超伝導入門

第8章: ギンスブルグ-ランダウ理論 (3)

抄録

For the intermediate state of Type I superconductors described in chapter 5 to exist stably, the boundary energy of the superconducting and normal regions must be positive because a negative boundary energy will induce fine division of the normal and superconducting regions immediately on entrance to the intermediate state to lower the energy as a whole. Magnetic properties and direct observation of the intermediate state show that this is not the case. The London theory, which assumes that the number of superelectrons is constant throughout the superconducting region, gives only a negative boundary energy because of the penetration of the magnetic field passing through the normal region which lowers the magnetic energy of the superconducting region. In the Ginzburg-Landau (GL) theory, however, the number of superelectrons is allowed to change over a coherence length ξ. Therefore, in contrast with the London theory where the number of superelectrons must decrease to zero stepwise at the boundary, the GL theory shows that the number of superelectrons near the boundary may decrease continuously to zero over the coherence length. The decrease of the number of superelectrons will give rise to a positive energy contribution to the boundary energy as a result of the loss in the superconducting condensation energy. It is shown that the boundary energy may be positive or negative, depending on the magnitude of the GL parameter κ=λ_L/ξ, where λ_L is the London penetration depth.