Is it possible to find a CAT(0) space on which the matrix group $SL(n,\mathbb{Z})$ acts properly discontinuously and cocompactly? Note: when the cocompactness is dropped , it is possible.
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If n=2, yes: it acts on its BassSerre tree. If n>2, no: your group contains distorted elements, i.e. elements conjugated to a proper power of themselves (look at unipotent matrices). Such an element will have zero displacement length, which is impossible for an infinite order element in a group acting discretely cocompactly. For this and much more, see the monograph of Bridson and Haefliger. For even more restrictions on SL_n actions, see Theorem 1.14 in: CapraceMonod, Isometry groups of nonpositively curved spaces: structure theory Journal of Topology 2 No. 4 (2009), 661–700 

