Temperature Dependence of the4fQuadrupole Moment of Yb in YbCu2Si2

An explanation is provided for the Mössbauer measurements on Yb${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$ by which the temperature-dependent quadrupole moment $Q\left(T\right)\mathrm{}$ of Yb is obtained. The quadruple splitting of the ${}_{}{}^{174}{}_{}{}^{}\mathrm{Yb}$ Mössbauer line is a direct probe of the noncubic crystalline electric field. We compute $Q\left(T\right)\mathrm{}$ by using the noncrossing approximation to a Green's-function formulation of the Anderson impurity problem. The observed $Q\left(T\right)\mathrm{}$ behavior is well reproduced by our theory. The zero-temperature value $Q\left(0\right)\mathrm{}$ is discussed within the frame of the variational ground state proposed by Varma and Yafet and by Gunnarsson and Schönhammer.