Competition of defect ordering and site disproportionation in strained LaCoO3 on SrTiO3 (001)

Abstract

The origin of the 3 × 1 reconstruction observed in epitaxial LaCoO3 films on SrTiO3(001) is assessed by using first-principles calculations including a Coulomb repulsion term. We compile a phase diagram as a function of the oxygen pressure, which shows that (3 × 1)-ordered oxygen vacancies (LaCoO2.67) are favored under commonly used growth conditions, while stoichiometric films emerge under oxygen-rich conditions. Growth of further reduced LaCoO2.5 brownmillerite films is impeded by phase separation. We report two competing ground-state candidates for stoichiometric films: a semimetallic phase with 3 × 1 low-spin/intermediatespin/intermediate-spin magnetic order and a semiconducting phase with intermediate-spin magnetic order. This demonstrates that tensile strain induces ferromagnetism even in the absence of oxygen vacancies. Both phases exhibit an intriguing (3 × 1)-reconstructed octahedral rotation pattern and accordingly modulated La-La distances. In particular, charge and bond disproportionation and concomitant orbital order of the t2g hole emerge at the Co sites that are also observed for unstrained bulk LaCoO3 in the intermediate-spin state and explain structural data obtained by x-ray diffraction at elevated temperature. Site disproportionation drives a metalto-semiconductor transition that reconciles the intermediate-spin state with the experimentally observed low conductivity during spin-state crossover without Jahn-Teller distortions.arXiv:2003.02317v1 [cond-mat.mtrl-sci]

Ferromagnetism of LaCoO$_3$ films

“…Should the magnetism of LaCoO 3 originate from the HS state of Co 3+ ions only, an anti-ferromagnetic (AFM) coupling between these is expected [26]. There were several attempts to explain emerging FM long-range order in insulating LaCoO 3 under tensile strain [25,[27][28][29][30]. The LDA+U density functional calculations using Hartree-Fock-like treatment of intra-atomic d-d interaction [28,29] argue in favor of the superexchange-based mechanism for FM ordering.…”

Correlated interface electron gas in infinite-layer nickelate versus cuprate films on SrTiO$_3$(001)

“…Wave functions and density were expanded into plane waves up to cutoff energies of 45 and 350 Ry, respectively. Ultrasoft pseudopotentials [40] as successfully employed in previous work [35][36][37][38][41][42][43][44][45], were used in conjunction with projector augmented wave datasets [46]. The Pr and Nd 4f electrons are frozen in the core, similar to previous studies involving Nd [4,13,19,34]; their explicit treatment leads to qualitatively similar results.…”

Ultra-low lattice thermal conductivity and anisotropic thermoelectric transport properties in Zintl compound β-K2Te2

“…However, it is still a challenge to optimize these TE parameters simultaneously because of their unavoidable conflict. The power factor (PF) can be improved via modulation doping, 6,7 applying strain, 8,9 and superlattices and alloying of materials. [10][11][12] On the other hand, the introduction of defects, [13][14][15] heavy element substitution, [16][17][18] using complex primitive cells, 19 boundary effects, 20,21 and reducing the dimensions are valid for decreasing the thermal conductivity.…”