Affine $\mathrm{ı}$quantum groups and Steinberg varieties of type C

We provide a geometric realization of the quasi-split affine $\mathrm{ı}$quantum group of type AIII${}_{2n-1}^{(\tau )}$ in terms of equivariant K-groups of non-connected Steinberg varieties of type C. This uses a new Drinfeld type presentation of this affine $\mathrm{ı}$quantum group which admits very nontrivial Serre relations. We then construct à la Springer a family of finite-dimensional standard modules and irreducible modules of this $\mathrm{ı}$quantum group, and provide a composition multiplicity formula of the standard modules.