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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

TM t2g splittings for tetragonal distortions of the oxygen octahedron sans SOC.(a) z axis compression of the octahedron corresponds to a tetragonal splitting δ<0, causes an orbital dublet to be lowest in energy and the g factors to be ordered as . (b) Elongation of the octahedron (δ>0) causes an orbital singlet to be lowest in energy and the g factors to be ordered as . Purple dashed lines indicate the conventional zero level used to define the sign of δ.
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f1: TM t2g splittings for tetragonal distortions of the oxygen octahedron sans SOC.(a) z axis compression of the octahedron corresponds to a tetragonal splitting δ<0, causes an orbital dublet to be lowest in energy and the g factors to be ordered as . (b) Elongation of the octahedron (δ>0) causes an orbital singlet to be lowest in energy and the g factors to be ordered as . Purple dashed lines indicate the conventional zero level used to define the sign of δ.

Mentions: Mott-Hubbard physics in d-metal compounds has been traditionally associated with first-series (3d) TM oxides. However, recently, one more ingredient entered the TM-oxide ‘Mottness' paradigm—large SOC's in 5d systems. SOC in 5d and to some extent 4d anisotropic oxides modifies the very nature of the correlation hole of an electron, by admixing the different t2g components1213, changes the conditions for localization7, the criteria of Mottness and further gives rise to new types of magnetic ground states and excitations911. While various measurements indicate that indeed spin-orbit-coupled jeff≈1/2 states form in A2IrO4 (refs 7, 9, 14), it has been also pointed out that off-diagonal SOC's may mix into the ground state (GS) wavefunction substantial amounts of character15161718. Such many-body interactions were shown to produce remarkable effects in X-ray absorption and X-ray magnetic circular dichroism (XMCD): the branching ratio between the L3 and L2 Ir 2p absorption edges reaches values as large as 4, nearly 50% higher than the 2.75 value for a ‘pure' jeff=1/2 system19. In addition, low-symmetry noncubic fields produce sizeable splittings of the 5d t2g levels, in some cases close to or even larger than ∼1/2 eV (refs 20, 21, 22), and therefore admix the jeff=1/2 and components1315. The structure of the spin-orbit GS depends on both the strength and sign of these splittings. Interestingly, the best fits of the X-ray absorption and XMCD data are achieved in Sr2IrO4 with a negative t2g tetragonal splitting19, although the oxygen octahedra in this material display a distinct positive tetragonal distortion—the IrO6 octahedra are substantially elongated23 (a negative tetragonal splitting should occur when the IrO6 octahedra are compressed1324, see Fig. 1). This is already a first indication of the level inversion that our ESR measurements and quantum chemistry calculations show to take place in Sr2IrO4.


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)

TM t2g splittings for tetragonal distortions of the oxygen octahedron sans SOC.(a) z axis compression of the octahedron corresponds to a tetragonal splitting δ<0, causes an orbital dublet to be lowest in energy and the g factors to be ordered as . (b) Elongation of the octahedron (δ>0) causes an orbital singlet to be lowest in energy and the g factors to be ordered as . Purple dashed lines indicate the conventional zero level used to define the sign of δ.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: TM t2g splittings for tetragonal distortions of the oxygen octahedron sans SOC.(a) z axis compression of the octahedron corresponds to a tetragonal splitting δ<0, causes an orbital dublet to be lowest in energy and the g factors to be ordered as . (b) Elongation of the octahedron (δ>0) causes an orbital singlet to be lowest in energy and the g factors to be ordered as . Purple dashed lines indicate the conventional zero level used to define the sign of δ.
Mentions: Mott-Hubbard physics in d-metal compounds has been traditionally associated with first-series (3d) TM oxides. However, recently, one more ingredient entered the TM-oxide ‘Mottness' paradigm—large SOC's in 5d systems. SOC in 5d and to some extent 4d anisotropic oxides modifies the very nature of the correlation hole of an electron, by admixing the different t2g components1213, changes the conditions for localization7, the criteria of Mottness and further gives rise to new types of magnetic ground states and excitations911. While various measurements indicate that indeed spin-orbit-coupled jeff≈1/2 states form in A2IrO4 (refs 7, 9, 14), it has been also pointed out that off-diagonal SOC's may mix into the ground state (GS) wavefunction substantial amounts of character15161718. Such many-body interactions were shown to produce remarkable effects in X-ray absorption and X-ray magnetic circular dichroism (XMCD): the branching ratio between the L3 and L2 Ir 2p absorption edges reaches values as large as 4, nearly 50% higher than the 2.75 value for a ‘pure' jeff=1/2 system19. In addition, low-symmetry noncubic fields produce sizeable splittings of the 5d t2g levels, in some cases close to or even larger than ∼1/2 eV (refs 20, 21, 22), and therefore admix the jeff=1/2 and components1315. The structure of the spin-orbit GS depends on both the strength and sign of these splittings. Interestingly, the best fits of the X-ray absorption and XMCD data are achieved in Sr2IrO4 with a negative t2g tetragonal splitting19, although the oxygen octahedra in this material display a distinct positive tetragonal distortion—the IrO6 octahedra are substantially elongated23 (a negative tetragonal splitting should occur when the IrO6 octahedra are compressed1324, see Fig. 1). This is already a first indication of the level inversion that our ESR measurements and quantum chemistry calculations show to take place in Sr2IrO4.

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