A new nonreduced moduli component of rank-$2$ semistable sheaves on ${\Bbb P}^{3}$

We describe a new irreducible component of the Gieseker–Maruyama moduli scheme $\mathcal{M}(14)$ of coherent rank-$2$ semistable sheaves with Chern classes $c_1=0$, $c_2=14$, and $c_3=0$ on ${\Bbb P}^{3}$ which is nonreduced at a general point. The construction of the component is based on Mumford's famous example of the nonreduced component of the Hilbert scheme of smooth space curves of degree $14$ and genus $24$ in ${\Bbb P}^{3}$.