Structure and properties of the la-doped bi-2201 system

In this article, the influence of La substitution for Sr on structure and physical properties of the 2201 phase is studied. First, the crystal microstructure of the Bi2.iSri.9_a, LaxCuOv (0< x <1.0) is characterized by means of X-ray diffraction and electron diffraction analyses, it is discovered that in the Lardoped 2201 system with increasing La content the 2201 phase undergoes tetragnoal-orthorhombic-monoclinic structural transition. With this phase transition, the modulation vector of the 2201 phase transforms from incommensurate to commensurate, and the period of the modulation wave decreases. Secondly, the superconductivity of this system is analyzed systematically. The results of the resistivity measurement show that the appropriate amount of La doping can raise the superconducting transition temperature (Tc) of the 2201 phase, with x=0.2, Tc reaches as high as 25 K. However the excessive doping of La (x >0.3) leads the 2201 phase to transit from metal to insulator and the superconductivity disappears. The measurement of thermoelectric power (TEP) of this system shows that with increasing La content the TEP value also increases gradually. But the interesting point is that the transition from positive to negative in TEP occurs for x=0.2 sample. In addition, the electronic structure and oxygen content of this system axe also studied. Based on these, we analyze the relation between the microstructure, electronic structure of the Lardoped 2201 phase and its superconductivity, indicating that the type of element substitution with higher valence, together with the microstructural distortion induced by the substitution affects the electronic states of Cu 3d seriously, that is, changes the degree of coulomb correlation of Cu 3d electrons, thus resulting in the transition of the type of carrier of this system and the decrease of hole carrier concentration, as well as the corresponding variation of the superconductivity of Lardoped 2201 phase.