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Thin films and coatings

Thin films and coatings

New theory explains why metallic oxides are transparent

19 Aug 2021 Isabelle Dumé
Photo of Mathieu Mirjolet in the lab
In the lab: Predoctoral researcher Mathieu Mirjolet inserts a strontium vanadium oxide (SrVO3) film into a probe station for electrical testing. (Courtesy: ICMAB-CSIC)

Few materials are both transparent and electrically conductive, and one of the rare examples that fits the bill – metallic oxides – may be transparent for reasons other than previously thought. While the usual explanation involves interactions between the material’s electrons, researchers at the Universitat Autonoma de Barcelona say that the conditions required for metallic transparency could instead arise from quasiparticles called polarons. As well as being important for fundamental science, this radically different view could aid the development of next-generation materials for touchscreens and displays.

At present, most smartphone and tablet touchscreens are made from indium tin oxide (ITO), a semiconductor that is also widely used in solar panels, LEDs in LCDs or OLEDs and for aircraft windshield coatings. Thanks to the scarcity of indium, however, ITO is becoming increasingly expensive, and researchers are looking for alternatives.

A substitute for ITO

One possible ITO substitute is vanadium strontium oxide (SrVO3), a transition-metal oxide that is metallic but becomes transparent when made into thin layers. The electrons in materials like SrVO3are confined in narrow 3orbitals, which enhances the electrostatic Coulomb interactions between them. These enhanced interactions are thought to increase the electrons’ effective mass to such an extent that they no longer resonate with the electric field of light – meaning that light particles (photons) pass straight through the material rather than interacting with electrons and being reflected.

However, researchers led by Josep Fontcuberta from the Institute of Materials Science of Barcelona (ICMAB, CSIC) say that their studies of SrVO3 suggest an alternative explanation. In their view, the effective mass of electrons is high in SrVO3not because of interactions between the electrons themselves, but because of the formation of polarons, which are couplings between electrons and the material’s ionic lattice. These couplings cause the lattice to distort around the electrons as they move through the material, which likewise increases the electrons’ effective mass.

“Serious difficulties”

The researchers obtained this result, which they describe in Advanced Scienceby analysing the optical and electronic properties of SrVO3epitaxial films grown under different conditions. “Our measurements revealed some serious difficulties when it came to describing the properties of the SrVO3with the ‘correlated electron’ scenario,” Fontcuberta tells Physics World. “Something was wrong and forced us to revise the whole picture.”

Preliminary first-principles calculations by collaborators at Germany’s Frankfurt University backed up the team’s findings, indicating that carrier-lattice coupling does indeed play an important role in the material’s transparency. According to Fontcuberta, this result may have profound implications not only for metallic oxides, but also for materials such as cuprates, which are high-temperature superconductors. These other materials, he says, “might also share the electron–polaron coupling feature we have observed”.

Looking forward, the researchers say they will now try to tune the coupling between the lattice and electrons in SrVO3. “We will also be extending our study to different materials to find out what mechanisms are responsible for the electron-lattice coupling becoming dominant as opposed to an electron–electron correlation, which should still surely be present,” Fontcuberta says.

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