We report high-resolution angle-resolved photoemission spectroscopy measurements on the honeycomb iridate Na2IrO3. Our measurements reveal the existence of a metallic surface band feature crossing the Fermi level with nearly linear dispersion and an estimated surface carrier density of 3.2 x 1013 cm-2, which has not been theoretically predicted or experimentally observed, and provides the first evidence for metallic behavior on the boundary of this material, whereas the bulk bands exhibit a robust insulating gap. We further show the lack of theoretically predicted Dirac cones at the M¯ points of the surface Brillouin zone, which confirms the absence of a stacked quantum spin Hall phase in this material. Our data indicates that the surface ground state of this material is exotic and metallic, unlike as predicted in theory, and establishes Na2IrO3 as a rare example of a strongly correlated spin-orbit insulator with surface metallicity.

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Published in Physical Review B, v. 93, issue 24, 245132, p. 1-6.

©2016 American Physical Society

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Work at Princeton University is supported by the US National Science Foundation Grant No. NSF-DMR-1006492. M.Z.H. acknowledges visiting scientist support from Lawrence Berkeley National Laboratory and additional partial support from the A. P. Sloan Foundation and NSF-DMR-0819860. G.C. acknowledges NSF support via Grant No. DMR 1265162. The photoemission measurements using synchrotron x-ray facilities are supported by the Basic Energy Sciences of the US Department of Energy under DE-FG-02-05ER46200.

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See Supplemental Material at http://link.aps.org/supplemental/10.1103/PhysRevB.93.245132 for analysis of the low-energy electron diffraction data, comparison to the bulk energy distribution curves previously reported, Lorentzians used to fit to the momentum distribution curves of the metallic in-gap states, and the linear fits to the peak positions of these Lorentzians.

SM_Na2IrO3.pdf (682 kB)
Supplemental Materials: Supplemental Figures S1-S4.