  It is generally believed that the quark-hadron transition at small values of
baryon chemical potentials $\mu_B$ is a crossover but changes to a first-order
phase transition with an associated critical endpoint (CEP) as $\mu_B$
increases. Such a $\mu_B$-dependent quark-hadron transition is expected to
result in a double-peak structure in the collision energy dependence of the
baryon density fluctuation in heavy-ion collisions with one at lower energy due
to the spinodal instability during the first-order phase transition and another
at higher energy due to the critical fluctuations in the vicinity of the CEP.
By analyzing the data on the $p$, d and $^3$H yields in central heavy-ion
collisions within the coalescence model for light nuclei production, we find
that the relative neutron density fluctuation $\Delta \rho_n=\langle(\delta
\rho_n)^2\rangle/\langle \rho_n\rangle^2$ at kinetic freeze-out indeed displays
a clear peak at $\sqrt{s_{NN}}=8.8$ GeV and a possible strong re-enhancement at
$\sqrt{s_{NN}}=4.86$ GeV. Our findings thus provide a strong support for the
existence of a first-order phase transition at large $\mu_B$ and its critical
endpoint at a smaller $\mu_B$ in the temperature versus baryon chemical
potential plane of the QCD phase diagram.
