Browsing by Author "Li, D."

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Hierarchical nature of the vortex matter in type II superconductors due to competition between interactions, thermal fluctuations and disorder
(2006-07-01)
Li, D.
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Rosenstein, B.
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Vinokur, V.
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電子物理學系
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Department of Electrophysics
We describe quantitatively the combined effects of both the thermal fluctuations and of the quenched disorder via the replica trick applied to the Ginzburg-Landau (GL) theory. We show that the vortex state can appear in either of the three disordered phases: (i) unpinned vortex liquid, (ii) amorphous vortex glass ( pinned), and (iii) the crystalline ( pinned but not containing topological defects) Bragg glass. The formation of the vortex glass is associated with the continuous replica symmetry breaking (RSB) reflecting the hierarchial structure of the potential barriers in a vortex glass state. An earlier analysis in the framework of London approximation have established that activation barriers controlling vortex dynamics obey the extreme value statistics within roughly the same domain of the phase diagram. We show that the disordered GL model in which only the coefficient at the quadratic term vertical bar psi vertical bar(2) is random, first considered by Dorsey et al., exhibits, in the gaussian approximation, an additional nonhierarchical state possessing certain glassy properties like nonzero Edwards - Anderson order parameter. We associate this state with the "marginal glass phase" predicted in the earlier work of one of the authors; the marginal glass state being characterized by the marginally glassy dynamics. We show further that when the random component of the coefficient of the quartic term vertical bar psi vertical bar(4) in GL free energy is taken into account, RSB effects appear. Application of the obtained results to description of various disorder-generated phenomena in vortex matter are briefly considered. The location of the glass transition line is determined and compared to experiments. This line is clearly different from both the melting line and the second peak line describing the translational and rotational symmetry breaking at high and low temperatures respectively. The phase diagram is separated by these two lines into the four phases described above.
Interplay of anisotropy and disorder in the doping-dependent melting and glass transitions of vortices in Bi2Sr2CaCu2O8+delta
(2007-04-20)
Beidenkopf, H.
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Verdene, T.
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Myasoedov, Y.
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Shtrikman, H.
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Zeldov, E.
We study the oxygen doping dependence of the equilibrium first-order melting and second-order glass transitions of vortices in Bi2Sr2CaCu2O8+delta. Doping affects both anisotropy and disorder. Anisotropy scaling is shown to collapse the melting lines only where thermal fluctuations are dominant. Yet, in the region where disorder breaks that scaling, the glass lines are still collapsed. A quantitative fit to melting and replica symmetry-breaking lines of a 2D Ginzburg-Landau model further reveals that disorder amplitude weakens with doping, but to a lesser degree than thermal fluctuations, enhancing the relative role of disorder.
Magnetic phase diagram of type-II superconductors: From high T-c to low T-c superconductors
(2008-09-15)
Li, D.
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Lin, P. J.
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Rosenstein, B.
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電子物理學系
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Department of Electrophysics
Phase diagram of both high temperature superconductors and low temperature superconductors are studied within the Ginzburg-Landau approach. Due to enhanced thermal fluctuation strength, disorder effects are relatively small in high T-c superconductors and consequently can be studied analytically. The vortex glass transition line is different and well separated from both the melting line and from the so-called second peak lines. On contrary, in low temperature superconductors, the disorder effect is dominant, as the thermal fluctuation strength is very small. Peak effect appears due to cross over of the collective pinning region to stronger pinning region. The location of the peak effect is obtained. The disorder and thermal fluctuation effects on structure phase transition are also studied. (C) 2008 Elsevier B.V. All rights reserved.
Theory of the fluctuation diamagnetism of strongly layered superconductor in strong magnetic field
(2009)
Lin, P. J.
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Zhuravlev, V.
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Li, D.
;
Rosenstein, B.
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電子物理學系
Recent experiments in Bi(2)Sr(1)Ca(1)Cu(2)O(8+delta) [7] demonstrated that the fluctuation diamagnetism in strong magnetic field at the vicinity of the critical temperature significantly differs from the behavior predicted by fluctuation diamagnetism theories based on Ginzburg-Landau model within the lowest Landau level approximation. In this study, we extend previous theoretical result to the T < T(c) region and incorporate to the theory contributions from higher Landau levels. Comparison with experimental results shows a good agreement with the theory for an optimally doped sample, while for under-doped sample the agreement is poor at low temperature.