Normalized solutions for critical Choquard systems

In this paper, we consider the critical Choquard system with prescribed mass \begin{equation*} \begin{aligned} \left\{ \begin{array}{lll} -\Delta u+\lambda_1u=(I_\mu\ast |u|^{2^*_\mu})|u|^{2^*_\mu-2}u+\nu p(I_\mu\ast |v|^q)|u|^{p-2}u\ & \text{in}\quad \mathbb{R}^N,\\ -\Delta v+\lambda_2v=(I_\mu\ast |v|^{2^*_\mu})|v|^{2^*_\mu-2}v+\nu q(I_\mu\ast |u|^p)|v|^{q-2}v\ & \text{in}\quad \mathbb{R}^N,\\ \int_{\mathbb{R}^N}u^2=a^2,\quad\int_{\mathbb{R}^N}v^2=b^2, \end{array}\right.\end{aligned} \end{equation*} where $N\geq3$, $0<\mu<N$, $\nu\in\mathbb{R}$, $I_\mu:\mathbb{R}^N\rightarrow\mathbb{R}$ is a Riesz potential, and $2_{\mu,*}:=\frac{2N-\mu}{N}<p,q<\frac{2N-\mu}{N-2}:=2^*_{\mu},$ with $2_{\mu,*}, 2^*_{\mu}$ called the lower and upper critical exponent in the sense of Hardy-Littlewood-Sobolev inequality respectively. When $\nu<0$, we prove that no normalized ground state exists. When $\nu>0$, we study the existence, non-existence and asymptotic behavior of normalized solutions by distinguishing three cases: $L^2$-subcritical case: $p+q<4+\frac{4-2\mu}{N}$; $L^2$-critical case: $p+q=4+\frac{4-2\mu}{N}$; $L^2$-supercritical case: $p+q>4+\frac{4-2\mu}{N}$. In particular, in $L^2$-subcritical case, and either $N\in\{3,4\}$ or $N\geq5$ with $(\frac N2-1)p+\frac {N}{2}q\leq 2N-\mu$ and $(\frac N2-1)q+\frac {N}{2}p\leq 2N-\mu$, we prove that there exists $\nu_0>0$ such that the system has a positive radial normalized ground state for $0<\nu<\nu_0$. In $L^2$-critical case and $N\in\{3,4\}$, we show there is $\nu'_0>0$ such that the system has a positive radial normalized ground state for $0<\nu<\nu'_0$. In $L^2$-supercritical case and $N\in\{3,4\}$, there are two thresholds $\nu_2\geq\nu_1\geq0$ such that a positive radial normalized solution exists if $\nu>\nu_2$, and no normalized ground state exists for $\nu<\nu_1$.