Schur positivity of the nabla operator on two-column modified Hall--Littlewood polynomials

In this paper, we investigate the Schur positivity of modified Hall–Littlewood polynomials indexed by two-column partitions under the action of the $\mathrm{\nabla }$ operator. Specifically, we resolve two conjectures posed by Bergeron, Garsia, Haiman, and Tesler in the two-column case. Furthermore, our approach demonstrates that these results can be extended to arbitrary powers ${\mathrm{\nabla }}^k$ for all integers $k\ge 1$.