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Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers.

Bogdanov NA, Katukuri VM, Romhányi J, Yushankhai V, Kataev V, Büchner B, van den Brink J, Hozoi L - Nat Commun (2015)

Bottom Line: We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance.This implies that the iridium d levels are inverted with respect to their normal ordering.State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered.

View Article: PubMed Central - PubMed

Affiliation: Institute for Theoretical Solid State Physics, IFW Dresden, Helmholtzstr. 20, 01069 Dresden, Germany.

ABSTRACT
A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance. While canonical ligand-field theory predicts g//-factors less than 2 for positive tetragonal distortions as present in Sr2IrO4, the experiment indicates g// is greater than 2. This implies that the iridium d levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital reconstruction in the context of oxide electronics.

No MeSH data available.


Related in: MedlinePlus

Effect of interlayer charge imbalance in A2IrO4 iridates.(a) The nearby surroundings of TM sites in A2TMO4-layered perovskites. In test calculations one can assign the adjacent (in-plane) TM ions the formal charge QTM−2Δq, which is compensated by assigning the NN A sites the charge QA+Δq. (b) Tetragonal crystal-field energy splitting between t2g orbitals (δ) as a function of the charge redistribution Δq for Sr2IrO4 and Ba2IrO4.
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f4: Effect of interlayer charge imbalance in A2IrO4 iridates.(a) The nearby surroundings of TM sites in A2TMO4-layered perovskites. In test calculations one can assign the adjacent (in-plane) TM ions the formal charge QTM−2Δq, which is compensated by assigning the NN A sites the charge QA+Δq. (b) Tetragonal crystal-field energy splitting between t2g orbitals (δ) as a function of the charge redistribution Δq for Sr2IrO4 and Ba2IrO4.

Mentions: The exact d-level order is of fundamental importance in TM oxides, dictating for instance the symmetry of the quasiparticle states in photoemission103241 and the nature of the magnetic ordering4243. In 214 iridates specifically, it determines the various isotropic as well as anisotropic contributions to the magnetic exchange couplings12353639, the evolution of those magnetic interactions with strain44 and/or pressure19 and most likely the nature of the intriguing transition to a nonmagnetic phase in Sr2IrO4 under high pressure19. Having established that in Sr2IrO4 the d levels are inverted and that in the closely related Ba2IrO4 they are not raises the question what actually drives the inversion. To address this, we performed an additional set of calculations, in which we change the charges around the reference IrO6 octahedron. As a simple numerical experiment that preserves charge neutrality of the A2IrO4 system, we assigned the 4 NN iridium sites (in-plane, see Fig. 4) the charge QTM−2Δq and the 8 closest A-site cations (out of plane) the valence QA+Δq. In a fully ionic picture, QTM and QA are 4+and 2+, respectively. However, since in our calculations the NN TM and A sites are not modelled as just formal point charges (see Methods), the actual valence states depart from their formal values, with larger ‘deviations' for QTM. The way we introduce Δq in the computations is therefore by appropriately modifying the nuclear charge at the respective site. For variable Δq, this interpolates linearly between nearby surroundings corresponding to 5d 214 layered perovskites (with Δq=0 and TM4+, A2+ formal valence states) and their cuprate 214 equivalents (with Δq=1, TM2+/A3+ formal ionic charges and ‘normal' order of the TM t2g and eg levels45).


Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers.

Bogdanov NA, Katukuri VM, Romhányi J, Yushankhai V, Kataev V, Büchner B, van den Brink J, Hozoi L - Nat Commun (2015)

Effect of interlayer charge imbalance in A2IrO4 iridates.(a) The nearby surroundings of TM sites in A2TMO4-layered perovskites. In test calculations one can assign the adjacent (in-plane) TM ions the formal charge QTM−2Δq, which is compensated by assigning the NN A sites the charge QA+Δq. (b) Tetragonal crystal-field energy splitting between t2g orbitals (δ) as a function of the charge redistribution Δq for Sr2IrO4 and Ba2IrO4.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4491190&req=5

f4: Effect of interlayer charge imbalance in A2IrO4 iridates.(a) The nearby surroundings of TM sites in A2TMO4-layered perovskites. In test calculations one can assign the adjacent (in-plane) TM ions the formal charge QTM−2Δq, which is compensated by assigning the NN A sites the charge QA+Δq. (b) Tetragonal crystal-field energy splitting between t2g orbitals (δ) as a function of the charge redistribution Δq for Sr2IrO4 and Ba2IrO4.
Mentions: The exact d-level order is of fundamental importance in TM oxides, dictating for instance the symmetry of the quasiparticle states in photoemission103241 and the nature of the magnetic ordering4243. In 214 iridates specifically, it determines the various isotropic as well as anisotropic contributions to the magnetic exchange couplings12353639, the evolution of those magnetic interactions with strain44 and/or pressure19 and most likely the nature of the intriguing transition to a nonmagnetic phase in Sr2IrO4 under high pressure19. Having established that in Sr2IrO4 the d levels are inverted and that in the closely related Ba2IrO4 they are not raises the question what actually drives the inversion. To address this, we performed an additional set of calculations, in which we change the charges around the reference IrO6 octahedron. As a simple numerical experiment that preserves charge neutrality of the A2IrO4 system, we assigned the 4 NN iridium sites (in-plane, see Fig. 4) the charge QTM−2Δq and the 8 closest A-site cations (out of plane) the valence QA+Δq. In a fully ionic picture, QTM and QA are 4+and 2+, respectively. However, since in our calculations the NN TM and A sites are not modelled as just formal point charges (see Methods), the actual valence states depart from their formal values, with larger ‘deviations' for QTM. The way we introduce Δq in the computations is therefore by appropriately modifying the nuclear charge at the respective site. For variable Δq, this interpolates linearly between nearby surroundings corresponding to 5d 214 layered perovskites (with Δq=0 and TM4+, A2+ formal valence states) and their cuprate 214 equivalents (with Δq=1, TM2+/A3+ formal ionic charges and ‘normal' order of the TM t2g and eg levels45).

Bottom Line: We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance.This implies that the iridium d levels are inverted with respect to their normal ordering.State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered.

View Article: PubMed Central - PubMed

Affiliation: Institute for Theoretical Solid State Physics, IFW Dresden, Helmholtzstr. 20, 01069 Dresden, Germany.

ABSTRACT
A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance. While canonical ligand-field theory predicts g//-factors less than 2 for positive tetragonal distortions as present in Sr2IrO4, the experiment indicates g// is greater than 2. This implies that the iridium d levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital reconstruction in the context of oxide electronics.

No MeSH data available.


Related in: MedlinePlus