As explained in the Graduate study FAQ, whether or not a staff member can take a new graduate student or appoint a new postdoc depends on whether he or she has obtained funding for such a position from one of our national granting agencies or the EU. So job opportunities in most of our groups depend strongly on future funding. See the faculty list for links to the home pages of the various groups in the Instituut-Lorentz (the theory division of the Leiden physics department).
Regardless of whether you find an opening listed or not, if you are looking for a PhD position with one of us and are exceptionally good, do contact us in any case if you really would like to work with one of us: for really very good candidates who we'd love to have in Leiden, we can apply for departmental PhD positions at any time. Decisions on such applications can be made in a few weeks. See the Graduate study FAQ for more information.