# what is meant by “crossover phenomena”?

In many articles I read the term "crossover phenomena" and a lot of methodology discussed according to it, with little or no description about what is meant by it. Sometimes there is a connection to networks, matrices, and then to other structures of elements. Can someone describe the concept of the phenomena?

• I've heard this, or similar terminology, in a broad array of sciences. Can you be more specific about what context you are talking about? – Colin K Apr 19 '11 at 16:36
• Do You read that in physics articles? I know it from nearly any field of science and arts. en.wikipedia.org/wiki/Crossover – Georg Apr 19 '11 at 16:36

The best example of a "crossover" phenomenon I can think of is the BEC-BCS crossover which occurs in cold Fermi gases. There one can tune the scattering length $a$ for pairwise interactions to go from being positive to negative. For $a < 0$ bound states can form and we get a BCS state (Bardeen-Cooper-Schrieffer) due to formation of Cooper pairs. As $a$ is tuned to go from being negative to positive the system "crosses over" from being a BCS to a BEC (Bose-Einstein Condensate).