Kondo scenario of the {\gamma}-{\alpha} phase transition in single crystalline Cerium thin films.

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/6H-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. 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 at low frequency not only confirmed that the Kondo singlet states emerged at $T_K$ $\geq$ 80 K, but also implied that the characteristic temperature of the RKKY interaction TRKKY among f electrons should be around 40 K. Our theoretical results indicate that the Fermi liquid coherence, i.e., f-f indirect bonding, happened after the Kondo dip formation upon cooling. Most importantly, TRKKY was comparable to $T_{\gamma-\alpha}$ observed by the XRD spectra, which suggested the $\gamma$-$\alpha$ phase transition in the fcc-Ce thin films could be explicated in the Kondo scenario.