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Imaging the evolution of metallic states in a correlated iridate

Abstract

The Ruddlesden–Popper series of iridates (Srn+1IrnO3n+1) have been the subject of much recent attention due to the anticipation of emergent phenomena arising from the cooperative action of spin–orbit-driven band splitting and Coulomb interactions1,2,3. However, an ongoing debate over the role of correlations in the formation of the charge gap and a lack of understanding of the effects of doping on the low-energy electronic structure have hindered experimental progress in realizing many of the predicted states4,5,6,7,8,9. Using scanning tunnelling spectroscopy we map out the spatially resolved density of states in Sr3Ir2O7 (Ir327). We show that its parent compound, argued to exist only as a weakly correlated band insulator, in fact possesses a substantial ~ 130 meV charge excitation gap driven by an interplay between structure, spin–orbit coupling and correlations. We find that single-atom defects are associated with a strong electronic inhomogeneity, creating an important distinction between the intrinsic and spatially averaged electronic structure. Combined with first-principles calculations, our measurements reveal how defects at specific atomic sites transfer spectral weight from higher energies to the gap energies, providing a possible route to obtaining metallic electronic states from the parent insulating states in the iridates.

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Figure 1: Topographic images of Sr3Ir2O7.
Figure 2: Tunnelling spectra across a chemical defect.
Figure 3: GGA+U band calculation along high-symmetry lines.
Figure 4: Visualization of the rotation angles of the underlying iridium oxide layer through crystal defects.
Figure 5: Spatial evolution of tunnelling spectra.

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Acknowledgements

V.M. gratefully acknowledges funding from US NSF-CAREER-0645299 for support of D.W., Y.O. and W.Z. S.D.W. acknowledges NSF DMR-1056625 for support of C.D. and S.K. Z.W. acknowledges the DOE grants: DE-FG02-99ER45747 and DOE DE-SC0002554. T.R.C. and H.T.J. are supported by the National Science Council, Taiwan. H.T.J also thanks NCHC, CINC-NTU and NCTS, Taiwan for technical support. The work at Northeastern University is supported by the US Department of Energy, Office of Science, Basic Energy Sciences contract DE-FG02-07ER46352, and benefited from Northeastern University’s Advanced Scientific Computation Center (ASCC), theory support at the Advanced Light Source, Berkeley and the allocation of supercomputer time at NERSC through DOE grant number DE-AC02-05CH11231.

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Y.O. and V.M. designed the experiments. Y.O., M.P., W.Z. and D.W. participated in the the experiments. V.M., Y.O., S.D.W., Z.W. and D.W. wrote the paper. C.D. and S.K. made single-crystal samples. Z.W., H-T.J., T-R.C., H.L. and A.B. carried out the theoretical calculations.

Corresponding author

Correspondence to Vidya Madhavan.

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The authors declare no competing financial interests.

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Okada, Y., Walkup, D., Lin, H. et al. Imaging the evolution of metallic states in a correlated iridate. Nature Mater 12, 707–713 (2013). https://doi.org/10.1038/nmat3653

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