Bulk and surface electronic states in the dosed semimetallic ${\mathrm{HfTe}}_{2}$

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Bulk and surface electronic states in the dosed semimetallic ${HfTe}_{2}$

Zakariae El Youbi, Sung Won Jung, Saumya Mukherjee, Mauro Fanciulli, Jakub Schusser, Olivier Heckmann, Christine Richter, Ján Minár, Karol Hricovini, Matthew D. Watson, and Cephise Cacho

Phys. Rev. B 101, 235431 – Published 19 June 2020

Abstract

The dosing of layered materials with alkali metals has become a commonly used strategy in ARPES experiments. However, precisely what occurs under such conditions, both structurally and electronically, has remained a matter of debate. Here we perform a systematic study of 1T- ${HfTe}_{2}$ , a prototypical semimetal of the transition metal dichalcogenide family. By utilizing photon energy-dependent angle-resolved photoemission spectroscopy (ARPES), we have investigated the electronic structure of this material as a function of potassium (K) deposition. From the $k_{z}$ maps, we observe the appearance of 2D dispersive bands after electron dosing, with an increasing sharpness of the bands, consistent with the wave-function confinement at the topmost layer. In our highest-dosing cases, a monolayerlike electronic structure emerges, presumably as a result of intercalation of the alkali metal. Here, by bringing the topmost valence band below $E_{F}$ , we can directly measure a band overlap of $\sim 0.2$ eV. However, 3D bulklike states still contribute to the spectra even after considerable dosing. Our work provides a reference point for the increasingly popular studies of the alkali metal dosing of semimetals using ARPES.

Received 3 April 2020
Revised 15 May 2020
Accepted 1 June 2020

DOI:https://doi.org/10.1103/PhysRevB.101.235431

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Electronic structure Fermi surface Spin-orbit coupling Surface states Topological materials

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Zakariae El Youbi ^1,2, Sung Won Jung¹, Saumya Mukherjee ^1,3, Mauro Fanciulli ^2,4, Jakub Schusser ^2,5, Olivier Heckmann^2,4, Christine Richter ^2,4, Ján Minár⁵, Karol Hricovini ^2,4, Matthew D. Watson ¹, and Cephise Cacho^1,*

¹Diamond Light Source, Harwell Campus, Didcot, OX11 0DE, United Kingdom
²Laboratoire de Physique des Matériaux et des Surfaces, CY Cergy-Paris Université, 5 mail Gay-Lussac, 95031 Cergy-Pontoise, France
³Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom
⁴Université Paris-Saclay, CEA, CNRS, LIDYL, 91191, Gif-sur-Yvette, France
⁵New Technologies-Research Center, University of West Bohemia, 30614 Pilsen, Czech Republic

^*cephise.cacho@diamond.ac.uk

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Vol. 101, Iss. 23 — 15 June 2020

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Figure 1
Electronic structure of 1T- ${HfTe}_{2}$ . (a) DFT calculations along M- $Γ$ -A-L direction with orbital character projection of the valence and conduction bands. (b) 3D Fermi surface of ${HfTe}_{2}$ calculated with a DFT code using the mBJ functional. (c) Constant energy map at $E_{F}$ (Fermi surface), measured at a photon energy $h ν = 100$ eV to probe the $A - H - L$ plane. The threefold in-plane intensity distribution of the electron pockets located at the $L$ points reflect the trigonal symmetry of the crystal structure. [(d), (f), and (h)] Valence band dispersion along $Γ - M$ ( $h ν = 80$ eV) (d), an arbitrary $k_{z}$ between $Γ$ and $A$ points ( $h ν = 87$ eV) (f) and $A - L$ direction ( $h ν = 100$ eV) (h). [(e), (g), and (i)] Simulations based on DFT calculations, averaging over a substantial fraction of the Brillouin zone width in the $k_{z}$ direction, as further described in the main text. (e), (g), and (i) correspond to an average $k_{z}$ value of $k_{z} = 0.0 π / c$ , $0.4 π$ /c, and $π / c$ , respectively.
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Figure 2
Dosing sequence of ${HfTe}_{2}$ at $T = 10$ K, measured at a photon energy of $h ν = 100$ eV. (a) Valence band dispersion along $A - L$ direction of the pristine surface, (b) at a carrier density of $n_{e} = 0.11$ and (c) at $n_{e} = 0.32$ el/u.c., together with ${HfTe}_{2}$ monolayer DFT calculations (white solid lines), shifted downwards by 650 meV. Due to the overestimated band overlap in DFT, the energy of the electron pocket is lower in the calculation than the data, however, the agreement on the valence band structure is more important here. (d) Energy distribution curves (EDCs) showing the evolution as a function of the charge carrier density. The labeled features correspond to the electron pockets at the $L$ point ( $p_{1}$ ), and the remnant bulklike bands ( $p_{3}$ ). (e) Equivalent EDCs at A, highlighting a peak from the inner valence band ( $p_{2}$ ). (f) Core level spectroscopy of Te $4 d$ as a function of charge carrier density. The spectra were fitted using two pairs of Voigt functions, depicted as blue (bulk) and red (surface) shaded areas. The SOC peak separation and areal ratio of Te $4 d_{3 / 2}$ to Te $4 d_{5 / 2}$ , were fixed to the values obtained from the pristine surface ( $n_{e} = 0.03$ ). (g) Comparison of the energy shift of shallow core levels of Te $4 d$ (blue and red circles), the inner Te $5 p$ -derived state of the valence band ( $p_{2}$ , filled red triangles) and the Hf $5 d$ -derived electron pockets ( $p_{1}$ , empty red triangles). The data reveal a similar trend of the $p_{2}$ valence band state (Te $5 p$ ) and Te $4 d$ shallow core levels.
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Figure 3
Photon energy-dependent ARPES. [(a) and (b)] $k_{z}$ maps, processed from photon-energy dependent data at $E_{F}$ before and after Potassium dosing on ${HfTe}_{2}$ surface along $M - Γ - M$ ( $L - A - L$ ) direction, providing clear evidence of the emergence of surface-confined states and the remnant bulklike contribution due to the non-negligible chance of the deep photoelectrons to escape. Photon energies into $k_{z}$ conversion was done by employing the standard free-electron final state approximation: $k_{z} = \frac{1}{ℏ} \sqrt{2 m_{e} (V_{0} + E_{k} {cos}^{2} θ)}$ , where $θ$ is the in-plane emission angle, $E_{k}$ is the photo-electron kinetic energy and $V_{0}$ is the inner potential. We use an inner potential of $11 eV$ . [(c) and (d)] Valence band dispersion at a photon energy $h ν = 87$ eV of the pristine and dosed surface, showing unambiguously the asymmetry disappearance and sharpness of the bands due the monolayerlike electronic structure after dosing. The observed features inside the electron pockets and the center are a clear evidence of the remnant bulk states of deeper photoelectrons in the material. [(e) and (f)] Band dispersion along the out-of-plane $k_{z}$ direction at the Brillouin zone center ( $Γ - A$ direction) of the pristine surface, highlighting the expected crossings of the Te $p_{z}$ orbital and Te $p_{x y}$ orbital-derived states, giving rise to a bulk Dirac point (BDP) (upper crossing) and an inverted bandgap (IBG) (lower crossing), as labeled in our DFT calculations.
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Physical Review B

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Bulk and surface electronic states in the dosed semimetallic ${HfTe}_{2}$

Zakariae El Youbi, Sung Won Jung, Saumya Mukherjee, Mauro Fanciulli, Jakub Schusser, Olivier Heckmann, Christine Richter, Ján Minár, Karol Hricovini, Matthew D. Watson, and Cephise Cacho

Phys. Rev. B 101, 235431 – Published 19 June 2020

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Physical Review B

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Bulk and surface electronic states in the dosed semimetallic HfTe2

Zakariae El Youbi, Sung Won Jung, Saumya Mukherjee, Mauro Fanciulli, Jakub Schusser, Olivier Heckmann, Christine Richter, Ján Minár, Karol Hricovini, Matthew D. Watson, and Cephise Cacho

Phys. Rev. B 101, 235431 – Published 19 June 2020

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Bulk and surface electronic states in the dosed semimetallic ${HfTe}_{2}$