Magnetic And Transport Properties In Low Dimentional Spin System

High quality CuGeO3 and Sr14-xCaxCu24O41 single crystals were successfully grown by floating-zone technique. We performed measurements of magnetic, transport properties and thermopower on these systems and found following new results.(1) The temperature dependence of magnetic susceptibility below the spin Peierls (SP) transition temperature (Tsp) under magnetic fields applying along both the a-and c-axis direction can be fitted well by a model of noninteracting dimmers. The spin gap derived from the fitting is consistent with other methods. A suppression of upturn Curie-Weiss term by magnetic field was observed and the possible origins were discussed.(2) Single crystals of spin-ladder compound Sr14-xCaxCu24O41 (SCCO) was successfully grown under 1atm oxygen atmosphere by floating-zone technique. The effect of oxygen pressure on the crystal growth, composition and the properties were investigated. The single crystals with x≤8 can be grown under 1atm oxygen pressure. But the high pressure oxygen is still necessary to grow the SCCO crystals with higher Ca content. We characterized the single crystals by EDX, XRD and resistivity measurements. We found that the difference among the SCCO single crystals grown under different oxygen pressure is not signigicant and high pressure oxygen favors the formation of SCCO crystals with higher Ca content.(3) Thermopower in Ca doped SCCO single crystals was first systematically investigated. A transition corresponding to the formation of charge-density waves (CDW) was observed in both the temperature dependence of resistivity and thermopower for x≤6. However, the temperature dependence of thermopower shows that the system become metallic for x≥8 although the resistivity is still semiconductor-like. We propose that the non-metallic behavior of the resistivity in the Ca doping range 8≤x≤11 could result from the localization of the charge carriers due to the disorder induced by Ca doping and a revised electronic phase diagram is derived based on our observations. We proposed that CDW order and superconducting order are competing with each orther,which does not support the coexistence of these two orders.