Exploring gapped metals: An emerging class of multi-functional quantum materials

The search for new quantum material phases has long been a major focus in condensed matter physics and materials science due to their intriguing physics and potential applications. Recently, a new class of multi-functional quantum materials has been emerged that combines the characteristic traits of both metals and insulators, known as gapped metals. These materials hold the Fermi level within the conduction or valence band and an internal bandgap between principal band edges. Owing to their high intrinsic conductivity and internal bandgap, these materials are considered as a promising substitute for heavily doped semiconductors without external doping. They exhibit potential for high-performance thermoelectric, transparent conducting, photocatalytic, and nano-electronic devices. Furthermore, gapped metals display properties resembling electrides, topological materials, and even dilute magnetic semiconductors without intentional transition-metal doping, due to their spin-dependent band gaps. Their intrinsic carrier concentration and internal bandgap can be tailored via spontaneous non-stoichiometry, enabling tunable functionalities. Motivated by these unique characteristics and promising applications, this review comprehensively examined the recent theoretical and experimental progress on gapped metals, highlighted their potential applications, elucidated their distinct physical properties, and outlined the future directions for their usages in the development of advanced technologies.