  We found that a single negative material has a character of zero-refraction
in near field, and point out that the mechanism of a metal superlens to image
with the resolution exceeding the diffraction limitation is different from that
of a perfect lens made of a double negative material. The principle of metal
superlens is disclosed. Our numerical results based on the zero-refraction
character coincide with the experimental results well. This work brings new
understanding about the single negative materials and will lead the
applications of metal superlens in right way.
