Heavy-fermion behavior inYbCu5−xAgxs

Abstract

The phase relation and physical properties of the ${\mathrm{YbCu}}_{5\mathrm{\ensuremath{-}}\mathrm{x}}$ ${\mathrm{Ag}}_{\mathrm{x}}$ system have been investigated as functions of temperature and composition x. Powder x-ray-diffraction experiments showed that ${\mathrm{YbCu}}_{5\mathrm{\ensuremath{-}}\mathrm{x}}$ ${\mathrm{Ag}}_{\mathrm{x}}$ crystallizes in a single phase of the cubic ${\mathrm{AuBe}}_{5}$ -type structure in the range of 0.125\ensuremath{\leqslant}x\ensuremath{\leqslant}1.0. Magnetic susceptibility \ensuremath{\chi}, electrical resistivity \ensuremath{\rho}, and specific heat C measurements revealed that this system belongs to a series of dense Kondo compounds, and the characteristic temperature of Kondo effect shifts to lower temperatures with decreasing x. It was also observed that the susceptibility at zero temperature, \ensuremath{\chi}(0), the ${\mathrm{T}}^{2}$ coefficient of resistivity, A, and the electronic specific heat coefficient, \ensuremath{\gamma}, are enhanced with the decrease of x (0.125\ensuremath{\leqslant}x\ensuremath{\leqslant}1.0). The \ensuremath{\gamma} value increases monotonically from 210 mJ/mol ${\mathrm{K}}^{2}$ for x=1.0 to 460 mJ/mol ${\mathrm{K}}^{2}$ for x=0.125 with decreasing x. This substantial increase of \ensuremath{\gamma} with decreasing x can be explained by the 'chemical pressure' effect, which is estimated from the composition dependence of the lattice parameters. Furthermore, the coefficient A depending on 'chemical pressure' is well parallel to that for x=1.0 under hydrostatic pressures. In the composition range of 0.0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.1, the system consists of two phases with the cubic ${\mathrm{AuBe}}_{5}$ -type and the hexagonal ${\mathrm{CaCu}}_{5}$ -type structures. These results mean that the ${\mathrm{YbCu}}_{5}$ exists with the cubic ${\mathrm{AuBe}}_{5}$ -type structure as well as with the hexagonal ${\mathrm{CaCu}}_{5}$ -type one, although only the hexagonal ${\mathrm{CaCu}}_{5}$ -type ${\mathrm{YbCu}}_{5}$ has been reported so far. The value of \ensuremath{\gamma} for the hypothetical cubic ${\mathrm{YbCu}}_{5}$ of single phase is expected to reach about 500 mJ/mol ${\mathrm{K}}^{2}$ from an extrapolation of the \ensuremath{\gamma} values of ${\mathrm{YbCu}}_{5\mathrm{\ensuremath{-}}\mathrm{x}}$ ${\mathrm{Ag}}_{\mathrm{x}}$ (0.125\ensuremath{\leqslant}x\ensuremath{\leqslant}1.0) to x=0.