Pressure influence on the valence and magnetic state of Yb ions in noncentrosymmetric heavy-fermion Yb Ni C 2

We present studies of neutron diffraction, electrical resistivity, magnetic susceptibility, magnetization, and specific heat of noncentrosymmetric $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_{2}$. At normal pressure, $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_{2}$ is a moderate heavy-fermion compound with Kondo lattice. At 16 K we observe an anomaly in the temperature dependence of specific heat which is ascribed to an abrupt valence change of Yb ions. At pressures above 7 GPa, the valence change and the Kondo lattice state are suppressed, and near a temperature of 10 K we detect the appearance of magnetic order. Above 5 GPa, the temperature dependence of the resistivity behaves similarly to other compounds of the $\mathrm{Ce}\mathrm{Ni}{\mathrm{C}}_{2}$-type family, indicating the formation of charge density waves. This is attributed to the nesting properties of the Fermi surface (FS) found within the density functional $\mathrm{theory}+\mathrm{dynamical}$ mean field theory treatment. Our ab initio calculations also show that the valence of Yb in $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_{2}$ at normal conditions is 2.85, which increases with temperature and pressure. The FS of $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_{2}$ is anisotropic in shape in comparison with the 3D surface of $\mathrm{Yb}\mathrm{Co}{\mathrm{C}}_{2}$.