Imaging the evolution of metallic states in a correlated iridate

• Published in: Nature materials Vol. 12; no. 8; pp. 707 - 713
• Main Authors: Okada, Yoshinori, Walkup, Daniel, Lin, Hsin, Dhital, Chetan, Chang, Tay-Rong, Khadka, Sovit, Zhou, Wenwen, Jeng, Horng-Tay, Paranjape, Mandar, Bansil, Arun, Wang, Ziqiang, Wilson, Stephen D., Madhavan, Vidya
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.08.2013; Nature Publishing Group
• Subjects: 639/301/1034/1038; 639/301/119/544; 639/301/119/995; 639/301/930/2735; Biomaterials; Charge; Condensed Matter Physics; Correlation; Defects; Doping; Electronic structure; Electronics; Energy gap; letter; Magnetism; Materials Science; Metals; Microscopy; Nanotechnology; Optical and Electronic Materials; Parents; Spectroscopy; Spectrum analysis
• ISSN: 1476-1122; 1476-4660; 1476-4660
• DOI: 10.1038/nmat3653

Iridate materials are at present the focus of interest because the combination of strong spin–orbit effects and many-body electronic correlations makes their physics non-trivial. Now, the density of states of Sr 3 Ir 2 O 7 is mapped out spatially using scanning tunnelling microscopy and spectroscopy, yielding insights into the influence of nanoscale heterogeneities on the electronic structure. The Ruddlesden–Popper series of iridates (Sr n +1 Ir n O 3 n +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 interactions 1 , 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 states 4 , 5 , 6 , 7 , 8 , 9 . Using scanning tunnelling spectroscopy we map out the spatially resolved density of states in Sr 3 Ir 2 O 7 (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.