Unconventional Superconductivity in electron-doped layered metal nitride halides $M$N$X$ ($M$ = Ti, Zr, Hf; $X$ = Cl, Br, I)

TL;DR

This review discusses superconductivity in electron-doped layered metal nitride halides, highlighting their unconventional features and comparing experimental and theoretical insights with other superconductors.

Contribution

It provides a comprehensive overview of the emergence and properties of superconductivity in layered metal nitride halides, emphasizing their unconventional mechanisms and recent research progress.

Findings

Superconductivity appears in doped metal nitride halides with layered structures.

Features suggest mechanisms beyond conventional electron-phonon interactions.

Comparison with other superconductors reveals unique electronic behaviors.

Abstract

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.