Hyper-gap transparent conductor.

An elusive conductor with perfect optical transparency holds revolutionary potential for fields such as optoelectronics and nanophotonics. Such a hypothetical metal would possess a spectral gap-a 'hyper-gap'-in its absorption spectrum, separating the intraband and interband absorptions, in which optical losses could vanish. Currently, this property is achievable only within the bandgap of insulators. However, realizing such a hyper-gap metal demands an exotic electronic structure in which the conducting bands have a bandwidth narrower than their energy separations from the remaining electronic states. Here we present such a hyper-gap in a family of organic metals-the Fabre charge-transfer salts-through first-principles predictions coupled with both electrical and optical measurements. A transparent window, spanning from red to near-infrared wavelengths, is identified in bulk single crystals that remain transmissive over a thickness of 30 µm. The corresponding absorption coefficient is the lowest among known stoichiometric metals, rivalling thin films of transparent conductive oxides. This finding introduces a path, beyond traditional doping strategies in insulators, to combine electronic conduction and optical transparency.