Optical absorption andsandpphase shifts of virtual-bound-state alloys

The results of optical studies of the nonmagnetic virtual-bound-state alloys CuNi, AuNi, AgPd, and AuPd are summarized. The importance of $s$ and $p$ phase shifts of the impurity potential is emphasized, and the results are reanalyzed in terms of an expression for the frequency-dependent relaxation time which includes $s$, $p$, and $d$ phase shifts and is consistent with the Friedel sum rule. CuNi appears anomalous. The various limitations of the theoretical expressions are discussed and it is suggested that CuNi may be understood if the effects of spin fluctuations are considered.