Ulrich Bundles on some threefold scrolls over $\mathbb{F}_e$

Abstract

We investigate the existence of Ulrich vector bundles on suitable $3$-fold scrolls $X_e$ over Hirzebruch surfaces $\mathbb{F}_e$, for any integer $e \geqslant 0$, which arise as tautological embeddings of projectivization of very-ample vector bundles on $\mathbb{F}_e$ that are uniform in the sense of Brosius and Aprodu--Brinzanescu. We explicitely describe components of moduli spaces of rank $r \geqslant 1$ vector bundles which are Ulrich with respect to the tautological polarization on $X_e$ and whose general point is a slope-stable, indecomposable vector bundle. We moreover determine the dimension of such components, proving also that they are generically smooth. As a direct consequence of these facts, we also compute the Ulrich complexity of any such $X_e$ and give an effective proof of the fact that these $X_e$'s turn out to be geometrically Ulrich wild. At last, the machinery developed for $3$--fold scrolls $X_e$ allows us to deduce Ulrichness results on rank $r \geqslant 1$ vector bundles on $\mathbb{F}_e$, for any $e \geqslant 0$, with respect to a naturally associated (very ample) polarization.