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Dirac states with knobs on: Interplay of external parameters and the surface electronic properties of three-dimensional topological insulators

E. Frantzeskakis, N. de Jong, B. Zwartsenberg, T. V. Bay, Y. K. Huang, S. V. Ramankutty, A. Tytarenko, D. Wu, Y. Pan, S. Hollanders, M. Radovic, N. C. Plumb, N. Xu, M. Shi, C. Lupulescu, T. Arion, R. Ovsyannikov, A. Varykhalov, W. Eberhardt, A. de Visser, E. van Heumen, and M. S. Golden
Phys. Rev. B 91, 205134 – Published 26 May 2015
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  • INTRODUCTION
  • RESULTS: THE EFFECT OF ILLUMINATION
  • DISCUSSION
  • CONCLUDING REMARKS
  • METHODS
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    Topological insulators are a novel materials platform with high applications potential in fields ranging from spintronics to quantum computation. In the ongoing scientific effort to demonstrate controlled manipulation of their electronic structure by external means, i.e., the provision of knobs with which to tune properties, stoichiometric variation and surface decoration are two effective approaches that have been followed. In angle-resolved photoelectron spectroscopy (ARPES) experiments, both approaches are seen to lead to electronic band-structure changes. Most importantly, such approaches result in variations of the energy position of bulk and surface-related features and the creation of two-dimensional electron gases. The data presented here demonstrate that a third manipulation handle is accessible by utilizing the amount of super-band-gap light a topological insulator surface has been exposed to under typical ARPES experimental conditions. Our results show that this third knob acts on an equal footing with stoichiometry and surface decoration as a modifier of the electronic band structure, and that it is in continuous and direct competition with the latter. The data clearly point towards surface photovoltage and photoinduced desorption as the physical phenomena behind modifications of the electronic band structure under exposure to high-flux photons. We show that the interplay of these phenomena can minimize and even eliminate the adsorbate-related surface band bending on typical binary, ternary, and quaternary Bi-based topological insulators. Including the influence of the sample temperature, these data set up a detailed framework for the external control of the electronic band structure in topological insulator compounds in an ARPES setting. Four external knobs are available: bulk stoichiometry, surface decoration, temperature, and photon exposure. These knobs can be used in conjunction to fine tune the band energies near the surface and consequently influence the topological properties of the relevant electronic states.

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    • Received 8 April 2015
    • Revised 6 May 2015

    DOI:https://doi.org/10.1103/PhysRevB.91.205134

    ©2015 American Physical Society

    Authors & Affiliations

    E. Frantzeskakis1,*, N. de Jong1, B. Zwartsenberg1, T. V. Bay1, Y. K. Huang1, S. V. Ramankutty1, A. Tytarenko1, D. Wu1, Y. Pan1, S. Hollanders1, M. Radovic2,3, N. C. Plumb2, N. Xu2, M. Shi2, C. Lupulescu4, T. Arion5,6, R. Ovsyannikov7, A. Varykhalov7, W. Eberhardt4,6, A. de Visser1, E. van Heumen1, and M. S. Golden1,†

    • 1Van der Waals-Zeeman Institute, Institute of Physics (IoP), University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, the Netherlands
    • 2Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland
    • 3SwissFEL, Paul Scherrer Institut, CH-5232 Villigen, Switzerland
    • 4Technische Universität Berlin, Institut für Optik und Atomare Physik, Strasse des 17. Juni 136, D-10623 Berlin, Germany
    • 5Institut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
    • 6Center for Free-Electron Laser Science/DESY, Notkestrasse 85, 22607 Hamburg, Germany
    • 7Helmholtz-Zentrum Berlin für Materialien und Energie, Elektronenspeicherring BESSY II, Albert-Einstein-Strasse 15, 12489 Berlin, Germany

    • *e.frantzeskakis@uva.nl
    • †m.s.golden@uva.nl

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    Issue

    Vol. 91, Iss. 20 — 15 May 2015

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    Images

    • Figure 1
      Figure 1

      Time-dependent changes in the electronic band structure of Bi2Se3 under weak and strong illumination. (a) Time-dependent energy position of the Dirac point (left axis) and the leading edge of the VB (right axis). The photon flux is increased by a factor of 100 at the time marked by the vertical line. Time intervals refer to the time since cleavage of the crystal in UHV. The error bars in the value of binding energy are estimated to be ±5 meV for both the Dirac point and VB. The green solid lines represent single (left panel) and double (right panel) exponential functions used to determine the time constants [see Eq. (1)] for the downward band bending and the fast (slow) upward reversal thereof under high-flux illumination. (b)–(d) The near-EF electronic band structure of Bi2Se3 at different time intervals and beam exposures: (b) a freshly cleaved sample without any prior exposure to the photon beam [i.e., the first point on curve (a)], (c) I(E,k) image acquired after 11 h from an area of the same sample that received only a limited amount of illumination, (d) same location on the sample as (b) but now after exposure to both the low- and high-flux beams [i.e., last point on the curve in the right-hand panel of (a)], (e) same location on the sample as (c) after additional exposure to higher-energy photons (hν=130 eV) until saturation is observed. All data have been acquired at 16 K.

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    • Figure 2
      Figure 2

      The effect of low-flux illumination on quaternary TIs with high bulk resistivity: Bi1.5Sb0.5Te1.7Se1.3 (BSTS1.5) and Bi1.46Sb0.54Te1.7Se1.3 (BSTS1.46). (a) T-dependent resistivity of BSTS1.5, BSTS1.46, and Bi2Se3. (b) The near-EF electronic band structure of BSTS1.46 after only minimal exposure to photons and UHV exposure as indicated. (c) BSTS1.5: time-dependent energy position of the Dirac point (left axis) and the intensity maximum of the k-integrated valence band spectrum within the energy window of measurement (right axis) under weak illumination [1.3×1019/(sm2)]. Error bars in the value of binding energy are estimated to be ±10 meV for the Dirac point and ±5 meV for the valence band. The green solid line represents an exponential fit to the time dependence of the downward band bending of the Dirac point [Eq. (1)]. (d) Comparison of the energy distribution curves at k∥=0 for freshly cleaved BSTS1.5 and Bi2Se3 (both 35–40 min after cleavage), revealing the absence of a low-binding-energy peak signaling the conduction band in BSTS. The Bi2Se3 spectrum is from the data shown in Fig. 1. All ARPES data were acquired at 16 K from single crystals from the same batch as those used for the resistivity measurements.

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    • Figure 3
      Figure 3

      The effect of high-flux illumination on quaternary TIs with high bulk resistivity: Bi1.5Sb0.5Te1.7Se1.3 (BSTS1.5) and Bi1.46Sb0.54Te1.7Se1.3 (BSTS1.46). (a) BSTS1.46: time-dependent energy position of the Dirac point (open circles) under the influence of strong illumination (yellow panel) and after continuous illumination ceased (gray panel). The energy position of the Dirac point recovers only partially once the light is off, and does so on a much longer time scale than the onset of the photoinduced upward shift in the yellow panel. Green solid lines are exponential fits on the data points using Eq. (1). As described in the text, the “illuminated” region is best fitted with a double-exponential function whose constituent curves are shown by pink and blue dashed lines. A single exponential fit can accurately describe the “dark” region. Arrows in panel (a) (i.e., BSTS 1.46) denote the time intervals where the analogous data on BSTS1.5 [panels (b)–(d)] have been acquired. The error bars in the value of binding energy are between ±5 and ±15 meV. (b) BSTS1.5: near-EF electronic dispersion upon minimal photon exposure after considerable UHV exposure (i.e., 10 h after cleavage). (c) Same as (b) but after 15 min high-flux photon exposure [1.3×1021photons/(sm2)]. (d) Same as (c) but with the total high-flux exposure time now increased to 2 h. All data have been acquired at 16 K.

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    • Figure 4
      Figure 4

      The effect of high-flux illumination on Bi2Te2Se (BTS) and BiSbTeSe2 (BSTS2). (a) T dependence of the resistivity of BSTS2 [both “type 1” and “type 2” (for definitions, see text)] and BTS. (b) Time-dependent energy position of the Dirac point of type-1 BSTS2 under the influence of a high-flux photon beam, with data from the valence band edge superimposed as described in the main text. (c) BSTS2: near-EF electronic band structure for a type-1 sample at times as indicated by the arrows in panel (b). (d) Time-dependent energy position of the Dirac points of BSTS2 (type 2, blue symbols) and BTS (black symbols) under the influence of a high-flux photon beam. The gray area denotes a time period in the BTS experiment that was without high-flux photon exposure. All data have been acquired at 16 K. Resistivity and ARPES measurements were performed on the same crystal pieces.

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    • Figure 5
      Figure 5

      The effect of photon flux and excitation energy in approaching flat-band conditions The vertical axis is the minimum binding energy (EB) of the Dirac point, arrived at by infinite extrapolation of exponential fits to the time dependence on high-flux illumination. Symbols represent the EB values for different 3D TI compounds. The leftmost data points underlaid in green show the maximally band-bent starting situation before high-flux illumination. The top axis gives the photon fluxes, and the bottom axis the product of photon flux and photon energy. The orange guides to the eye show how the Dirac point binding energy decreases as the flux is increased and the purple guides to the eye illustrate further band flattening when, already at high flux, the photon energy (and hence the product flux×hν) is increased. The rightmost data point of both BSTS samples and the BSTS2 sample lie on the light blue background and, thus, from a close-to-flat-band ARPES perspective can be considered to be p type. On the other hand, for Bi2Se3 and BTS the analogous Dirac point EB remains on the gray background, making these materials n type.

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    • Figure 6
      Figure 6

      From saturated UHV-induced band bending to flat-band conditions by high-flux illumination. (Top row) Experimental I(E,k) images arrived at by exposure of different compounds to residual gases in UHV (P<2×10−10mbar) for time periods in the range of 10–48 h, during which external illumination of these specific sample locations was kept to a minimum and the downward band bending was seen to saturate (see also row 7 of Table 1). (Bottom row) Experimental I(E,k) images recorded after high-flux, high-photon-energy exposure, ensuring close to flat-band conditions. The energy of the Dirac point for each data panel in the lower row differs by less than 20 meV from last (rightmost) data point for each corresponding material plotted in Fig. 5 (see also row 3 of Table 1). Samples whose data are on a gray (light blue) background correspond to n-type (p-type) compounds and their Fermi energies are pinned within a few meV of the bottom of the conduction band (top of the valence band). The four schematic illustrations show the electronic band structure for band-bending (top row) and flat-band (bottom row) conditions, for both n-type (left) and p-type (right) compounds. Data have been acquired at temperatures between 16 and 35 K, and all I(E,k) images are plotted on the same axes and scales as Bi2Se3.

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    • Figure 7
      Figure 7

      The effect of temperature on the electronic band structure of Bi1.5Sb0.5Te1.7Se1.3 (BSTS1.5) (a) T-dependent energy position of the Dirac point and the conduction band (CB) minimum on a temperature ramp from 20 to 300 K (filled circles). After 18 h at 300 K, the data shown by the open green symbol were measured prior to recooling of the sample to 20 K on which the binding energy of both the Dirac point and the CB minimum quickly recover to the previous, low-T values (uppermost open symbols at T=20 K). Both features continue shifting to higher binding energy as time passes (in this case a further 60 min), signaled by the small green arrows. (b) Time-dependent energy position of the Dirac point (left axis) and the valence band (VB) maximum (right axis) when the cryostat temperature is quickly increased to 150 K but before full thermalization of the sample is attained. The error bars in the value of binding energy are estimated to be ±5 meV for the valence band and range between ±10 and ±15 meV for the Dirac point. (c) Near-EF electronic band structure at 300 K measured 10 min after cleavage (left), 15 h after cleavage (middle), and upon subsequent cooling over 45 min to 40 K, followed by a thermalization period (waiting time) of 10 min (right panel).

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    • Figure 8
      Figure 8

      The effect of high-flux photons on Bi1.5Sb0.5Te1.7Se1.3 (BSTS1.5) band bent due to metal adatoms and held at different temperatures. (a), (b) The electronic band structure of BSTS1.5 after the deposition of 0.1 ML Ag at 35 K. The spectra are acquired after (a) 1 min and (b) 20 min exposure to high-flux 27 eV photons at 35 K, at which temperature the images were also measured. (c) The evolution of the Dirac point binding energy for the same band-bent BSTS1.5 after exposure to high-flux photons at 35 K (blue data points) and at RT (red data points). The error bars in the value of binding energy are estimated to be ±5 meV. (d), (e) The electronic band structure of BSTS1.5 after the deposition of 0.8 ML Nb at RT. The spectra are acquired at room temperature after (c) 1 min and (d) 23 min exposure to high-flux photons at RT.

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