Temperature dependence of the 4f quadrupole moment of Yb in YbCu2Si2

An explanation is provided for the Mössbauer measurements on YbCu2Si2 by which the temperature-dependent quadrupole moment Q(T) of Yb is obtained. The quadruple splitting of the Yb174 Mössbauer line is a direct probe of the noncubic crystalline electric field. We compute Q(T) by using the noncrossing approximation to a Green's-function formulation of the Anderson impurity problem. The observed Q(T) behavior is well reproduced by our theory. The zero-temperature value Q(0) is discussed within the frame of the variational ground state proposed by Varma and Yafet and by Gunnarsson and Schönhammer.