Unconventional Superconductivity in electron-doped layered metal nitride   halides $M$N$X$ ($M$ = Ti, Zr, Hf; $X$ = Cl, Br, I)
  In this review, we present a comprehensive overview of superconductivity in electron-doped metal nitride halides $M$N$X$ ($M$ = Ti, Zr, Hf; $X$ = Cl, Br, I) with layered crystal structure and two-dimensional electronic states. The parent compounds are band insulators with no discernible long-range ordered state. Upon doping tiny amount of electrons, superconductivity emerges with several anomalous features beyond the conventional electron-phonon mechanism, which stimulate theoretical investigations. We will discuss experimental and theoretical results reported thus far and compare the electron-doped layered nitride superconductors with other superconductors. 
Yuichi Kasahara, Kazuhiko Kuroki, Shoji Yamanaka, Yasujiro Taguchi
cond-mat.supr-con cond-mat.str-el