  We present in this letter terahertz (THz) metamaterials with low ohmic losses
made from low-temperature superconductor niobium nitride (NbN) films. The
resonance properties are characterized by THz time-domain spectroscopy. The
unloaded quality factor reaches as high as about 178 at 8 K with the resonance
frequency at around 0.58 THz, which is about 24 times as many as gold
metamaterials with the same structure. The unloaded quality factor also keeps
high as the resonance frequency increases, which is about 90 at 1.02 THz that
is close the gap frequency of NbN film. All these experimental observations are
well understood in the framework of Bardeen-Cooper-Schrieffer theory and
equivalent circuit model. Our work offers an efficient way to design and make
high-performance THz electronic devices.
