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Gap-like feature observed in the non-magnetic topological insulators

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Journal of Physics: Condensed Matter
Authors:
Turgut Yilmaz at Xiamen University Malaysia
Turgut Yilmaz
Anna Pertsova at Nordic Institute for Theoretical Physics
Anna Pertsova
W Hines
W Hines
W Hines
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Elio Vescovo at Brookhaven National Laboratory
Elio Vescovo
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Abstract and Figures

Non-magnetic gap at the Dirac point of topological insulators remains an open question in the field. Here, we present angle-resolved photoemission spectroscopy experiments performed on Cr-doped Bi2Se3 and showed that the Dirac point is progressively buried by the bulk bands and a low spectral weight region in the vicinity of the Dirac point appears. These two mechanisms lead to spectral weight suppression region being mistakenly identified as an energy gap in earlier studies. We further calculated the band structure and found that the original Dirac point splits into two nodes due to the impurity resonant states and the energy separation between the nodes is the low density of state region which appears to be like an energy gap in potoemission experiments. We supported our arguments by presenting photoemission experiments carried out with on- and off- resonant photon energies. Our observation resolves the widely debated questions of apparent energy gap opening at the Dirac point without long range ferromagnetic order in topological insulators.
ARPES maps of Bi1.95Cr0.05Se3 films with different thicknesses grown on the 12 QL Bi2Se3 films. Thicknesses of the Bi1.95Cr0.05Se3 films are given above the images. Green and the red lines represent bulk states of E1–E3 and the gap feature, respectively. ARPES maps were collected at room temperature with a He I-α (21.2 eV).
ARPES maps of Bi1.95Cr0.05Se3 films with different thicknesses grown on the 12 QL Bi2Se3 films. Thicknesses of the Bi1.95Cr0.05Se3 films are given above the images. Green and the red lines represent bulk states of E1–E3 and the gap feature, respectively. ARPES maps were collected at room temperature with a He I-α (21.2 eV).
… 
(a) EDC for different thicknesses of Bi1.95Cr0.05Se3 on pristine Bi2Se3 along the k∥=0 A⁻¹ in figure 1. Dashed gray rectangular area represents the gap region. pink, black, and blue lines represent the DP, E1, and E2, respectively. Orange lines are the bulk valance band (BVB) high intensity points. Low intensity point of the gap feature is indicated with cyan line. Full set of ARPES maps can be found in supplementary figure 3. (b) Dispersion of the bulk and the surface states as a function of the thickness.
(a) EDC for different thicknesses of Bi1.95Cr0.05Se3 on pristine Bi2Se3 along the k∥=0 A⁻¹ in figure 1. Dashed gray rectangular area represents the gap region. pink, black, and blue lines represent the DP, E1, and E2, respectively. Orange lines are the bulk valance band (BVB) high intensity points. Low intensity point of the gap feature is indicated with cyan line. Full set of ARPES maps can be found in supplementary figure 3. (b) Dispersion of the bulk and the surface states as a function of the thickness.
… 
(a)–(c) EDC for the 3-6-8 QL Bi1.95Cr0.05Se3 films with fitting lines. Black arrows show the low-intensity points within the gap feature. Pink and black fitting lines are the DP and E1, respectively. Blue and green fitting lines are the bulk bands and gray fitting line represents the lower surface state of the gapped Dirac cone (see figure 4).
(a)–(c) EDC for the 3-6-8 QL Bi1.95Cr0.05Se3 films with fitting lines. Black arrows show the low-intensity points within the gap feature. Pink and black fitting lines are the DP and E1, respectively. Blue and green fitting lines are the bulk bands and gray fitting line represents the lower surface state of the gapped Dirac cone (see figure 4).
… 
Calculated band structures of a Bi2Se3 thin film: (a) pristine structure, (b) potential impurity with U  =  6 eV and M  =  0. Horizontal dashed line in (a) marks the position of the Dirac point. In panel (b), only the top Dirac states are shown since the bottom surface states are unaffected by the dopants. Horizontal dashed lines in (b) mark the energy region where the gap feature appears. The vertical red arrow in (b) demonstrates the gap feature.
Calculated band structures of a Bi2Se3 thin film: (a) pristine structure, (b) potential impurity with U  =  6 eV and M  =  0. Horizontal dashed line in (a) marks the position of the Dirac point. In panel (b), only the top Dirac states are shown since the bottom surface states are unaffected by the dopants. Horizontal dashed lines in (b) mark the energy region where the gap feature appears. The vertical red arrow in (b) demonstrates the gap feature.
… 
(a)–(c) ARPES spectra of 3 QL Bi1.78Cr0.22Se3 on a 12 QL Bi2Se3 taken with 30 eV, 45 eV, and 60 eV photons. (d) Corresponding EDCs taken along the k∥=0 A⁻¹. (e) Integrated EDCs within k∥=∓0.45 A⁻¹. Dashed black and blue lines are to guide eyes.
(a)–(c) ARPES spectra of 3 QL Bi1.78Cr0.22Se3 on a 12 QL Bi2Se3 taken with 30 eV, 45 eV, and 60 eV photons. (d) Corresponding EDCs taken along the k∥=0 A⁻¹. (e) Integrated EDCs within k∥=∓0.45 A⁻¹. Dashed black and blue lines are to guide eyes.
… 
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In topological insulators (TIs), the surface states are protected
against non-magnetic perturbations by time reversal sym-
metry (TRS) [1, 2]. These surface states have helical spin tex-
ture and disperse linearly with respect to energy forming the
well know Dirac point (DP). Majority of the work in the eld
concerns the breaking of TRS by a net out-of-plane magn-
etic moment to open an energy gap at the DP to drive a TI
into quantum anomalous Hall state which can be achieved by
doping magnetic impurities into the bulk [3–8]. Ambiguously,
many angle resolved photoemission spectroscopy (ARPES)
experiments reported the existence of a large energy gap
(>100 meV) in Mn-, V-, and Cr-doped Bi2Se3 far above the
ferromagnetic transition temperature (
Tc
) [9–11]. This violates
the fundamental theory of TIs which asserts the robustness of
the gapless Dirac cone in the non-magnetic state. In theory,
impurity resonant states were proposed to be responsible for
such modication of the local electronic structure and the
non-magnetic gap at the DP [12–21]. At present, however, the
debate about exact nature of the non-magnetic gap continues
due to the lack of photoemission experiments addressing the
appearance of an energy gap in photoemission experiments
without ferromagnetic order, which is inconsistency is still
awaiting to be experimentally resolved.
Here, we present a schematic ARPES and tight binding
model calculation in Cr-doped Bi2Se3 with varied thickness
and show that the gap is not due to the magnetization but it
is derived from the impurity resonant states which splits the
original DP into the two nodes apart from each other by >100
meV. Lifetime broadening due to the low crystal quality
induced by Cr-incorporation into the bulk leads nodes and the
surface states to be buried by the bulk bands. This give rise to
misinterpreting the energy separation between the two nodes
Journal of Physics: Condensed Matter
Gap-like feature observed in the
non-magnetic topological insulators
TYilmaz1,2, APertsova3, WHines1, EVescovo2, KKaznatcheev2,
AVBalatsky1,3 and BSinkovic1
1 Department of Physics, University of Connecticut, Storrs, CT 06269, United States of America
2 Brookhaven National Laboratory, National Synchrotron Light Source-II, Upton, NY 11973,
United States of America
3 Nordita, Roslagstullsbacken 23, SE-106 91 Stockholm, Sweden
E-mail: tyilmaz@bnl.gov
Received 17 September 2019, revised 1 December 2019
Accepted for publication 18 December 2019
Published 8 January 2020
Abstract
Non-magnetic gap at the Dirac point of topological insulators remains an open question in the
eld. Here, we present angle-resolved photoemission spectroscopy experiments performed on
Cr-doped Bi2Se3 and showed that the Dirac point is progressively buried by the bulk bands and
a low spectral weight region in the vicinity of the Dirac point appears. These two mechanisms
lead to spectral weight suppression region being mistakenly identied as an energy gap in
earlier studies. We further calculated the band structure and found that the original Dirac point
splits into two nodes due to the impurity resonant states and the energy separation between the
nodes is the low density of state region which appears to be like an energy gap in potoemission
experiments. We supported our arguments by presenting photoemission experiments carried
out with on- and off- resonant photon energies. Our observation resolves the widely debated
questions of apparent energy gap opening at the Dirac point without long range ferromagnetic
order in topological insulators.
Keywords: topological insulators, time reversal symmetry, ARPES
S
Supplementary material for this article is available online
(Some guresmay appear in colour only in the online journal)
T Yilmaz etal
Gap-like feature observed in the non-magnetic topological insulators
Printed in the UK
145503
JCOMEL
© 2020 IOP Publishing Ltd
32
J. Phys.: Condens. Matter
CM
10.1088/1361-648X/ab6349
Paper
14
Journal of Physics: Condensed Matter
IOP
2020
1361-648X
1361-648X/ 20 /145503+7$33.00
https://doi.org/10.1088/1361-648X/ab6349
J. Phys.: Condens. Matter 32 (2020) 145503 (7pp)
... Although it can be problematic to achieve a regular lattice of impurities on a TI surface, there is strong evidence of impurity-induced states in typical doped TI samples [26]. Moreover, recent angle-resolved photoemission spectroscopy studies suggested that overlapping of impurity resonances with the Dirac node, a situation depicted in Fig. 1(b), could be achieved in the experiment as the binding energy of the node changes with increasing the film thickness [27]. It is also possible to resolve, at least partially, the impurity bands by adjusting the photon energy of the pulse. ...
... The location of the impurity resonances varies greatly with the type of material and dopant [26]. Recent experiments indicated that favorable conditions can be achieved for at least some samples and dopants, e.g., for Cr in Bi 2 Se 3 above the magnetic ordering temperature [27]. ...
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