Negative Charge-Transfer Energy in SrCoO_2.5 Thin Films: An Interplay between O-2p Hole Density, Charge-Transfer Energy, Charge Disproportionation, and Ferromagnetic Ordering

Abstract

SrCoO2 .5 (SCO) bulk is found to be a charge-transfer insulator with a positive value of charge-transfer energy (Δ) and 3d6 ground state, which achieves the antiferromagnetic nature through conventional Co3+–O–Co3+ superexchange interaction. However, in the epitaxial SCO thin film, it is observed, with the help of resonant photoemission spectroscopy and X-ray absorption spectroscopy, that substrate-induced strain modifies the ground state of SCO film to 3d7 L (L: O-2p hole), causing a negative value of Δ. The presence of the O-2p hole in the negative Δ SCO film arises because of strong O 2p–Co 3d hybridization and induces charge disproportionation (CD) in the system. Consequently, the Co3+–O–Co3+ superexchange interaction is modified, and a hole-mediated unconventional ferromagnetic (or ferrimagnetic) ordering is observed in the SCO film. The magnetic moment is found to depend on the values of Δ and CD, which are controlled by the lattice-induced strain. This is manifested from the strain-dependent valence band and conduction band spectra of the SCO films. Because of the negative Δ and CD, additional spectral features appear in the Co L-edge of the films, which are absent in its bulk counterpart. The energy positions and intensities of such features vary with the film thickness, divulging the role of strain in modifying Δ (and CD). Tuning SCO from the positive Δ regime in the bulk to the negative Δ regime in the thin film provides an opportunity to modulate the electronic structure vis-à-vis magnetic property via strain engineering with huge technological potential applications.

Electronic, Orbital, and Spin Reconstructions at the Interface between Metallic La0.7Sr0.3MnO3 and Charge-Disproportionated Insulator SrCoO2.5

“…Two features R 1 and R 2, highlighted by the shaded regions, are also observed at the higher photon energy sides of the L 3 and L 2 features, respectively. Such features are indicative of the negative Δ in the SCO/LSAT film, as suggested by Abbate et al for the Sr 3 FeCoO 7 compound, and are also verified theoretically in our previous study . Besides, double-peak features, assigned as S 1 and S 2, are observed in the L 3 -edge of the SCO/LSAT film, which is absent in SCO bulk reference.…”

“…These S 1 and S 2 features divulge that formal charge state Co 3+ in the SCO/LSAT film undergoes charge disproportionation (CD) into Co (3−δ)+ and Co (3+δ)+ ; 2Co +3 → Co +(3−δ) + Co +(3+δ) . A detailed investigation and discussion of CD can be found elsewhere. , …”

“…Also, the CD features ( S 1 and S 2) disappear in the heterostructure. The CD in the SCO/LSAT single-layer film is associated with the HS Co 2+ and LS Co 4+ charge states. , Integration of LSMO with the SCO allows electron transfer from Mn to the LS (t 2g 5 e g 0 ) Co 4+ sites, which is prohibited for the HS (t 2g 5 e g 2 ) Co 2+ sites. This minimizes the charge difference between the HS Co 2+ and LS Co 4+ sites and destroys the Co CD in the LSMO/SCO/LSAT heterostructure.…”

Revealing the Correlation of OER with Magnetism: A New Descriptor of Curie/Neel Temperature for Magnetic Electrocatalysts

“…As shown in Figure 3h , it is remarkable that the uppermost region in the valance band and the lowermost region in the conduction band near the E F are both strongly hybridized O 2p−Co 3d band in nature for all the samples, suggesting a p−p type of charge fluctuations. [ 36 ] In the strong electron‐correlated material systems, the on‐site Coulomb potential ( U dd ) localizes the d bands, leading to a d−d gap between occupied and empty ones. [ 37 ] Due to the strong hybridization of p‐d orbits, the gap between the highest occupied states and the lowest unoccupied states is narrowed utmost in the LSCO10 sample.…”

The Mystery behind Dynamic Charge Disproportionation in BaBiO3

“…Charge ordering (CO) in perovskites (ABO 3 ) has provided an impetus for exploring quantum phenomena like the insulator–metal/superconductor transition in cuprates and doped bismuthates, as well as charge density wave (CDW) in rare earth nickelates, cobaltates, and bismuthates. − Interestingly, in these materials, charge difference between the neighboring metal ions is fractional, e.g., 2 ± δ (with δ ≠ 1) in cuprates, called partial CO, , or simply charge disproportionation (CD). The complexities arising from the coupling among charge, spin, orbital, and lattice degrees of freedom in TMOs serve as a major bottleneck in understanding the CO and CD mechanisms. , Absence of spin degree of freedom as well as strong electron correlations in BaBiO 3 (BBO) makes it an ideal material to probe the mechanism of CO and CD.…”