Crystals and intersecting branes
  We show that the index of BPS bound states of D4, D2 and D0 branes in IIA theory compactified on a toric Calabi Yau are encoded in the combinatoric counting of restricted three dimensional partitions. Using the torus symmetry, we demonstrate that the Euler character of the moduli space of bound states localizes to the number of invariant configurations that can be obtained by gluing D0 bound states in the C^3 vertex along the D2 brane wrapped P^1 legs of the toric diagram. We obtain a geometric realization of these configurations as a crystal associated to the extra bound states of D0 branes at the singular points of a single D4 brane wrapping a high degree equivariant surface that carries the total D4 charge. We reproduce some known examples of the partition function computed in the opposite regime where D0 and D2 charge are dissolved into D4 flux, as well as significantly generalize these results. The crystal representation of the BPS bound states provides a direct realization of the OSV relation to the square of the topological string partition function, which in toric Calabi Yau is also described by a theory of three dimensional partitions. 
Daniel Jafferis