Simple approximation scheme for the Anderson impurity Hamiltonian

A simple approximation scheme is presented for solving the Anderson impurity model within the Noncrossing Approximation (NCA). It is shown that with it the computations reduce to an extent that the scheme can be readily applied to interpret different experiments. The theory is applied to calculate the temperature dependence of the quadrupole moment and of the static and dynamic magnetic susceptibility. The effects of the crystalline electric field (CEF) are thereby incorporated. A comparison of the static susceptibility with exact Bethe ansatz results is given, when the effect of the CEF is neglected. Comparisons with the numerically exact solutions of the NCA are made where they are available. The application of the present theory to YbCu₂Si₂ is discussed.