Kenji Takanaka

Transition Temperature and Upper Critical Fields for Superconducting Superlattices

We numerically calculate the transition temperature and the upper critical fields for superconducting superlattices with spatial variations of the BCS interaction, the density of states and the diffusion constant. For the superlattices composed of thin layers, the Debye cutoff frequency plays a role to make the states with large eigenvalues less important. By virtue of this, a simple expression for the perpendicular upper critical field H c2⊥ is obtained, if the diffusion constants are same in both thin layers.

Angular Dependence of Superconducting Critical Fields

We consider the angular derivatives d H c2 (θ)/dθ and d H c3 (θ)/dθ at θ=0, where H c2 (θ) and H c3 (θ) are the upper critical field and the sheath critical field, and θ is the angle to denote the magnetic field direction measured from the maximum critical field. It is found that the angular dependence of the critical fields exhibits a cusp if the nucleation of the order parameter takes place at the definite position of the sample.

Upper Critical Fields for Superconducting Superlattices

For the superconducting superlattices whose diffusion constant changes continuously, we study the upper critical field H c2 and its angular derivative d H c2 (θ)/ dθ at θ=0, where θ is the angle between the layer plane and the applied magnetic field. The latter quantity shows whether the order parameter has a preferential position relative to the sample. From H c2 and d H c2 (θ=0)/ dθ , we discuss the difference of this model from the model whose diffusion constant changes discontinuously at the interfaces and the Josephson coupling model.

Upper Critical Field Hc2 of Dirty Layered Superconductors

The upper critical field H c2 is calculated in the dirty limit for layered superconductors whose Fermi surface is assumed to consist of two distinct types. While the upper critical field perpendicular to the layer has the same temperature dependence as that for the bulk isotropic dirty superconductors, H c2 parallel to the layer is found to have a positive curvature in the temperature dependence and to be finite even if there is no Pauli paramagnetism.

Superconducting properties of superlattices containing ferromagnetic layers

Using the microscopic theory formulated by de Gennes and extended by Takahashi and Tachiki, we calculate the transition temperatureTc and the pair functionF for the superlattices consisting of superconducting and ferromagnetic layers. Superconducting layers. (s) and ferromagnetic layers (f) are modeled byVs≠0 andIm,s=0 andVf=0 andIm,f≠0, whereVs.(Vf) is the BCS coupling constant andIm,s (Im,f) is the molecular field fors (f) layers.

Surface critical field of superconducting superlattice I

Abstract We investigate the surface effects on the parallel critical field H c3 ( T ) and the pair function F 0 ( z ) of the superlattice with spatial variations of the BCS coupling constant and the diffusion constant. We make use of the lowest eigenvalues and eigenfunctions up to 30 to obtain the temperature dependence of the critical field. The effect of the BCS coupling constant appears significantly in our results. It is found that the positive curvature in H c3 (T) is related to the disappearance of the sub-peaks of F 0 ( z ).

Surface effects on the critical field of superconducting superlattices

Abstract Assuming that two types of layers in a superlattice are characterized by different diffusion constants D1 and D2, we study theoretically surface effects on the critical field Hc of dirty superconducting superlattices.

Hc3 of Superconducting Films in the Presence of Electron-Spin and Spin-Orbit Effects

The critical field H c 3 of dirty superconducting films is calculated in the presence of Pauli spin paramagnetism and spin-orbit impurity scattering. It is shown that the ratio H c 3 / H c 2 for thin films near the transition temperature has larger values than 1.695 and it decreases with decreasing temperature by both temperature and spin paramagnetic effects.

Stable Zero Field Mode in Superconducting Josephson Tunnel Junctions

Assuming that the superconducting phase difference in voltage biased Josephson tunnel junctions has only a single frequency component, the current-voltage ( I – V ) characteristics are studied and their stability is discussed. On the n -th step which occurs at \(\textit{eV}{=}\hbar\omega_{n}\) with the applied voltage V and the n -th resonance frequency ω n (= n ω 1 ), it is found that the mode with the frequency ω 1 has a lower energy than that with the frequency ω n . On all steps the radiation frequency from the junctions is therefore equal to ω 1 (the fundamental frequency) in accord with the experimental observations.

Stable states of superconducting superlattices

Abstract The temperature dependence of the upper critical field Hc2(T) perpendicular to the planes of the superconducting superlattice with magnetic layers is calculated. The results show that the phase difference ϕ(L) of the order parameter in one spatial period L changes from 0 to π with decreasing temperature for an appropriate set of the physical quantities. Further, we derive the analytic expression for the third-order term of the order parameter of the Ginzburg–Landau equation as the first step to study the stable state at (H,T) below Hc2(T).