Researchers from the Institute of Materials Science of Barcelona (ICMAB-CSIC) Gyanendra Singh and Josep Fontcuberta (MULFOX group) have experimentally confirmed a theory they proposed in 2021 that explains the mechanism by which some metals are transparent. Their findings, published in the journal Physical Review Letters, show that electrons in these materials move more slowly because they interact strongly with the vibrations of the material, so they cannot respond to visible light in the way electrons normally do in common metals. This work has been carried out in close collaboration with the Paul Scherrer Institut and ETH Zurich (both in Switzerland).

Researchers discovered that these transparent metallic oxides display this property because their reflectivity to visible light is largely suppressed at infrared, and so visible light can propagate through them. This is in sharp contrast with conventional metals that largely reflect light at visible range. This property allows to use transparent metallic oxides in many high-tech applications; for instance, as electrodes in photovoltaic applications or screens in mobiles, to mention a couple of examples.

Traditionally, it was accepted the electrons were dressed with a heavy effective mass by the electron-electron interactions in these metals, thus decreasing the plasma frequency (the edge of reflectivity) to infrared, contrasting with most common metallic systems where the plasma frequency is a higher energy.

One of these transparent metal oxides is the one named SrVO3. Back in 2021, after a systematic analysis of transport and optical data of films of this compound, researchers came to the conclusion that atomic vibrations, phonons, play a major role of the mass enhancement, overruling the conventional electron-electron correlations. This hypothesis allowed a description and understanding of experimental data.

As any hypothesis should be, it was falsifiable and subject to empirical validation. This has taken sometime. Now, publishing in Physical Rev Letters, new experiments involving a challenging ¹⁸O isotopic substitution in SrVO₃ films, these authors have conclusively demonstrated the prominent role of phonons on carrier dynamics, enhancing their effective mass and ultimately governing their optical transparency.

Therefore, these results provide strong empirical support of the relevance of electron phonon coupling to understand the origin of the astonishing and much soaked transparency of metallic oxide thin films.

Josep Fontcuberta, leader of the ICMAB research group, inspiring and guiding the experiments that confirmed their early hypothesis, said: "This last step (the isotopic substitution instrumental on falsifying our hypothesis)) has required extensive and dedicated work. If phonons are responsible for dressing the carriers, then if the mass of ions in the lattice is changed the corresponding atomic vibrations should be modified and therefore the coupling the itinerant carriers (electrons) altered. To do so we have undertaken a long path that, in collaboration with our partners from PSI-Villingen (Switzerland), involves a partial substitution of ¹⁶O in SrVO3 thin films by its heavier isotope ¹⁸O, while keeping to total electron density constant."

These findings challenge the conventional view centered on electron–electron interactions, establishing instead that electron–phonon coupling is key to understanding the remarkable transparency of metallic oxide thin films.

This disruptive view overrules conventional electron-electron interaction scenario, and the results reported here and their implications must be reckoned with in any future development.

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