Yang-Mills fields on $B$-branes

Considering the $B$-branes over a complex manifold $Y$ as objects of the bounded derived category $D^b(Y)$, we define holomorphic gauge fields on $B$-branes and the Yang-Mills functional for these fields.These definitions are a generalization to $B$-branes of concepts that are well known in the context of vector bundles. Given ${\mathscr F}^{\bullet}\in D^b(Y)$, we show that the Atiyah class $a({\mathscr F}^{\bullet})\in{\rm Ext}^1({\mathscr F}^{\bullet},\,\Omega^1({\mathscr F}^{\bullet}))$ is the obstruction to the existence of gauge fields on ${\mathscr F}^{\bullet}$. We determine the $B$-branes over $\mathbb{ CP}^n$ that admit holomorphic gauge fields. We prove that the set of Yang-Mills fields on the $B$-brane ${\mathscr F}^{\bullet}$, if it is nonempty, is in bijective correspondence with the points of an algebraic subset of ${\mathbb C}^m$ defined by $m\cdot s$ polynomial equations of degree $\leq 3$, where $m={\rm dim}\,{\rm Hom}({\mathscr F}^{\bullet},\,\Omega^1({\mathscr F}^{\bullet}))$ and $s$ is the number of non-zero cohomology sheaves ${\mathscr H}^i({\mathscr F}^{\bullet})$. We show sufficient conditions under them any Yang-Mills field on a reflexive sheaf of rank $1$ is flat.