Abstract
The physical mechanism driving the γ–α 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 γ–α phase transition at Tγ−α ≈ 50 K, which is retarded by strain effect from substrate comparing with Tγ−α (about 140 K) of the bulk Ce metal. However, APRES spectra did not show any signature of α-phase emerging in the surface-layer from 300 to 17 K, which implied that α-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 γ–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 γ–Ce, the behavior of the self-energy’s imaginary part at low frequency not only confirmed that the Kondo singlet states emerged at TKS ≥ 80 K, but also implied that they became coherent states at a lower characteristic temperature (Tcoh ~40 K) due to the indirect RKKY interaction among f–f electrons. Besides, Tcoh from the theoretical simulation was close to Tγ−α from the XRD spectra. These issues suggested that the Kondo scenario might play an important role in the γ–α phase transition of cerium thin films.
Highlights
Cerium is among one of the most amazing elements across the periodic table, due to its complex phase diagram in which up to seven allotropic phases (γ, β, α, α0, α′′, η, and δ) could be realized in a modest pressure and temperature range[1,2,3,4,5,6,7]
Five streaks are indicated by white arrows
It down to 15 K, we found that the as-grown Ce thin films should be noted that the stacking sequence along the c-axis of surprisingly sustained the γ-phase from 300 K down to around 50 K, which is controversial to commonly accepted phase β–Ce is –ABACABAC, while it is –ABCABC– perpendicular to the γ–Ce(111) surface
Summary
Cerium is among one of the most amazing elements across the periodic table, due to its complex phase diagram in which up to seven allotropic phases (γ, β, α, α0, α′′, η, and δ) could be realized in a modest pressure and temperature range[1,2,3,4,5,6,7]. Therefor the electronic configuration changed from 4f 11⁄25d6s23þ to 1⁄24f 5d6s24þ. This argument was proved to be inconsistent with positronannihilation experiments[12,13], Compton scattering[14] and inelastic neutron scattering[15,16], as almost no obvious change in the 4f occupancy was observed. (3) the Kondo Volume Collapse (KVC) model[18,19], in which the Kondo hybridization (Jeff) between the 4f-electron and the conducting (c) valence electrons was assumed to vary with volume between the γ and α phases. It should be stressed that the key ingredient in the KVC model is the f–c hybridization, which means f-electrons become indirectly bonding due to the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction[20,21,22,23]. A direct and clear observation of the f–c hybridization effect is inevitable for the understanding of the γ to α phase transition of Ce metal
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