# What is clean limit and dirty limit?

In superconductor there has two limits, one called as clean and other one called dirty limit. But what are that meaning? What means clean and dirty? How the coherence length depends on the electron mean free path, what are these limit using for? For that I had read sth, in which the surface energy was mentioned. What kind of influences this energy has for superconductor? I hope you can understand my English.

Clean and dirty limits refer to the comparison of the carriers mean free path ($$\ell$$) to the coherence length of the pairs ($$\xi$$). Note that $$\xi$$ is, roughly speaking, the size of the Cooper pair (superconducting state), and that $$\ell$$ refers to unpaired carriers.
The clean limit is the case where $$\xi \ll \ell$$ and the dirty limit is, obviously, the case $$\xi \gg \ell$$.