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Kondo Lattice Behavior Observed in the CeCu$_9$In$_2$ Compound

We report systematic studies of CeCu$_9$In$_2$, which appears to be a new Kondo lattice system. Electrical resistivity exhibits a logarithmic law characteristic of Kondo systems with a broad maximum at $T_{coh}\approx$45 K and it obeys the Fermi liquid theory at low temperature. Specific heat of CeCu$_9$In$_2$ is well described by the Einstein and Debye models with electronic part at high temperature. Fitting of the Schottky formula to low temperature 4f contribution to specific heat yielded crystal field splitting of 50.2 K between a doublet and quasi-quartet. The Schotte-Schotte model estimates roughly Kondo temperature as $T_K\approx$5 K, but does not reproduce well the data due to a sharp peak at 1.6 K. This structure should be attributed to a phase transition, a nature of which is possibly antiferromagnetic. Specific heat is characterized with increased Sommerfeld coefficient estimated as $γ\approx$132 mJ/(mole$\cdot$K$^2$). Spectra of the valence band, which have been collected with ultraviolet photoelectron spectroscopy (UPS), show a peak at binding energy$\approx$250 meV, which originates from the Ce 4f electrons and is related to the 4f$^1$$_{7/2}$ final state. Extracted 4f contribution to the spectral function exhibits also the enhancement of intensity in the vicinity of the Fermi level. Satellite structure of the Ce 3d levels spectra measured by X-ray photoelectron spectroscopy (XPS) has been analyzed within the framework of the Gunnarsson-Schönhammer theory. Theoretical calculations based on density functional theory (FPLO method with LDA+U approach) delivered densities of states, band structures and Fermi surfaces for CeCu$_9$In$_2$ and LaCu$_9$In$_2$. The results indicate that Fermi surface nesting takes place in CeCu$_9$In$_2$.