Large $N$ Schur index of $\mathcal N=4$ SYM from semiclassical D3 brane

We consider the refined Schur superconformal index of 4d $\mathcal N=4$ $U(N)$ SYM and the first term of its giant-graviton expansion, first predicted in arXiv:2001.11667 using indirect superconformal algebra considerations and analytic continuation of fugacities. This correction is the leading non-perturbative correction to the index at large $N$ and we reproduce it from the semiclassical partition function of quantum D3 brane wrapped on $S^{1}\times S^{3}$ in a twisted modification of the $AdS_{5}\times S^{5}$ string background, depending on the index R-symmetry fugacity. Our calculation does not exploit directly supersymmetry. It is based on the determination of the partition function of the various bosonic and fermionic fluctuations on the wrapped brane whose action is conformal with specific constant holonomies along thermal cycle. We show how those partition functions may be obtained by adapting the operator counting method of Cardy to the twisted background.