Large (bi)Polarons for Novel Energy Conversion and Superconductivity

Abstract

Materials containing high densities of exceptionally displaceable ions (e.g. perovskites) have extremely large ratios of their static to high-frequency dielectric constants, > 2. Large polarons form in such materials as their electronic charge carriers self-trap by displacing surrounding ions. Large polarons are very heavy-massed slow-moving quasi-particles that are very weakly scattered by ambient phonons. Large-polaron mobilities, e.g. 1 cm2/V-sec at 300 K, are much smaller than the minimum possible for conventional electronic charge carriers. The minimum mobility for an itinerant charge carrier of effective mass m, eh/mkT, occurs when its mean-free-path falls to its de Broglie wavelength, e.g. 300 cm2/V-sec at room temperature for m equaling the free-electron mass. Distinctively, large-polarons frequency-dependent conductivities consist of two contributions that separate as the temperature is reduced. Large polarons Drude-like contributions are relegated to frequencies below those of characteristic phonons. Contributions from excitations of large polarons self-trapped electronic carriers occur above those of characteristic phonons. Oppositely charged large polarons repel one another at short range. The resulting suppressed recombination facilitates exceptionally efficient solar cells. Large polarons of the same charge attract one another at short range to enable their real-space pairing into singlet bipolarons. Additional attractions between large bipolarons facilitates their condensation into liquids that can exhibit superconductivity.