Plumbene on a Magnetic Substrate: A Combined Scanning Tunneling Microscopy and Density Functional Theory Study

Gustav Bihlmayer, Jonas Sassmannshausen, André Kubetzka, Stefan Blügel, Kirsten von Bergmann, and Roland Wiesendanger

Phys. Rev. Lett. 124, 126401 – Published 24 March 2020

Abstract

As a heavy analog of graphene, plumbene is a two-dimensional material with strong spin-orbit coupling effects. Using scanning tunneling microscopy, we observe that Pb forms a flat honeycomb lattice on an Fe monolayer on Ir(111). In contrast, without the Fe layer, a $c (2 \times 4)$ structure of Pb on Ir(111) is found. We use density-functional theory calculations to rationalize these findings and analyze the impact of the hybridization on the plumbene band structure. In the unoccupied states the splitting of the Dirac cone by spin-orbit interaction is clearly observed, while the occupied Pb states are strongly hybridized with the substrate. In a freestanding plumbene we find a band inversion below the Fermi level that leads to the formation of a topologically nontrivial gap. Exchange splitting as mediated by the strong hybridization with the Fe layer drives a quantum spin Hall to quantum anomalous Hall state transition.

Received 5 September 2019
Accepted 11 February 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.126401

Physics Subject Headings (PhySH)

Edge states Spin-orbit coupling Topological materials

Chern insulators Honeycomb lattice Topological insulators

Density functional calculations Spin-polarized scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Gustav Bihlmayer ^1,*, Jonas Sassmannshausen², André Kubetzka², Stefan Blügel¹, Kirsten von Bergmann ^2,†, and Roland Wiesendanger²

¹Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA, 52425 Jülich, Germany
²Department of Physics, University of Hamburg, D-20355 Hamburg, Germany

^*g.bihlmayer@fz-juelich.de
^†kbergman@physnet.uni-hamburg.de

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Vol. 124, Iss. 12 — 27 March 2020

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Images

Figure 1
Pseudo three-dimensional STM image of a sample of Pb on Fe on Ir(111); the different exposed surfaces are labeled. Pb was deposited well below room temperature and grows as monolayer high ordered islands both on the bare Ir surface and on the Fe monolayer on Ir. The superstructure on the pseudomorphic Fe monolayer on Ir is not due to the structure but corresponds to the magnetic signal of the nanoskyrmion lattice. Measurement parameters are $U = + 5 mV$ , $I = 3 nA$ , $T = 4 K$ , Cr-bulk tip.
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Figure 2
Closer view constant-current STM images of the Pb monolayer on (a) the bare Ir(111) surface and (b) the Fe monolayer on Ir(111). Both the symmetry and the atomic distances for the two differently ordered Pb monolayers are different and the corresponding structure models are presented in (c) and (d). The white dashed rectangle in (c) refers to the primitive unit cell of the $c (2 \times 4)$ structure and the white dashed diamond in (d) marks the $p (2 \times 2)$ unit cell; blue dashed circles mark positions that are empty for a lower atom-density configuration and occupied for a higher atom-density configuration (see text). Panels (e) and (f) show the height profiles along the lines indicated in (a) and (b), respectively. Measurement parameters are (a) $U = + 10 mV$ , $I = 1.5 nA$ , (b) $U = + 5 mV$ , $I = 2 nA$ , both (a) and (b) $T = 4 K$ , Cr-bulk tip. Information on the bias dependence is provided in the Supplemental Material [21], Fig. S1.
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Figure 3
(a) Orbitally resolved DOS of the Pb atoms in the honeycomb structure on Fe/Ir(111). The total Pb DOS is shown as gray background, and the $s$ , $p_{x}$ , $p_{y}$ , and $p_{z}$ contributions are shown in black, blue, green, and red, respectively. Positive and negative values correspond to majority and minority spins, respectively. (b) The same quantities for the Pb honeycomb lattice on Ir(111).
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Figure 4
Red and blue: Sign of the spin polarization of the Pb states of a Pb honeycomb structure on Fe/Ir(111). Spin-orbit coupling is included and the spin-quantization axis is assumed to be normal to the surface. In black the band structure of a freestanding plumbene layer with the same structure as the supported one is shown.
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Figure 5
(a) Band structure of a freestanding, edge-hydrogenated plumbene ribbon in an external magnetic field. The size of the dots marks the weight of the states on one of the zigzag edges, the color indicates the spin character. (b) Constant-current STM image of the typical zigzag configuration of the edge of a plumbene island on Fe/Ir; measurement parameters are $U = - 10 mV$ , $I = 2 nA$ , $T = 4 K$ , Cr-bulk tip.
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