Moduli spaces of rank 2 sheaves on $\mathbb P^3$ via Quot schemes

Abstract

We study the Gieseker--Maruyama moduli space $M_{c_3}$ of stable rank 2 torsion-free sheaves on $\mathbb{P}^3$ with Chern classes $c_1=-1$, $c_2=1$, and odd $c_3 \le 1$. Every such sheaf $E$ has reflexive hull $E^{\ast\ast}$ parametrised by $M_1$, and the quotient $E^{\ast\ast}/E$ is a 0-dimensional sheaf of length $n = (1-c_3)/2$. We prove that $M_{c_3}$ is isomorphic to the relative Quot scheme $\mathsf{Quot}_{\mathbb{P}^3 \times M_1/M_1}(\mathscr{R},n)$, where $\mathscr{R}$ is the universal reflexive sheaf over $M_1$. Our technique combines (i) Bridgeland-type wall-crossing in Donaldson--Thomas theory, (ii) the theory of good moduli spaces, (iii) deformation theory in dgla and Ext-quiver language. We deduce that $M_{c_3}$ is connected, and we analyse its \emph{smoothable component} $M_{c_3}^{\mathsf{main}}$, proving it is rational and generically smooth of dimension $4n+3$. We further prove that $\dim \mathrm{Sing} M_{c_3} \geq 4n-2$ for all $n \geq 2$. Finally, we provide a geometric characterisation for the cases $n\leq 2$. We show that $M_{-1}$ is a smooth rational projective variety of dimension 7 and compute its motive in the Grothendieck ring of varieties, while $M_{-3}$ is shown to be an integral, normal variety of dimension 11 that is regular in codimension 4.