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

研究了高温超导体混合态的相图，对涡旋物质物理有了系统的认识。主要研究结果如下：电阻率测量结果分别显示了低场和高场下的涡晶格熔化和涡玻璃化转变。另一方面，在中间场中观察到非欧姆行为，但结果仍然遵循旋涡玻璃转变的标度行为，其转变温度与弱电流相关。在相同的归一化温度下，随着载流子密度的增加，熔化场归一化到第二峰。熔化场的温度依赖性与修正的解耦理论一致。随着载流子密度的变化，玻璃化跃迁场的温度依赖性也随之变化。对于低载流子密度，跃迁场几乎与温度无关，这表明该体系是准二维的。由于涡旋熔化和涡旋玻璃化转变，交流磁化率的一、三次谐波对温度和外场的依赖表现出明显的异常。该行为在低场高温区频率依赖性较弱，而在高场低温区频率依赖性较强，分别对应一阶和二阶相变。结果表明，各向异性因素主要决定了相图的低场高温区，而高场低温区则主要取决于杂质水平。研究发现，脉冲场下的涡旋动力学受通量线黏度的强烈影响。这一课题与实际应用的关系有待进一步研究。

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.