Semi-classical states for the nonlinear Choquard equations: existence, multiplicity and concentration at a potential well

We study existence and multiplicity of semi-classical states for the nonlinear Choquard equation: $$-\varepsilon^2\Delta v+V(x)v = \frac{1}{\varepsilon^\alpha}(I_\alpha*F(v))f(v) \quad \hbox{in}\ \mathbb{R}^N,$$ where $N\geq 3$, $\alpha\in (0,N)$, $I_\alpha(x)={A_\alpha\over |x|^{N-\alpha}}$ is the Riesz potential, $F\in C^1(\mathbb{R},\mathbb{R})$, $F'(s) = f(s)$ and $\varepsilon>0$ is a small parameter. We develop a new variational approach and we show the existence of a family of solutions concentrating, as $\varepsilon\to 0$, to a local minima of $V(x)$ under general conditions on $F(s)$. Our result is new also for $f(s)=|s|^{p-2}s$ and applicable for $p\in (\frac{N+\alpha}{N}, \frac{N+\alpha}{N-2})$. Especially, we can give the existence result for locally sublinear case $p\in (\frac{N+\alpha}{N}, 2)$, which gives a positive answer to an open problem arisen in recent works of Moroz and Van Schaftingen. We also study the multiplicity of positive single-peak solutions and we show the existence of at least $\hbox{cupl}(K)+1$ solutions concentrating around $K$ as $\varepsilon\to 0$, where $K\subset \Omega$ is the set of minima of $V(x)$ in a bounded potential well $\Omega$, that is, $m_0 \equiv \inf_{x\in \Omega} V(x) < \inf_{x\in \partial\Omega}V(x)$ and $K=\{x\in\Omega;\, V(x)=m_0\}$.