Extended Data Figure 10 : Comparison of the optical absorptions and calculated densities-of-states for the three phases.

From: Electric-field control of tri-state phase transformation with a selective dual-ion switch

Extended Data Figure 10

a, Optical absorptions of the three phases, plotting α (the optical absorption) versus ħω (the photon energy). Below a photon energy of 4.0 eV, there are two main absorption features in all three phases, which can be attributed to the intraband dd transition (lower-energy end; the absorption peaks α, σ and δ) and interband pd transition (higher-energy end; the absorption peaks β, ε and γ). Consistent with our electrical transport studies, we find that the SrCoO3−δ is metallic, with strong absorption among the whole optical range studied here. However, both SrCoO2.5 and HSrCoO2.5 show insulating behaviour, with strong absorption spectra emerging around the interband transitions (β and ε). Furthermore, the absorption by SrCoO2.5 is even greater than that of the metallic SrCoO3−δ phase at higher energies (greater than 2.5 eV), owing to the large pd excitation. However, the absorption by HSrCoO2.5 is strongly suppressed compared with that by SrCoO2.5, owing to enhancement of the direct bandgap. b–d, Calculated total and projected density of states (DOS) onto oxygen 2p, CoT (tetragonal layer) and CoO (octahedral layer) orbitals for: b, SrCoO2.5; c, HSrCoO2.5; and d, SrCoO3. In comparison with SrCoO2.5, the bandgap of HSrCoO2.5 becomes larger, owing to the ascension of the unoccupied cobalt 3d state. For SrCoO3, the Fermi energy level cuts into the oxygen 2p and cobalt 3d DOS, consistent with the metallic nature of this phase suggested by our electrical transport studies.