Observation of metallic surface states in the strongly correlated Kitaev-Heisenberg candidate ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$

Observation of metallic surface states in the strongly correlated Kitaev-Heisenberg candidate ${Na}_{2} {IrO}_{3}$

Nasser Alidoust, Chang Liu, Su-Yang Xu, Ilya Belopolski, Tongfei Qi, Minggang Zeng, Daniel S. Sanchez, Hao Zheng, Guang Bian, Madhab Neupane, Yu-Tzu Liu, Stephen D. Wilson, Hsin Lin, Arun Bansil, Gang Cao, and M. Zahid Hasan

Phys. Rev. B 93, 245132 – Published 14 June 2016

Abstract

We report high-resolution angle-resolved photoemission spectroscopy measurements on the honeycomb iridate ${Na}_{2} {IrO}_{3}$ . 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 $3.2 \times 10^{13} {cm}^{- 2}$ , 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 $\bar{M}$ 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 ${Na}_{2} {IrO}_{3}$ as a rare example of a strongly correlated spin-orbit insulator with surface metallicity.

Received 23 October 2014
Revised 20 May 2016

DOI:https://doi.org/10.1103/PhysRevB.93.245132

Physics Subject Headings (PhySH)

Honeycomb lattice Iridates

Angle-resolved photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Nasser Alidoust^1,*, Chang Liu^1,2, Su-Yang Xu¹, Ilya Belopolski¹, Tongfei Qi³, Minggang Zeng^4,5, Daniel S. Sanchez¹, Hao Zheng¹, Guang Bian¹, Madhab Neupane^1,6, Yu-Tzu Liu^4,5, Stephen D. Wilson⁷, Hsin Lin^4,5, Arun Bansil⁸, Gang Cao³, and M. Zahid Hasan^1,†

¹Laboratory for Topological Quantum Matter and Spectroscopy (B7), Department of Physics, Princeton University, Princeton, New Jersey 08544, USA
²Department of Physics, South University of Science and Technology of China, No 1088, Xueyuan Rd., Xili, Nanshan District, Shenzhen, Guangdong, China 518055
³Center for Advanced Materials, Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506, USA
⁴Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546
⁵Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542
⁶Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
⁷Materials Department, University of California, Santa Barbara, California 93106, USA
⁸Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA

^*Corresponding author: nassera@princeton.edu
^†Corresponding author: mzhasan@princeton.edu

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Vol. 93, Iss. 24 — 15 June 2016

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Figure 1
(a) Side and (b) top views of the crystal structure and (c) the corresponding Brillouin zone (BZ) of magnetic ${Na}_{2} {IrO}_{3}$ with the zigzag-AFM spin configuration. The arrows indicate magnetic moments of Ir atoms. (d)–(f) Same as (a)–(c) for nonmagnetic ${Na}_{2} {IrO}_{3}$ . The projection of the bulk BZ on the (001) surface is indicated by the red and green rectangles in (c) and (f), respectively. (g) Relationship between the (001) surface BZ of the nonmagnetic (green) and zigzag-AFM magnetic (red) ${Na}_{2} {IrO}_{3}$ . (h) Measured low-energy electron diffraction (LEED) patterns of the crystals used in our studies at various electron energies as marked on each panel.
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Figure 2
(a) Core level spectra of ${Na}_{2} {IrO}_{3}$ . (b) ARPES electronic band structure of ${Na}_{2} {IrO}_{3}$ along the high symmetry direction $\bar{Γ} − \bar{K}$ of the BZ. (c) Energy distribution curves (EDCs) of the $k − E$ intensity profile in (b). The dashed line indicates the position of $E_{F}$ . (d) Constant binding energy cuts at various binding energies between $E_{F}$ and $E_{B} = 3 eV$ . The corresponding binding energies are indicated at the top of each panel. The high symmetry directions of the BZ, $\bar{Γ} − \bar{K}$ and $\bar{Γ} − \bar{M}$ , are marked in the last panel. (e) Replica of (d) with dashed guide-to-the-eye lines drawn at the place of each Fermi surface pocket to emphasize the sixfold symmetry of the BZ. Green lines mark the pockets formed by the intensities crossing the $E_{F}$ , blue lines the pockets formed by the lower binding energy bulk valence bands, and red lines the pockets formed by the higher binding energy bulk valence bands. These measurements were conducted at $T$ = 300 K with $h ν = 90 eV$ .
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Figure 3
ARPES electronic band structure of ${Na}_{2} {IrO}_{3}$ along the high symmetry directions of (a) $\bar{Γ} − \bar{K}$ and (b) $\bar{Γ} − \bar{M}$ of the BZ, respectively. The momentum distribution curves (MDCs) at the Fermi level are plotted on top of each panel in purple. (c) MDCs (red) and heuristic fits to the MDCs (blue) of the metallic in-gap states around the $\bar{Γ}$ point of the Brillouin zone using two Lorentzians. (d) Zoomed-in version of the metallic in-gap states and (e) their corresponding MDCs. The bold curve in (e) indicates the MDC at $E_{F}$ , and the purple dots in (d) show the peak positions of the Lorentzians used to fit to the MDCs of these metallic in-gap states. The horizontal black dashed lines in (a), (b), and (d) denote the position of $E_{F}$ . (f) Comparison between the EDCs of polycrystalline gold and that of the ${Na}_{2} {IrO}_{3}$ samples in our study around the $\bar{Γ}$ point of the BZ. Note that the numerical values on the color bars may not be compared across figures, since they are in arbitrary units specific to a certain color plot.
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Figure 4
(a) Photon energy $h ν$ ( $\propto k_{z}$ ) dependence measurements of the observed metallic linearlike in-gap electronic states and (b) the corresponding MDCs in the region indicated by the purple rectangular dashed box in the first panel of (a). The horizontal black dashed lines in (a) show the position of $E_{F}$ .
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Physical Review B

covering condensed matter and materials physics

Observation of metallic surface states in the strongly correlated Kitaev-Heisenberg candidate ${Na}_{2} {IrO}_{3}$

Nasser Alidoust, Chang Liu, Su-Yang Xu, Ilya Belopolski, Tongfei Qi, Minggang Zeng, Daniel S. Sanchez, Hao Zheng, Guang Bian, Madhab Neupane, Yu-Tzu Liu, Stephen D. Wilson, Hsin Lin, Arun Bansil, Gang Cao, and M. Zahid Hasan

Phys. Rev. B 93, 245132 – Published 14 June 2016

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Physical Review B

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Observation of metallic surface states in the strongly correlated Kitaev-Heisenberg candidate Na2IrO3

Nasser Alidoust, Chang Liu, Su-Yang Xu, Ilya Belopolski, Tongfei Qi, Minggang Zeng, Daniel S. Sanchez, Hao Zheng, Guang Bian, Madhab Neupane, Yu-Tzu Liu, Stephen D. Wilson, Hsin Lin, Arun Bansil, Gang Cao, and M. Zahid Hasan

Phys. Rev. B 93, 245132 – Published 14 June 2016

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Observation of metallic surface states in the strongly correlated Kitaev-Heisenberg candidate ${Na}_{2} {IrO}_{3}$