Browsing by Author "Chang, D"

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Dynamical analysis of survival of Kosterlitz-Thouless pairs due to pinning
(1997-01-01)
Chang, D
;
Rosenstein, B
;
Wu, CL
;
電子物理學系
;
Department of Electrophysics
We investigate the process of annihilation between vortices of opposite flux in superconducting thin films in the presence of randomly distributed pinning centers. The number and distribution of pairs of vortex-antivortex that avoid annihilation due to pinning on two pinning centers is estimated analytically and compared with numerical simulations. The situation can be realized, for example, when the system is quenched from high temperature (above or below the Kosterlitz-Thouless temperature) or when the current is reduced abruptly. Qualitatively two different cases can be distinguished. The simpler one is when the pairs are well separated. In this case, the problem under mild assumptions can be solved analytically. When the vortex pairs are not very rare, the situation is much more complicated because a great variety of processes become significant. We isolate the important ones and construct an iterative (real-space renormalization-group) scheme to describe the behavior of the interacting multivortex system. Some dynamical aspects of pinning and vortex motion are discussed.
Dynamical analysis of survival of Kosterlitz-Thouless pairs due to pinning
(1997-01-01)
Chang, D
;
Rosenstein, B
;
Wu, CL
;
電子物理學系
;
Department of Electrophysics
We investigate the process of annihilation between vortices of opposite flux in superconducting thin films in the presence of randomly distributed pinning centers. The number and distribution of pairs of vortex-antivortex that avoid annihilation due to pinning on two pinning centers is estimated analytically and compared with numerical simulations. The situation can be realized, for example, when the system is quenched from high temperature (above or below the Kosterlitz-Thouless temperature) or when the current is reduced abruptly. Qualitatively two different cases can be distinguished. The simpler one is when the pairs are well separated. In this case, the problem under mild assumptions can be solved analytically. When the vortex pairs are not very rare, the situation is much more complicated because a great variety of processes become significant. We isolate the important ones and construct an iterative (real-space renormalization-group) scheme to describe the behavior of the interacting multivortex system. Some dynamical aspects of pinning and vortex motion are discussed.
Explicit four-fold symmetry breaking effect on the vortex lattice in D-wave superconductors
(1998-04-01)
Chang, D
;
Mou, CY
;
Rosenstein, B
;
Wu, CL
;
電子物理學系
We use a recently developed effective Ginzberg-Landau equation to study the vortex lattice structure in d-wave superconductors. By introducing explicit four-fold symmetry breaking terms we are able to reproduce the small angle neutron scattering data on YBCO by Keimer et al. [9]. A phase diagram is also constructed and shows quite rich structures.
New findings of the electromagnetic response in superconductors
(1998-04-01)
Mou, CY
;
Chang, D
;
Rosenstein, B
;
Wu, CL
;
電子物理學系
Based on the Ginzburg-Landau theory, two theoretical findings in the electromagnetic response of superconductors are described. First, we derive the most general linear response theory and then apply it to the charged super fluid regime, where the magnetic fluctuation is relevant. In particular, we show that a novel effect arises, in which the divergence of the conductivity tensor is suppressed by the diamagnetic susceptibilities. Secondly, we show that because of the approximated fourfold symmetry of the crystal lattice in high T-c cuprates, a non-vanishing Hall current can arise near Hc(2). This Hall current is non-linear and can become large to be observed if the applied electric field is large enough.
Static and dynamical anisotropy effects in the mixed state of d-wave superconductors
(1998-04-01)
Chang, D
;
Mou, CY
;
Rosenstein, B
;
Wu, CL
;
電子物理學系
We describe the effects of anisotropy caused by the crystal lattice in d-wave superconductors using an effective foe-energy approach in which only one order parameter, the d-wave order-parameter field, is used. The Abrikosov parameter beta(A) is calculated analytically for both static and moving vortex lattices. The analytic expression provides an unambiguous determination of the vortex-lattice structure. We also calculate both direct and Hall I-V curves as functions of the angle between the current and the crystal-lattice orientation. In particular, we show that near H-c2 the fourfold symmetry of the crystal lattice causes asymmetric motions of particles and holes, resulting in a nonvanishing Hall current.
The effect of anisotropy on vortex lattice structure and flux flow in d-wave superconductors
(1998-04-01)
Chang, D
;
Mou, CY
;
Rosenstein, B
;
Wu, CL
;
電子物理學系
We describe effects of anisotropy caused by the crystal lattice in d-wave superconductors using effective free energy approach in which only one order parameter, the d-wave order parameter field, is used. The Abrikosov parameter beta(A) is calculated analytically for both static and moving vortex lattices. The analytic expression provides an unambiguous determination of the vortex lattice structure. We also calculate both direct and Hall I-V curves as functions of the angle between the current and the crystal lattice orientation. In particular, we show that near H-c2 the fourfold symmetry of the crystal lattice causes asymmetric motions of particles and holes, resulting in a non-vanishing Hall current.