How to contact Researchers once you are accepted I'm a physics student from Argentina and I've just been accepted for a Physics PhD in Cornell University. There are a few research groups that I really like and I was wondering if it is a common thing to mail them introducing myself and asking if it is possible that there is going to be an available position for me in the future. I wouldn't like to study there for a year to find out that the groups I like wouldn't have taken me in the first place. Thanks!

P.D.: Also, does anybody know when does research begins? I thought that the PhD program was 1 year of courses and then research, but now I've read that there are actually 2 years of courses before you join a research group.
Most major graduate institutions, including Cornell, have extensive written materials that lay out how some of these things are done in a specific program. For example, waiting 1 year to join a research group sounds like a long time for me, but apparently it is normal for your institution (maybe normal for physics in general).

For Cornell Physics, see their handbook at http://physics.cornell.edu/sites/physics/files/Yellow-Book-Revised-July-2013.pdf and the rest of their graduate website at http://physics.cornell.edu/graduate

Verbatim from the 'yellow book':


  As to research, students should, during the first year of graduate work, spur themselves (and be
  encouraged by the faculty) to find out about some of the Cornell research going on around them.
  By the summer following the first year they should associate themselves personally with the
  research of some professor or research group, even if this association is only temporary. The
  normal time for a reasonably firm commitment for a Ph.D. thesis area, and for such an
  understanding with a supervising professor, is at about the end of the second year.


You may certainly approach professors who you may want to work with during and after acceptance into the program. I am not sure what the funding norms are in physics in terms of how far in advance professors know they will be able to take on a student, but I see no reason why it would hurt to contact professors you are interested in working with as part of the application process and decision to accept admission if granted.

Cornell offers visits to prospective Ph.D students: http://physics.cornell.edu/prospective-graduate-students#plan-a-visit which includes meeting with professors, presumably who you would be interested in working with.

It doesn't seem like a visit is necessary just based on what I see on their website (and I would expect this sort of thing is normally done before acceptance, maybe you have even already done this), but you could certainly manage a more informal visit via email, particularly if travel is a hardship.