Abstract
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 gap1,2—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 salts3—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.
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Data availability
The data presented in the main text are available via Zenodo at https://doi.org/10.5281/zenodo.15228102 (ref. 37). All other data are available from the corresponding author on reasonable request.
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Acknowledgements
We thank X. Wang, S. Gao, T. Qian, G. Miao and B. Liu for helpful discussions. This work was supported by the Natural Science Foundation of China (12025409) and by the Chinese Academy of Sciences through the Project for Young Scientists in Basic Research (YSBR-021), the Strategic Priority Research Program (XDB33000000) and the IOP-HKUST-Joint Laboratory for Wave Functional Materials Research. A portion of this work was carried out at the Synergetic Extreme Condition User Facility (SECUF) and the Laboratory of Microfabrication, IOP CAS.
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Z.W. performed the DFT calculations with the help of X.H.; conducted the crystal synthesis with the help of C.L.; conducted the electrical measurements; performed various optical measurements and analysis with the help of Y.L., X.G. and K.C.; and wrote the manuscript with L.L. who supervised the entire project.
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Nature Materials thanks Kazushi Kanoda and Jacob Khurgin for their contribution to the peer review of this work.
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Extended data
Extended Data Fig. 1 DFT results of (TMTTF)2X with different anions X = SbF6, PF6, AsF6 and BF4.
a, Band structure. Eintra: the bandwidth of the metallic band; Einter: the distance from Fermi energy EF to the band maximum of bands below -2 eV; Δℏω: the width of the hyper-gap. b, The dependence of Eintra, Einter, Δℏω, and the effective mass m* (me is the free electron mass) on different anions.
Extended Data Fig. 2 Density of states (DOS) and Partial density of states (PDOS) of (TMTTF)2SbF6.
PDOS for s, p, d orbitals are contributed from different atoms.
Extended Data Fig. 3 (TMTTF)2SbF6 samples under the microscope.
a, Samples illuminated by white light from front side. b, Samples illuminated by white light from back side.
Extended Data Fig. 4 Polarization dependent transmittance of 6.5 μm-thick (TMTTF)2SbF6.
a, Transmittance spectra from 0° to 90° polarization. b, Transmittance spectra from 90° to 180° polarization.
Extended Data Fig. 5 Experimental electrical and optical properties of (TMTTF)2X with different anions X = SbF6, PF6, AsF6 and BF4 at room temperature.
a, σ: conductivity along x direction; μ: carrier mobility. μ is derived from σ and calculated carrier density n using μ = σ/(ne). b, optical properties with x and y polarized incident light, extracted from Supplementary Fig. 4. α: lowest absorption coefficient; EU,intra and EU,inter: Urbach energies of the two tails (lower and higher energy sides) of the absorption spectra.
Supplementary information
Supplementary Information
Supplementary Notes 1–6, Figs. 1–9, Table 1 and discussion.
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Wu, Z., Li, C., Hu, X. et al. Hyper-gap transparent conductor. Nat. Mater. 24, 1387–1392 (2025). https://doi.org/10.1038/s41563-025-02248-0
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DOI: https://doi.org/10.1038/s41563-025-02248-0
