Browsing by Author "Li, D. P."

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Covariant gaussian approximation in Ginzburg-Landau model
(2017-05-01)
Wang, J. F.
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Li, D. P.
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Kao, H. C.
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Rosenstein, B.
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電子物理學系
Condensed matter systems undergoing second order transition away from the critical fluctuation region are usually described sufficiently well by the mean field approximation. The critical fluctuation region, determined by the Ginzburg criterion, vertical bar T/T-c, 1 vertical bar << Gi, is narrow even in high T, superconductors and has universal features well captured by the renormalization group method. However recent experiments on magnetization, conductivity and Nernst effect suggest that fluctuations effects are large in a wider region both above and below T-c. In particular some "pseudogap" phenomena and strong renormalization of the mean field critical temperature T-mf can be interpreted as strong fluctuations effects that are nonperturbative (cannot be accounted for by "gaussian fluctuations"). The physics in a broader region therefore requires more accurate approach. Self consistent methods are generally "non-conserving" in the sense that the Ward identities are not obeyed. This is especially detrimental in the symmetry broken phase where, for example, Goldstone bosons become massive. Covariant gaussian approximation remedies these problems. The Green's functions obey all the Ward identities and describe the fluctuations much better. The results for the order parameter correlator and magnetic penetration depth of the Ginzburg-Landau model of superconductivity are compared with both Monte Carlo simulations and experiments in high T-c, cuprates. (C) 2017 Elsevier Inc. All rights reserved.
Dynamics of disordered type-II superconductors: Peak effect and the I-V curves
(2007-09-01)
Bel, G.
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Li, D. P.
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Rosenstein, B.
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Vinokur, V.
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Zhuravlev, V.
We quantitatively describe the competition between the thermal fluctuations and disorder by the Ginzburg-Landau approach using both the replica method in statics and the dynamical Martin-Siggia-Rose approach which allows generalization beyond linear response. The two methods are consistent in static, while the dynamical method allows calculation of the critical current as function of magnetic field and temperature. The surface in the J-B-T space defined by this function separates between a dissipative moving vortex matter regime and vortex glass. The non-Ohmic I-V curve is obtained. (c) 2007 Elsevier B.V. All rights reserved.
Influence of quenched disorder on the square-to-rhombohedral structural transformation of the vortex lattice of type-II superconductors
(2006-11-01)
Li, D. P.
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Lin, P. -J.
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Rosenstein, B.
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Shapiro, B. Ya.
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Shapiro, I.
A theory of structural transformations of the vortex lattice in a disordered fourfold symmetric type-II superconductor is constructed using two complementary descriptions of the vortex matter. At low temperatures we use a nonlocal London approach, while near H-c2(T) the lowest Landau level Ginzburg-Landau model with an asymmetric high derivative term is utilized. The quenched disorder influences the location of the square-to-rhombohedral structural transition line, the simplest transformation of that kind. In the clean case the slope of the line in the T-B plane is generally negative, as thermal fluctuations favor a more symmetric square lattice. We calculated the transition line in the presence of pinning and find that the disorder, which plays an important role in low-T-c materials, makes the slope positive, as has been observed in recent experiments.