well,, now I see that 'Superconducting instability' just means that the formation of the pair cannot happen above the critical temperature in a dense limit..
Thanks anyway.
HJ
I am sorry if my question was vague. I didn't know that Superconducting instability is used in many other situations.
I was reading the article 'Bose Condensation in an Attractive Fermion Gas'
http://www.springerlink.com/content/n200841124207n14/fulltext.pdf
and I got stuck with this word...