  This article study the fractional Hamiltonian systems
\begin{eqnarray}\label{00}
{_{t}}D_{\infty}^{\alpha}({_{-\infty}}D_{t}^{\alpha}u) + \lambda L(t)u = \nabla
W(t, u), \;\;t\in \mathbb{R}, \end{eqnarray} where $\alpha \in (1/2, 1)$,
$\lambda >0$ is a parameter, $L\in C(\mathbb{R}, \mathbb{R}^{n\times n})$ and
$W \in C^{1}(\mathbb{R} \times \mathbb{R}^n, \mathbb{R})$. Unlike most other
papers on this problem, we require that $L(t)$ is a positive semi-definite
symmetric matrix for all $t\in \mathbb{R}$, that is, $L(t) \equiv 0$ is allowed
to occur in some finite interval $\mathbb{I}$ of $\mathbb{R}$. Under some mild
assumptions on $W$, we establish the existence of nontrivial weak solution,
which vanish on $\mathbb{R} \setminus \mathbb{I}$ as $\lambda \to \infty,$ and
converge to $\tilde{u}$ in $H^{\alpha}(\mathbb{R})$; here $\tilde{u} \in
E_{0}^{\alpha}$ is nontrivial weak solution of the Dirichlet BVP for fractional
Hamiltonian systems on the finite interval $\mathbb{I}$.
