurement, and were found to be of good quality.2. The low temperature specific heat of purple bronze KMo6O17 was studied. The anomaly observed at 16 K in magnetoreistance measurements was investigated, meanwhile, the phason contribution to the low temperature specific heat was found in a 2D system for the first time. Using a modified Debye model, a pinning frequency for KMo6O17 is estimated from the phason contribution.3. We measured specific heats of Tl0.3MoO₃ and K_0.3MoO₃ near Peierls phase transition. We find the scaling relation of ?C p dχ/dT between excess specific heat and susceptibility. Two kinds of fluctuations near Peierls phase transition were found: one is the Gauss fluctuation and the other is critical fluctuation. By analysis of the Gauss fluctuations, the critical regions of these two compounds are 8 K and 10 K, respectively. Analysis of the critical behaviors show that the specific heats can be well described by the 3D XY model with the critcal exponent of 0.007.4. Research of the thermoelectric power (TEP) of Rb_0.3MoO₃ and Rb0.15K0.15MoO₃ reveals that two kinds of carriers (electron and hole) are crucial in the transport. Above TP, the hole is dominant, the TEP mainly comes from the carriers'diffusion and can be described as S = AT+B, from which we calculate the Fermi energies of Rb_0.3MoO₃ and Rb0.15K0.15MoO₃ to be 1.55 eV and 0.53 eV, respectively; below TP, the electron is dominant, the TEP comes from the carriers'diffusion and phonon-drag and can be described as S = AT+B/T. The effect of acceptor dopant makes the Fermi energy decease and weakens the coupling of the chains, therefore it increases the effective area of Fermi surface and results in S=0 appearing above TP.5. We have successfully prepared polycrystalline singe-phase K_0.3MoO₃ by Hot Isostatic Pressing sintering.6. We studied the extrinsic MR in magnetic SiO₂-coated Fe₃O₄ granular system. Monodispersed magnetite nanoparticles were first prepared by a solvothermal method, and then were coated with a thin SiO₂ layer by etraethyl orthosilicate hydrolysis. Finally the MR of the pressed powder was studied compared with the uncoated sample. We find an enhanced low-field MR in coated samples. The high-field MR shows that MR of the uncoated sample origins from the spin related scattering of the electrons on the grain boundaries while MR of the coated sample results from the spin polarized tunneling of the electrons through the insulating barriers. Further more, two kinds of conductance channels were found: one is direct tunneling and the other is inelastic tunneling which decrease the MR value with the temperature increasing.