Type I and Type II Superconductors

I don't really expect much of an answer but anyway, I've been researching superconductivity and I still don't have a good qualitative understanding of what causes Type I and Type II superconductors.

I've seen the equations and understand that it's due to contributions to the free energy from the penetration depth and coherence length, but what does that mean physically? Why does a greater penetration depth reduce the free energy and a greater coherence length increase it?

Recall that Type I superconductors are usually pure metals and because of this they have a ordered lattice, promoting Cooper pairings.

The Cooper paring is related to the coherence length, i.e. pure metals typically have larger coherence lengths than composites. Because of this, Type I superconductors are expected to have large coherence lengths (they also have small penetration depths).

The GL parameter is the ratio of the penetration depth to the coherence length. Since a Type I has small penetration depths and large coherence lengths with respect to a Type II, the GL parameter will be less in a Type I than in a Type II.

Hope this helps.

Maybe I should be a little more specific.

The explanations I've seen consider the interface between a superconducting and normal phase of some material. For a large GL parameter (i.e. penetration depth is greater than coherence length), said interface has a negative surface energy, and thus more interfaces (such as through the quantized regions of magnetic flux in Type II's mixed state) minimize the free energy.

What I want is a qualitative understanding of why a large GL parameter gives rise to this negative surface energy.