  Metamaterials are engineered materials composed of small electrical circuits
producing novel interactions with electromagnetic waves. Recently, a new class
of metamaterials has been created to mimic the behavior of media displaying
electromagnetically induced transparency (EIT). Here we introduce a planar EIT
metamaterial that creates a very large loss contrast between the dark and
radiative resonators by employing a superconducting Nb film in the dark element
and a normal-metal Au film in the radiative element. Below the critical
temperature of Nb, the resistance contrast opens up a transparency window along
with a large enhancement in group delay, enabling a significant slowdown of
waves. We further demonstrate precise control of the EIT response through
changes in the superfluid density. Such tunable metamaterials may be useful for
telecommunication because of their large delay-bandwidth products.
