Dynamic Property Of Vortex Lattice In Hign Temperature Superconductors

The discovery of high-temperature superconductivity in the Copper Oxide by Bednorz and Müller in 1986 opened a new chapter in the field of solid-state physics in general and in superconductivity in particular. The new high-Tc superconductors are strongly type-Ⅱsuperconductors, exhibiting many electromagnetic properties similar to those of the conventional superconductors. While, high-Tc superconductors also show many properties different from those of the conventional superconductors, among which are high critical temperature, extremely short coherence length and high anisotropic properties. In this dissertation, based on the Maxwell phenomenological theory and Langevin flux flow model and by means of Monte-Carlo numerical simulation methods, we study the flux dynamic properties of high-Tc superconductors in the mixed state. The main results are listed as follows:(1)By calculating the vortex velocity, differential resistance and z-axis correlation function in 3D disorder pinning vortex lattices, the dynamical phase versus driving force FL has been obtained. With the driving force FL(flow velocity)increased, phase of vortices transforms from pinned phase into disordered plastic flow, then from smectic flow into moving crystal. In plastic flow phase, two-step depinning critical points is observed where vortices conquer pinning function formed by their collective correlation and by disordered pin centers. We find two factors: vortex flow velocity and vortex density, which make vortices into order. We observe hysteresis in the plastic flow and smectic flow regime when the driving force is cycled. This phenomenon is distinct in middling pinning strength. With temperature increased, the depinning transition changes from first-order transition to continuous transition, and the hysteresis regime disappears gradually .the width of hysteresis gradually narrows with the vortex density increased, vortex-vortex interactions prevail over vortex-pin interactions. As the vortex density is increased beyond a critical value, the hysteresis disappears.(2) By using the temperature dependent of vortex-vortex interaction potential, and discussing the competition among vortex-vortex, vortex-pin interaction and thermal fluctuation, we calculate the z-axis correlation function of vortices and finally obtain a phase diagram between disordered pinning density Fp0 and temperature t. In the phase diagram there exists three different phases: ordered Bragg glass phase,disordered vortex glass phase and disordered vortex liquid phase. In the low temperature and weak pinning system, vortices form into Bragg glass because of vortex-vortex interactions. By fixing the pinning density and increasing the temperature beyond a melting value, a first-order melting transition occurs, from ordered Bragg glass into disordered vortex liquid. However, by fixing the temperature and increasing the pinning density, we find a solid-solid transition between ordered Bragg glass and disordered vortex glass because of disordered collective pinning effect. In the low temperature region, the BG-VG transition line is parallel with temperature axis. But in the intermediate region, it shows an upturn. Such order-disorder transitions resemble the inverse melting in vortex latticesobserved recently by Avraham et al. [Nature 411, 451 (2001)]. We attribute the unusual inverse melting behavior to temperature dependence of the vortex-vortex interactions.