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

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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.