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 $\xi \gg \ell$.
There are several uses for these definitions either, in particular in what concerns the electrodynamics (interaction with electromagnetic fields) of superconductors. One particular interest of the notion of clean and dirty is the effect of impurities in the superconducting properties. Opposite to clean superconductors, the introduction of additional impurities or defects in a "dirty" material do not have a major influence in the superconducting properties.