  The physical mechanism driving the $\gamma$-$\alpha$ phase transition of
face-centre-cubic (fcc) cerium (Ce) remains controversial until now. In this
work, high quality single crystalline fcc-Ce thin films were grown on
Graphene/6$H$-SiC(0001) substrate, and explored by XRD and ARPES measurement.
XRD spectra showed a clear $\gamma$-$\alpha$ phase transition at
$T_{\gamma-\alpha}\approx$ 50 K, which is retarded by strain effect from
substrate comparing with $T_{\gamma-\alpha}$ (about 140 K) of the bulk Ce
metal. However, APRES spectra did not show any signature of $\alpha$-phase
emerging in the surface-layer from 300 K to 17 K, which implied that
$\alpha$-phase might form at the bulk-layer of our Ce thin films. Besides, an
evident Kondo dip near Fermi energy was observed in the APRES spectrum at 80 K,
indicting the formation of Kondo singlet states in $\gamma$-Ce. Furthermore,
the DFT+DMFT calculations were performed to simulate the electronic structures
and the theoretical spectral functions agreed well with the experimental ARPES
spectra. In $\gamma$-Ce, the behavior of the self-energy's imaginary part at
low frequency not only confirmed that the Kondo singlet states emerged at
$T_{\rm KS} \geq 80$ K, but also implied that they became coherent states at a
lower characteristic temperature ($T_{\rm coh}\sim 40$ K) due to the indirect
RKKY interaction among $f$-$f$ electrons. Besides, $T_{\rm coh}$ from the
theoretical simulation was close to $T_{\gamma-\alpha}$ from the XRD spectra.
These issues suggested that the Kondo scenario might play an important role in
the $\gamma$-$\alpha$ phase transition of cerium thin films.
