Systematic study of the phase diagram of the mixed state of high temperature superconductors

本文研究了高温超导体混合态的相图，以便系统地理解涡旋物质物理。主要研究结果如下：1.电阻率测量结果表明，在低场和高场区分别存在涡旋晶格熔化和涡旋玻璃化转变。另一方面，在中间场的情况下，观察到了非欧姆行为，但结果仍然遵循涡旋玻璃化转变的标度行为，其转变温度具有弱的电流依赖性。2.在相同的归一化温度下，归一化到第二峰场的熔化场随着载流子密度的增加而增加。熔化场的温度依赖性与修正的退耦理论一致。3.玻璃化转变场的温度依赖性随载流子密度的变化而变化。对于低载流子密度，跃迁场几乎与温度无关，表明系统是准二维的。4.由于涡旋熔化和涡旋玻璃化，交流磁化率的一次和三次谐波的温度和外场依赖性表现出明显的异常。这种行为在低场高温区的频率依赖性较弱，而在高场低温区的频率依赖性较强，分别对应于一级和二级相变。5.可以得出结论，各向异性因子主要决定相图的低场高温区，而高场低温区则强烈依赖于杂质能级。6.磁通线的粘性对脉冲场下的涡旋动力学有很大的影响。这个问题需要进一步研究与实际应用的关系。

The phase diagram of the mixed state of high temperature superconductors has been investigated for a systematical understanding of vortex matter physics. The main results can be summarized as following :1.The resistivity measurements revealed evidences of vortex lattice melting and vortex glass transitions in low and high field regimes, respectively. On the other hand, a non-Ohmic behavior was observed for intermediate field, but the results still followed the scaling behavior of vortex glass transition with an transition temperature that has a weak current dependence.2.The melting field normalized to the second-peak field increased with the carrier density when compared at the same normalized temperature. The temperature dependence of the melting field agrees well with a modified decoupling theory.3.The temperature dependence of the glass transition field varied with the change in carrier densities. For low carrier density the transition field is almost temperature independent, which suggests that the system is quasi-two-dimensional.4.The temperature and external field dependence of the first and third harmonics of the ac-susceptibility showed distinct anomalies due to the vortex melting and vortex glass transitions. The frequency dependence of this behavior was weak in the low-field high-temperature region while it was strongly frequency dependent in the high-field low-temperature region, corresponding to phase transitions of first and second orders, respectively.5.It was concluded that the anisotropy factor mostly determines the low-field high-temperature region of the phase diagram, while the high-field low-temperature region depends strongly on the impurity level.6.The vortex dynamics under pulsed field was found to be strongly affected by the viscosity of flux lines. This subject requires further studies in the relation with practical applications.