T.J. Yang

Current Distribution and Vortex-Vortex Interaction in a Superconducting Film of Finite Thickness

The current distribution due to a single vortex in a superconducting film of finite thickness is calculated, based on the London model. At large distances from the vortex core, the current density on the surface of the superconductor was found to follow the 1/r2 law which has a simple expression, J s(r, 0)≈0[coth (d/λ)+ csch (d/λ)]/2πµ0λr2. We also find that the interaction energy between two vortices has the form, U int(r12)=02[H0(r12/Λ)-Y0(r12/Λ)], when the film is not too thick and at large distances between two vortices. This potential differs from Pearls result by the new effective penetration depth Λ=2λcoth (d/λ).

Analysis of transport critical current density in a bent BiPbSrCaCuO silver-sheathed tape

Abstract The experimental results on the magnetic behavior of the transport critical current density in a bent Bi based superconducting tape have shown that the irreversible strain limit eirrev for the onset of permanent strain damage to Ag sheathed superconductors is not dependent on the magnetic field, nor does the normalized I c (H, e) I c (0, e) current depend on the strain at least up to 0.5 T at 77 K. Such a behavior has been attributed to two reasons: (1) The intrinsic pinning properties are unchanged in a bent tape; (2) The strain effect is extrinsic and arises from superconductor cracks. Thus, based on these arguments a Josephson junction tunneling model with cracks in between the grain boundaries is proposed to explain the Jc behavior quite well for a bent granular high-Tc superconducting tape. Our model calculation shows that the critical current density of high-Tc superconductor may be enhanced by increasing the strain tolerance eirrev and the eirrev is determined by the properties of Ag sheathed tape and its related material parameters, which are dependent on the connectivity of the grain boundaries.

Evidence of d-wave behavior in Pr1.85Ce0.15CuO3.97 aligned superconductors

Abstract The c -axis and in-plane alignment of 23 K Pr 1.85 Ce 0.15 CuO 3.97 superconducting powder were achieved with powder/epoxy cylindrical holder perpendicular and parallel to the alignment field. The temperature dependencies of basal-plane penetration depth λ ab ( T ) and c -axis Josephson penetration depth λ c ( T ) were derived using zero-field-cooled diamagnetic susceptibility χ d ( T ) in a low applied field ( B a H c1 ) parallel or perpendicular to the aligned powder where small Pr Curie–Weiss paramagnetic susceptibility χ p ( T ) was subtracted. Up to 0.5 T c , Δλ ab ( T )= λ ab ( T )− λ ab (0) as well as Josephson Δλ c ( T ) can be well fitted with a d-wave-like power law c 2 T 2 + c 4 T 4 , where T 2 -term is dominated below 0.3 T c . The present result agrees with recent phase-sensitive experiment, which indicates d-wave nature for electron-doped R 1.85 Ce 0.15 CuO 4− δ (R=Pr, Nd) cuprates.

Nature of the 45 degrees vortex lattice reorientation in tetragonal superconductors

The transformation of the vortex lattice in a tetragonal superconductor which consists of the

Magnetic force signal of vortex creation in type-II superconducting thin films

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Thickness dependence of magnetic force and vortex creation in type-II superconducting thin film

reorientation relative to the crystal axes is studied using the nonlocal London model. It is shown that the reorientation occurs as two successive second order (continuous) phase transitions. The transition magnetic fields are calculated for a range of parameters relevant for borocarbide superconductors in which the reorientation has been observed.

INFLUENCE OF AN EXTERNAL ELECTRIC FIELD ON HIGH-TC SUPERCONDUCTIVITY

Abstract The possibility of the formation of a vortex in thin film due to the field of magnetic dipole moment when a point dipole is vertically approaching the surface of a thin film is discussed. Calculations are performed for certain discrete values of the critical position of the point dipole for creating the vortex in the thin film and the equilibrium positions of the vortex, both of which depend on the strength of magnetic dipole moment. The creation of a new single vortex in the thin film causes an abrupt change in the vertical force.

Magnetic properties for anisotropic dx2−y2-wave superconductivity

Abstract We examined a single vortex line in an infinite type-II superconducting thin film with finite thickness. The thickness dependence of the magnetic force acting on a point dipole placed above a superconducting film is calculated as the negative of the gradient of the interaction energy, coupled to the London theory of superconductivity with electromagnetics. We also considered the situation of a single vortex creation in the thin film, caused by the field of the magnetic dipole tip itself. The thickness dependence of the critical position of the point dipole for creating the first vortex line in the thin film is calculated.

Thickness dependence of magnetic force acting on a magnetic dipole over a type-II superconducting thin film

Abstract The surface superconductivity induced by applying an electric field perpendicular to the surface of a high-Tc superconductor covered by an insulator and a gate electrode is studied theoretically. Taking the idea into account that the change of the surface carrier density confined in a thin screening layer modulates the transport properties of superconductors, we develop a simple method based on the Chen and Yang model to study the Tc shift dependence on the applied voltage in the frameworks of a Thomas-Fermi approximation and BCS theory. In such a case a novel boundary condition is introduced to modify the G-L equations. Solving the modified one, the Tc shift derived by the present model is similar in qualitative behaviour to the Shapiro result, but much simpler in form. In addition, it is shown to be proportional to the applied field (the square of applied field) for thin film (thick film) for a given material. Base on our simple results, the critical current density Jc and nucleation field Hc3 as functions of the applied voltage are easily derived.

Formation of multiple-quanta vortices at circular defects in a superconducting thin film

Abstract The single vortex structure is derived in the presence of applied magnetic field. According to our results, the winding number of the s-wave does not change regardless of the vector potential. The generic London equation for a d x 2 − y 2 -wave superconductor with mass anisotropy is expressed. By neglecting higher order terms, this work analyzes the magnetic-field distribution with and without a vortex. The interaction force between two parallel vortices is derived as well. Our results further reveal the presence of a torque between vortices irrespective of s - or d -wave order parameter, which is expected to vanish for isotropic cases.