A superpotential for Grassmannian Schubert varieties

Abstract: While mirror symmetry for flag varieties and Grassmannians has been extensively studied, Schubert varieties in the Grassmannian are singular, and hence standard mirror symmetry statements are not well-defined. Nevertheless, in this article we introduce a superpotential'' $W^{\lambda}$ for each Grassmannian Schubert variety $X_{\lambda}$, generalizing the Marsh-Rietsch superpotential for Grassmannians, and we show that $W^{\lambda}$ governs many toric degenerations of $X_{\lambda}$. We also generalize thepolytopal mirror theorem'' for Grassmannians from our previous work: namely, for any cluster seed $G$ for $X_{\lambda}$, we construct a corresponding Newton-Okounkov convex body $\Delta_G^{\lambda}$, and show that it coincides with the superpotential polytope $\Gamma_G^{\lambda}$, that is, it is cut out by the inequalities obtained by tropicalizing an associated Laurent expansion of $W^{\lambda}$. This gives us a toric degeneration of the Schubert variety $X_{\lambda}$ to the (singular) toric variety $Y(\mathcal{N}{\lambda})$ of the Newton-Okounkov body. Finally, for a particular cluster seed $G=G^\lambda{\mathrm{rec}}$ we show that the toric variety $Y(\mathcal{N}{\lambda})$ has a small toric desingularisation, and we describe an intermediate partial desingularisation $Y(\mathcal{F}\lambda)$ that is Gorenstein Fano. Many of our results extend to more general varieties in the Grassmannian.