Comment on Total Negative Refraction in Real Crystals for Ballistic Electrons and Light (Phys. Rev. Lett. 91, 157404 (2003))

Recently, Zhang et al. (Phys. Rev. Lett. 91, 157404 (2003)) have demonstrated that an amphoteric refraction, i. e. both positive and negative refraction, may prevail at the interface of two uniaxial anisotropic crystals when their optical axes are in different directions. The authors subsequently made a correspondence between such a refraction with the negative refraction expected for Left Handed Materials (LHMs). Here we comment that the amphoteric refraction can be observed even with one uniaxial crystal, and the refraction is not related to the negative refraction expected for the much debated LHM. Rather, the phenomenon is a natural result of anisotropic media.