Abstract Body

The depairing current density J_d is the theoretical limit of the current density that can flow in the superconducting state [1]. Although the typically observed critical current density is often considerably smaller than J_d, critical current densities comparable to J_d have actually been observed in superconducting nanostrips with thin thickness and narrow width.

In this study, we numerically investigate the dependence of the critical current density J_c in type-II superconducting films on the magnetic field B_a and on the film thickness d_s. Here, the applied magnetic field is parallel to the film surfaces, and we consider the case of longitudinal (transverse) magnetic field parallel (perpendicular) to the transport current. We restrict to the case of extreme type-II superconductors (λξ ≫ 1, where λ is the penetration length and ξ is the coherence length) in the Meissner state where no quantized magnetic flux exists, and we numerically solve the nonlinear London equation derived from the one-dimensional Ginzburg-Landau equation [2].

The dependence of the critical current density J_c on the longitudinal magnetic fields B_a for various film thicknesses d_s is shown in Fig. 1. For thin films (d_s ≲ λ), J_c is close to J_d near B_a=0 and decreases nonlinearly and slowly with increasing B_a. For thick films (d_s ≫ λ), on the other hand, J_c < J_d at B_a= 0, and J_c decreases rapidly with increasing B_a. The B_a dependence of J_c for d_s≫λ fits well with the theoretical J_c=J_c0 [1-(B_a / B_c)² ]^1/2. Where J_c0 is the zero-field critical current density and B_c is the thermodynamic critical magnetic field.

References

[1] V. L. Ginzburg and L. D. Landau, Zh. Eksp. Teor. Fiz. 20, 1064 (1950).
[2] P. G. de Gennes, Solid Stat. Commun. 3, 127 (1965).

Acknowledgment

This work has been supported by JSPS KAKENHI, GrantNo. JP20K05314.

Fig. 1 Dependence of the critical current density J_c on the longitudinal magnetic field B_a, (a) where J_c is scaled by J_d and B_a is scaled by B_c and (b) where J_c is scaled by J_c0 and B_a is scaled by the superheating field B_sh