Optimal spectrum for the borocarbides YNi2B2C and LuNi2B2C

The concept of an optimal electron-phonon interaction spectral density as an Einstein spectrum which allows to describe all physical properties of a superconductor in an optimal way is developed from Carbotte's original definition of an optimum spectrum. It is shown, using the borocarbides YNi2B2C and LuNi2B2C as examples, that such a concept is meaningful even for anisotropic systems. An Einstein spectrum is sufficient for clean-limit systems, a 2$\delta$-peak spectrum is better suited for anisotropic systems with impurities.