Just trying to clarify what you are asking for. If you've already identified the special field in which you want to do a postdoc, I would assume you've already read a large number of research papers in that special field; and, from the authors on those papers, would have identified potential advisors.
I looked at your profile and from what I can tell, you have completed your PhD at a university in Brazil in the area of physics (or materials science) related to phase transitions.
Are you currently
(a) employed as a postdoc and seeking projects to work in complex systems,
(b) seeking postdoctoral positions outside of Brazil in complex systems?
If (b), then some places I can think of for opportunities include the following:
1. Santa Fe Institute (www.santafe.edu) - research institute devoted to complex systems. Try and see if there are any postdoc opportunities available there.
2. Center for the Study of Complex Systems, University of Michigan (lsa.umich.edu/cscs). According to the website, they are seeking tenure track position there.
3. Complex Systems Society (https://cssociety.org/job-openings) has a number of postings in EU countries for postdoc positions. Two of those positions are based in Spain -- I'm curious as a Brazilian citizen what your fluency would be in Spanish (I've been told that Portuguese speakers have little trouble picking up Spanish).