氧掺杂Y_(0.8)Ca_(0.2)Ba_2Cu_3O_y超导体正交-四方相变的正电子湮没实验研究

Study on oxygen doping Y_(0.8)Ca_(0.2)Ba_2Cu_3O_y superconductors and O-T transition by positron annihilation experiment

利用正电子湮没技术以及X射线衍射等实验手段对Y0.8Ca0.2Ba2Cu3Oy(y=6.32～6.84)体系进行了系统研究。结果表明,在氧含量y=(6.50±0.05)附近发生正交四方相变。正电子寿命参数在正交四方相变发生前后均有明显变化,说明样品内部微观电子结构在发生正交四方相变附近区域发生了显著变化。表征样品内部电子结构整体平均效应的体寿命参数τb随y的降低而逐渐增大,表明由于氧含量降低导致了样品内部局域电子密度的降低,这和Tc随y降低而降低是一致的。认为该体系在y约为6.50附近正交四方相变区域Tc出现平台式响应可能与相变过程中区域体系内部电子结构变化上的某种弛豫效应存在某种关联。且讨论了YBCO体系中正电子湮没机制以及它与正交四方相变与和高温超导电性之间的关联。

Y0.8Ca0.2Ba2Cu3 Oy with large range of oxygen content (y=6.84-6.32) have been studied by means of positrons annihilation technology (PAT) and X-ray diffraction (XRD). The results show that Y0.8Ca0.2Ba2Cu3 Oy, undergoes a structure phase transition from orthorhombic to tetragonal (O-T) around y=(6.50±0.05). Positron lifetime parameters show obvious change during the O-T structure transition, which indicate that the local electronic structure changes a lot and in the different crystal phase the positron at different trapping states. On the whole, the bulk lifetime τb, that represents the average variation effect of the electron structure in the sample increases with the decrease of oxygen content, which means that the local electron density decreases with the decreases of sample oxygen content. This result was consistent with the change of Tc. The platform of Tc-65K near y-6.50 where the O-T phase transition occurs indicates that it might related to some relaxation effect of the electron structure during the phase transition in the sample. Meanwhile, the mechanism of positron annihilation in YBCO system and its relation with O-T phase transition and superconductivity were also discussed.