The sup-norm problem for automorphic cusp forms of $\mathrm{PGL}(n,\mathbb{Z}[i])$

Let $ϕ$ be an $L^2$-normalized Hecke--Maaß cusp form for $\mathrm{PGL}_n(\mathbb{Z}[i])$ on the locally symmetric space $X:=\mathrm{PGL}_n(\mathbb{Z}[i])\backslash \mathrm{PGL}_n(\mathbb{C}) / \mathrm{PU}_n$. If $Ω$ is a compact subset of $X$, then we prove the bound $\|ϕ|_Ω\|_{\infty}\ll_Ω λ_ϕ^{n(n-1)/4-δ}$ for some $δ>0$ depending only on $n$, where $λ_ϕ$ is the Laplace eigenvalue of $ϕ$.

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