Observation of surface states on heavily indium-doped SnTe(111), a superconducting topological crystalline insulator

C. M. Polley, V. Jovic, T.-Y. Su, M. Saghir, D. Newby, Jr., B. J. Kowalski, R. Jakiela, A. Barcz, M. Guziewicz, T. Balasubramanian, G. Balakrishnan, J. Laverock, and K. E. Smith

Phys. Rev. B 93, 075132 – Published 18 February 2016

Abstract

The topological crystalline insulator tin telluride is known to host superconductivity when doped with indium ( ${Sn}_{1 - x} {In}_{x} Te$ ), and for low indium content ( $x = 0.04$ ) it is known that the topological surface states are preserved. Here we present the growth, characterization, and angle resolved photoemission spectroscopy analysis of samples with much heavier In doping (up to $x \approx 0.4$ ), a regime where the superconducting temperature is increased nearly fourfold. We demonstrate that despite strong $p$ -type doping, Dirac-like surface states persist.

Received 18 August 2015
Revised 25 January 2016

DOI:https://doi.org/10.1103/PhysRevB.93.075132

Physics Subject Headings (PhySH)

Electronic structure Superconductivity Topological insulators

Doped semiconductors Thin films

Angle-resolved photoemission spectroscopy Energy-dispersive x-ray spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

C. M. Polley^1,*, V. Jovic², T.-Y. Su³, M. Saghir⁴, D. Newby, Jr.³, B. J. Kowalski⁵, R. Jakiela⁵, A. Barcz⁵, M. Guziewicz⁶, T. Balasubramanian¹, G. Balakrishnan⁴, J. Laverock^3,7, and K. E. Smith^2,3

¹MAX IV Laboratory, Lund University, 221 00 Lund, Sweden
²School of Chemical Sciences and MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Auckland, Auckland 1142, New Zealand
³Department of Physics, Boston University, 590 Commonwealth Avenue, Boston, Massachusetts 02215, USA
⁴Physics Department, University of Warwick, Coventry, CV4 7AL, United Kingdom
⁵Institute of Physics, Polish Academy of Sciences, 02-668 Warszawa, Poland
⁶Institute of Electron Technology, Al. Lotnikow 32/46, 02-668 Warszawa, Poland
⁷H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, United Kingdom

^*craig.polley@maxlab.lu.se

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Issue

Vol. 93, Iss. 7 — 15 February 2016

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Images

Figure 1
(a) Low-energy (55 eV) electron diffraction pattern of a typical film. (b) Normal emission UPS spectra of the $4 d$ core levels of Sn and In for film 1 ( $h ν = 70$ eV) and film 2 ( $h ν = 94$ eV); both films exhibit a large indium signal. (c)–(e) The same core levels excited with high energy photons ( $h ν = 2$ –4 keV) for both the ${Sn}_{0.6} {In}_{0.4} Te$ source material and a third thin film preparation. The reduction of the In $4 d$ peak with increasing photon energy in the thin film is indicative of indium segregation during film growth. The valence band spectra are labeled according to the scheme of Kemeny [20], and demonstrate that the films grow as In-doped SnTe. (e) SIMS profile of film 1, confirming that indium segregates to the surface.
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Figure 2
(a) Room-temperature, normal emission photoemission spectra for the $x = 0.41$ sample as a function of photon energy. Such a measurement maps the $k_{Z}$ dispersion along the bulk $L - Γ - L$ high-symmetry direction. The experimental dispersions agree well with tight binding calculations for bulk SnTe (b), adapted with permission from Littlewood et al. [24]). (c) A schematic depiction of the relationship between the (111) surface and the bulk Brillouin zone illustrates the projection of Fermi ellipsoids at the bulk $L$ points onto $\bar{Γ}$ and $\bar{M}$ in the surface Brillouin zone.
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Figure 3
Photon energy-dependent ARPES of the hole pockets at $\bar{Γ}$ for the $x = 0.41$ sample ( $T \approx 230$ K). Raw spectra (a) and corresponding second derivative images after box smoothing along the energy direction (b). The outermost bands do not disperse with increasing photon energy, identifying them as a surface states. The innermost pair broadens and moves to higher binding energy, consistent with the bulk valence band at bulk $L$ .
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Figure 4
Quantitative dispersion mapping of the $\bar{Γ}$ hole pockets for two different indium content samples (a,b). By fitting individual momentum distribution curves, the dispersion of both the bulk and surface bands can be tracked. A linear extrapolation of the surface state bands gives a measure of the doping level and Fermi velocity for the two different samples.
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Figure 5
Room-temperature three-dimensional ARPES mapping of the $\bar{Γ}$ hole pockets in the $x = 0.23$ film at $h ν = 18$ eV. Above the Fermi level the contours resemble an isotropic two-dimensional hole gas, but at higher binding energies the surface state becomes severely warped towards $\bar{M}$ . This becomes particularly clear in second derivative images [(b) and (c)].
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Figure 6
Fermi surface maps covering several surface Brillouin zones of the $x = 0.41$ (a) and $x = 0.23$ (c) samples. The intensity of different features within the maps is highly nonuniform, but cuts along $\bar{M} - \bar{Γ} - \bar{M} - \bar{K}$ in both samples [(b) and (c)] indicate anisotropic hole pockets located at $\bar{M}$ .
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