Letterpubs.acs.org/NanoLett

Large-Gap Magnetic Topological Heterostructure Formed by
Subsurface Incorporation of a Ferromagnetic Layer
Toru Hirahara,*,† Sergey V. Eremeev,‡,¶,§,∥ Tetsuroh Shirasawa,⊥ Yuma Okuyama,† Takayuki Kubo,#
Ryosuke Nakanishi,# Ryota Akiyama,# Akari Takayama,# Tetsuya Hajiri,○ Shin-ichiro Ideta,○
Masaharu Matsunami,○ Kazuki Sumida,△ Koji Miyamoto,∇ Yasumasa Takagi,◆ Kiyohisa Tanaka,○
Taichi Okuda,∇ Toshihiko Yokoyama,◆ Shin-ichi Kimura,○ Shuji Hasegawa,#
and Evgueni V. Chulkov∥,▲,□,¶,§†

Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
Institute of Strength Physics and Materials Science, Tomsk 634055, Russia¶
Tomsk State University, Tomsk 634050, Russia§
Saint Petersburg State University, Saint Petersburg 198504, Russia∥
Donostia International Physics Center (DIPC), Paseo de Manuel Lardizabal, 4, 20018 San Sebastián/Donostia, Basque Country,
Spain⊥
Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan#
Department of Physics, University of Tokyo, Tokyo 113-0033, Japan○UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan△
Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan∇
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-8526, Japan◆
Department of Materials Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan▲
Departamento de Física de Materiales, Facultad de Ciencias Químicas, UPV/EHU, Apdo. 1072, 20080 San Sebastián, BasqueCountry, Spain□
Centro de Física de Materiales, CFM-MPC, Centro Mixto CSIC-UPV/EHU, Apdo.1072, 20080 San Sebastián/Donostia, BasqueCountry, Spain‡

S Supporting Information*

ABSTRACT: Inducing magnetism into topological insulators
is intriguing for utilizing exotic phenomena such as thequantum anomalous Hall eﬀect (QAHE) for technologicalapplications. While most studies have focused on doping
magnetic impurities to open a gap at the surface-state Diracpoint, many undesirable eﬀects have been reported to appear
in some cases that makes it diﬃcult to determine whether thegap opening is due to the time-reversal symmetry breaking or
not. Furthermore, the realization of the QAHE has been
limited to low temperatures. Here we have succeeded ingenerating a massive Dirac cone in a MnBi2Se4/Bi2Se3 heterostructure, which was fabricated by self-assembling a MnBi2Se4 layer
on top of the Bi2Se3 surface as a result of the codeposition of Mn and Se. Our experimental results, supported by relativistic ab
initio calculations, demonstrate that the fabricated MnBi2Se4/Bi2Se3 heterostructure shows ferromagnetism up to room
temperature and a clear Dirac cone gap opening of ∼100 meV without any other signiﬁcant changes in the rest of the band
structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than amagnetic proximity eﬀect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum,
characterized by a nontrivial Chern number C = −1, has a potential to realize the QAHE at signiﬁcantly higher temperatures thanreported up to now and can serve as a platform for developing future “topotronics” devices.
KEYWORDS: Topological insulators, magnetism, massive Dirac cone, quantum anomalous Hall eﬀect

Classiﬁcation of materials and phases based on the“topological properties” of the system has become oneof the most extensively studied research ﬁelds in physics and© 2017 American Chemical Society

Received: February 10, 2017
Revised: May 11, 2017
Published: May 26, 2017
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Figure 1. (a) Band dispersion of the substrate Bi2Se3 ﬁlm measured along the Γ̅ −M̅ direction taken at hν = 21 eV. The red line shows the energy
distribution curve (EDC) at the Γ̅ point. (b) Band dispersion of the heterostructure for sample A measured along the Γ̅ −M̅ direction taken at hν =21 eV. (c) The second derivative with respect to the energy of the band dispersion in (b). (d) Raw spectra (EDCs) near the Γ̅ point of the band
dispersion in (b). The gap size of the Dirac cone is 85 meV. (e) Band dispersion of the heterostructure for sample A measured along the Γ̅ −M̅direction taken at hν = 11 eV. The gap size of the Dirac cone is 80 meV. The red line shows the EDC at the Γ̅ point. (f) Band dispersion of theheterostructure for sample B measured along the Γ̅ −M̅ direction taken at hν = 50 eV. The gap size of the Dirac cone is 110 meV. The red line shows
the EDC at the Γ̅ point. All the measurements were performed at 30 K.

was the topic for the Nobel prize in physics in 2016.1
Topological insulators (TIs) are insulating materials that havemetallic surface states, whose electron spins are locked to their
momentum.2,3 In the simplest case, the surface states are helical
Dirac Fermions and the Dirac point is robust due to theprotection by time-reversal symmetry. When the time-reversal
symmetry is broken in TIs, novel quantum phenomena havebeen predicted to occur including the formation of magnetic
monopoles,4 the quantum anomalous Hall eﬀect (QAHE),5and the topological magnetoelectric eﬀect.6
In terms of the electronic structure, a magnetic TI is expected
to host a massive Dirac cone with a band gap. Many researches
have been performed to induce magnetism in TIs by magnetic
impurity doping when growing thin ﬁlms.7,8 Although it
seemed that this was the most eﬃcient way with the successful
observation of the QAHE in Cr- or V-doped (Bi,Sb)2Te3 thinﬁlms,8−10 the precise quantization of the Hall resistance at zeroﬁeld is still only limited to low temperature (10−100 mK). The
temperature for the realization of the QAHE (TQAHE) depends
on the Curie temperature as well as the size of the Dirac conegap of the system. Up to now, the main obstacle in enhancing
TQAHE is an inhomogeneous distribution of magnetic atoms
over the TI ﬁlm that results in strong ﬂuctuations of themagnetic energy gap.11,12 Modulation doping was shown toincrease TQAHE and the QAHE was observed at 2 K for Crdoped (Bi,Sb)2Te3 thin ﬁlms when the magnetic-doped layer
was placed 1 nm below the surface.13 A recent theoretical worksuggests that this can be enhanced by V and I codoping of
Sb2Te3.14
Another drawback of the magnetically doped systems is that
there has been no direct observation of the massive Dirac cone
for the samples that show the QAHE using angle-resolved
photoemission spectroscopy (ARPES).15 With local probes
such as scanning tunneling spectroscopy (STS), an inhomogeneous Dirac cone gap was observed for Cr-doped(Bi,Sb)2Te3.11 However, for V-doped Sb2Te3, it was reported
that impurity states will reside within the Dirac cone gap, and as
a result, the density of states will look gapless.16 Moreover,
impurity bands will emerge near the Dirac point, and it wasreported that a massive Dirac cone can arise even due to anonmagnetic origin for impurity-doped Bi2Se3 ﬁlms with
ARPES.17 Thus, one can say that magnetic impurity doped
TI ﬁlms are not promising in terms of (i) enhancing TQAHE to
the region where application becomes possible and (ii) clearlyobserving the gapped Dirac cone as a result of the time-reversal
symmetry breaking in a macroscopic scale.
An alternative way to induce magnetism in TIs is to make aheterostructure of TIs and magnets. Such systems are also used
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Figure 2. (a) Experimental IV spectra of LEED spots measured at 100 K for the heterostructure (red solid lines), and the calculated spectra of the
optimized model shown in (b) (blue dotted lines). The inset shows the LEED pattern at 200 eV. (b) Cross-sectional (left) and top (right) view ofthe optimized model of the heterostructure, which turns out to be MnBi2Se4/Bi2Se3. Blue, pink, and green atoms represent Mn, Bi, and Se,
respectively. The solid lines show the mirror plane and the dotted parallelogram shows the unit cell.

in experiments to observe the surface-state spin transport.18−21
However, there has not been a direct observation of the QAHE
as well as the existence of a gapped Dirac cone at the interfaces
in these heterostructures. One reason for this is the absence anatomically sharp interface that makes the gap small and diﬃcult
to observe, which should be also responsible for the low spincharge conversion eﬃciency. Even in some cases with a sharp
interface (EuS/Bi2Se3), the Dirac cone gap was shown not tobe related with the magnetic proximity eﬀect,22 althoughmagnetization measurements showed evidence of a magnetic
interaction.23 In another case of MnSe/Bi2Se3, it was shown
from ab initio calculations that states other than the gapped
Dirac cone will cross EF24 and make the QAHE impossible torealize. However, the measured band structure was completely
diﬀerent from the calculation, but no clear explanation wasgiven.25 Thus, the interplay between the topological properties
and magnetism in an ordered layered system needs to be
examined further.
In the present study, we take advantage of self-organization
to produce an ideal system of a magnetic insulator/TI
heterostructure, which is formed spontaneously as a hexagonal
MnBi2Se4 septuple layer (SL) on the basis of the topmost
quintuple layer (QL) of Bi2Se3 ﬁlm under Mn and Se
codeposition. Due to the ferromagnetism of MnBi2Se4 up to
room temperature as revealed by superconducting quantum
interference device (SQUID) and X-ray magnetic circulardichroism (XMCD) measurements, the Dirac cone becomes
massive with a gap size of ∼100 meV. First-principles
calculations show that the calculated Chern number is C =−1, and the heterostructure is identiﬁed as a quantum
anomalous Hall phase. Our results not only show the ﬁrst
example of a clear Dirac cone gap opening at the ferromagnet/
TI interface but imply that this should be a suitable system torealize the QAHE at higher temperatures than previously
reported by tuning the Fermi level as well as to develop novel
devices based on the topological magnetoelectric eﬀect.
Figure 1a shows the band dispersion of the Bi2Se3 thin ﬁlm,
which was used as the substrate for the heterostructureformation. The well-known Dirac cone of the surface states inthe bulk band gap can be seen together with the bulk
conduction band at the Fermi level due to the unintentional
doping. The Dirac point is recognized as shown in the energy
distribution curve (EDC) at the Γ̅ point (red line). Figure 1b
shows the band structure of the heterostructure (sample A)
after Mn and Se were codeposited on Bi2Se3. It was measured
immediately after cooling down the sample. A clear gap
opening is observed at the Dirac point. This becomes more
prominent by making the second derivative with respect to theenergy of the image in Figure 1b, which is shown in Figure 1c.
The gap size is deduced as ∼85 meV from the EDCs in Figure
1d. A slightly smaller gap is observed when the photon energy
is changed to 11 eV, as shown in Figure 1e. The estimated gap
size is ∼80 meV, which is nearly the same as that shown in
Figure 1d. It can also be noticed that the midpoint of theenergy gap has shifted down by about 70 meV from Figure 1b−
d to Figure 1e, due to the band bending near the surface causedby the residual gas or defect formation26,27 since Figure 1e wasmeasured 12 h after Figure 1b−d. The band dispersion images
taken with other photon energies for sample A are shown in
Figure S1. Even though the gap exists irrespective of the Fermi
level position, the gap size slightly changes depending on themeasurement condition. However, there is no correlation
between the midgap position and the gap size. Figure 1f shows
the band dispersion image of a diﬀerent sample (sample B)
taken at hν = 50 eV just after cooling down the sample
posterior to the preparation. The gap size is ∼110 meV, larger
than that for sample A although the midpoint of the gap isnearly the same. We have also measured the gap size for othersamples, and the gap value is in the range of 75−120 meV when
the Dirac point is located at the binding energy of 0.31−0.39
eV, although there is no correlation between the two quantities.
We conclude that when Mn and Se are codeposited on Bi2Se3
and a heterostructure is formed, the Dirac cone becomesgapped. At ﬁrst glance, it should be due to the magnetic
proximity eﬀect from antiferromagnetic MnSe.
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Figure 3. (a) Calculated band dispersion of a MnBi2Se4/6QL Bi2Se3 heterostructure. The states marked in red and blue are localized at the topmost
SL and the next Bi2Se3 QL and spin-polarized in the in-plane direction perpendicular to the wavenumber (positive and negative, respectively). The
states marked in green are the ones mostly localized at MnBi2Se4 SL and labeled “Mn”. The states localized at the bottom surface of the slab are
indicated in gray. The shaded area shows the bulk band projection of Bi2Se3. The inset shows a close-up of the Dirac state dispersion. A gap of 81
meV is found for the Dirac cone at the top surface. (b) The real space atomic conﬁguration used in the calculation together with the interlayer
spacings and layer-resolved magnetic moments. (c) Wave function (|Ψ|2) plot for the gapped Dirac state at the top surface (violet) and that at thebottom surface (gray), respectively. (d) The calculated band dispersion overlapped with the experimental data of Figure 1c. Pink circles show thestates localized in the SL.

that the MnBi2Se4 SL is 630 meV energetically more favorable
as compared to the MnSe bilayer (BL)/Bi2Se3 QL interface.
MnBi2Se4 in the bulk form is reported to have the monoclinic
structure with C2/m symmetry. It is shown to be a narrow gap
semiconductor and antiferromagnetic with TNeel ≈ 14 K.28
However, the MnBi2Se4 SL that we have fabricated has ahexagonal structure and is a semiconductor with a gap of ∼0.4
eV (Figure S3).
The electronic structure of the heterostructure was calculated
based on the determined atomic structure of Figure 2b.
Namely, a six QL ﬁlm of Bi2Se3 was sandwiched between
MnBi2Se4 and vacuum. The interlayer spacing values obtained
after structural relaxation of the heterostructure and shown in
Figure 3b are in excellent agreement with the experimentally
determined values shown in Figure 2b. We found that the spin
structure with the Mn magnetic moment pointing out-of-plane
is 0.2 meV energetically more favorable than the one with inplane spin polarization, and the magnetic moment of Mn was
found to be 4.61 μB (Figure 3b). The calculated band
dispersion is shown in Figure 3a. The red and blue points
correspond to the states with in-plane spin polarization
perpendicular to the wavenumber (positive and negative,
respectively) that are localized at the topmost SL and theneighboring QL (refer to the Supporting Information for theexperimental results of the Dirac cone spin-polarization). Near
the Fermi level, the massless Dirac cone of pristine Bi2Se3 at thebottom surface can be found (gray points and gray line in inset)
together with the massive Dirac state with a gap Dg = 81 meVat the top surface. The “gray” band can be a good reference fortracking the changes in the Dirac state. As can be seen, thelower part of the Dirac cone of the gapped state shows almost
no change in the energy position, and the gap is formed byshifting up the upper branch. The calculations performed for a

However, there is one contradictory point about this band
dispersion of the heterostructure. According to ab initio
calculations, 24 the band dispersion of a MnSe/Bi 2 Se 3
heterostructure should have the following characteristics: (i) amassive Dirac cone with a gap of 8.5 meV and (ii) othermetallic states in the Bi2Se3 bulk band gap. In our experimental
data, we do not ﬁnd any additional states in the Dirac stateenergy region. Moreover, the gap size is an order of magnitudelarger than that predicted in the calculation. Therefore, thecalculation and the experiment are not consistent with eachother. Various factors may be responsible for this inconsistency,such as the diﬀerence in the interface structure or the thicknessof the MnSe layers. We have therefore performed structure
analyses of the heterostructure based on the LEED IV
technique to resolve this inconsistency.
Figure 2a shows the experimentally observed IV curves (redsolid lines). To reproduce these curves, we have ﬁrst assumed
the MnSe/Bi2Se3 heterostructure that is expected from thesample preparation method. However, we could not obtain Rfactor (Rp) values smaller than 0.4. Thus, it was proven that theheterostructure we have fabricated was not MnSe/Bi2Se3. We
tried other structures and ﬁnally found one that was in striking
agreement with the experimental LEED IV curves. The
calculated IV curves for the optimized structure are shown by
the blue-dotted lines in Figure 2a. The agreement between thetwo curves is excellent with Rp = 0.18 ± 0.03. The structure
thus determined is shown in Figure 2b. What is remarkableabout this structure is that the deposited Mn and Se are not on
top of the Bi2Se3 substrate, but they are incorporated inside thetopmost QL of Bi2Se3. So we can say that a MnBi2Se4 SL
spontaneously forms on top of Bi2Se3 by self-organization.
Although the microscopic mechanism of MnSe bilayer
immersion into the QL is not clear, our calculations show
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Figure 4. (a,b) Magnetization curves measured with SQUID for the Se-capped sample B taken at 4 K (a) and at 300 K (b), respectively. The
magnetic ﬁeld was applied perpendicular to the sample surface. The inset shows the close-up near zero-ﬁeld. Clear hysteresis loops can be observed
both at 4 and 300 K. (c) X-ray absorption spectra (XAS) at 7.2 K for a circularly polarized incident light when a ±5 T magnetic ﬁeld was appliedalong the surface-normal direction. μ+ and μ− correspond to the spectrum obtained at +5 T and -5 T, respectively. The corresponding XMCD
spectra is also shown. The Se capping layer was removed by annealing the sample in UHV. (d) Same as that in (c) but measured at zero ﬁeld. A
remanent XMCD signal can be observed (shown by the arrow).

Upon further decrease of the spin−orbit interaction strength,
the gap reopens and the system becomes a topologically trivial
insulator.
Next, we have tried to experimentally verify the absence oftime-reversal symmetry in the heterostructure as the calculationsuggests. Figure 4a,b shows the magnetization curves measured
with SQUID for the Se-capped sample B taken at 4 K (a) andat 300 K (b), respectively. The magnetic ﬁeld was applied
perpendicular to the sample surface. (The data for the in-planemagnetization can be found in Figure S7). In order to display
the situation near zero-ﬁeld clearly, we show in the inset theclose-up for −0.1 to 0.1 T. There is a vivid hysteresis loop bothat 4 and 300 K, demonstrating the ferromagnetic nature of theheterostructure.31 Actually, the gapped Dirac cone can also be
observed at room temperature (Figure S2), and thus, the Curie
temperature is conﬁrmed to be above 300 K also from ARPES.
We also performed XMCD measurements after removing the
Se cap in UHV to make sure that the Se cap is not aﬀecting themagnetic properties. Figure 4c shows the X-ray absorption(XAS) spectra at the Mn L edge measured at 7.2 K for acircularly polarized incident light when a ±5 T magnetic ﬁeld
was applied along the surface-normal direction. μ+ and μ−
correspond to the spectrum obtained at +5 T and −5 T,
respectively. The corresponding XMCD spectrum is also
shown, and there is a clear signal. The quantitative analysis of
the XMCD measurements using the sum-rule is discussed in
the Supplementary Information, see Figure S5. The measurements were also performed at remanence (0 T appliedmagnetic ﬁeld), namely, the signal was obtained after switchingoﬀ the ±5 T magnetic ﬁeld applied perpendicular to the surface(Figure 4d). Although weak (the peak at the L3 edge is ∼7%
compared to the data of Figure 4c), we still can notice the
XMCD signal (the arrow in Figure 4d), and this conﬁrms thatthe system is ferromagnetic. We believe that the SQUID data of
Figure 4a,b and that obtained by XMCD in Figures 4c,d areboth showing the ferromagnetism of the Mn layer despite thediﬀerent probing depth of the two measurements since it canbe the only source of ferromagnetism in the present
heterostructure system.
The heterostructure we have fabricated demonstrates various
advantages compared to previously studied quantum anomalous Hall systems. First, it forms spontaneously by depositing
Mn and Se on top of Bi2Se3. We have done this using thin ﬁlms,
but in principle, it should also be possible to do this for single

symmetric heterostructure, where Bi2Se3 is sandwiched
between two MnBi2Se4 SLs, result in the same value of the
Dirac cone gap. Figure 3d shows this calculated spectrum
overlapped with the experimental result (the calculation was
rigidly shifted by −0.38 eV to ﬁt to the experimental EF). The
agreement between the two is excellent with exception of ﬂatter
dispersion in the bottom part of the Dirac cone in thecalculation whereas the gap size in the Dirac cone, as well as
ﬁne structures other than the Dirac cone, are reproduced. The
green points in Figure 3a are states localized at the topmost SL
and labeled “Mn” because it has the maximum contribution
from the Mn pz orbitals. They are located along the Γ̅ −K̅
direction in the Bi2Se3 bulk band gap and a band that resembles
this is also found in the calculated dispersion of the freestanding MnBi2Se4 SL (Figure S3). This was actually observed
experimentally when we performed ARPES measurements
along the Γ̅ −K̅ , as shown in Figure S9a. Altogether, thecalculation reproduces the experimentally observed band
structure nicely, and we are sure that the heterostructure wehave fabricated is MnBi2Se4/Bi2Se3. Moreover, one can say thatthe mass acquisition of the Dirac cone is not due to a magnetic
proximity eﬀect because the Dirac state and the magnetic layer
are both at the topmost SL (70% of the Dirac state is localized
in the SL, see Figure 3c) We should rather say that this is a“direct interaction” between the magnetic layer and the surface
Dirac cone, since they overlap in real space and some part of
the wave-function of the Dirac cone is within the Mn layers.
The original Dirac cone of Bi2Se3 has been pushed up to the
newly formed MnBi2Se4 capping layer, which has been
discussed previously for other topological-insulator based
heterostructures.29 This is the reason why there is a large gapof ∼80 meV at the Dirac point in contrast to the case of MnSe/
Bi2Se3 where a magnetic proximity eﬀect was considered.24
The gapped Dirac state has a potential to realize the QAHE.
The quantized Hall conductance σyx = Ce2/h is related to the
topological characteristic of the band structure known as the
Chern number C.30 Our calculations show that the gapped
spectrum of the MnBi2Se4/Bi2Se3 heterostructure is characterized by the Chern number C = −1, which deﬁnes theobtained heterostructure unambiguously as a quantum
anomalous Hall phase. To conﬁrm the topological characterof the heterostructure spectrum, we artiﬁcially decreased thespin−orbit interaction strength λ and found that it leads to the
gap narrowing and its closing at λ/λ0 ≈ 0.75 (see Figure S4).
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crystals or even thinner ﬁlms. This is in contrast to theextensively studied systems with an intentional magnetic
impurity doping7−9,16,17 in which ﬁne-tuning of the impurity
concentration is needed to achieve a long-range magnetic
order.32
Second, in our samples, we have an ideal situation where theDirac cone and the magnetic layer interact strongly due to thespatial overlap of the respective wave functions, thus leading toa large magnetic gap but with minimal eﬀects on other bands.
This is in contrast to heterostructures that utilize the magnetic
proximity eﬀect.22−24
Third, our samples are free from inhomogeneity since thestructure was clearly determined by LEED measurements. This
allowed us to observe a clear Dirac cone gap with ARPES andassign its origin unambiguously as due to the time-reversal
symmetry breaking. Recalling that the presence of the impurity
states makes it diﬃcult to determine the gap opening in the
Dirac cone as well as its origin,16,17 this is a merit. Furthermore,
the clear gap opening in ARPES means that our heterostructure
does not show strong ﬂuctuations of the magnetic energy gap.11
Finally, the Curie temperature of the heterostructure is above
room temperature. Furthermore, the Dirac cone gap is larger
than the thermal ﬂuctuation at room temperature. Therefore,
we can expect to observe the QAHE for our samples at higher
temperatures than those previously reported8−10 by moving the
Fermi level into the induced gap either chemically7 or by
gating.8,10,32 So our heterostructure may be used in application
for developing “topotronics” devices.
Experimental and Calculation Methods. The heterostructure samples were prepared by molecular beam epitaxy inultrahigh vacuum (UHV) chambers equipped with a reﬂectionhigh-energy electron diﬀraction (RHEED) system. First, a clean
Si(111)-7 × 7 surface was prepared on an n-type substrate by acycle of resistive heat treatments. The Si(111)β √3 × √3-Bi
surface was formed by 1 ML (7.83 × 1014 cm−2) of Bi
deposition on the 7 × 7 surface at 620 K monitored by
RHEED. Then Bi was deposited on the β √3 × √3-Bi
structure at ∼200 °C in a Se-rich condition. Such a procedure isreported to result in a smooth epitaxial ﬁlm formation with thestoichiometric ratio of Bi/Se = 2:3.33,34 The grown Bi2Se3 ﬁlms
were annealed at ∼240 °C for 5 min. The thickness of the
Bi2Se3 ﬁlms in this work is ∼15 QL, which does not show the
gap opening in the Dirac cone due to ﬁnite-size eﬀects,34,35 asshown in Figure 1a. Finally, Mn was deposited on Bi2Se3 in a
Se-rich condition at ∼240 °C. As reported in ref 25, this results
in a ﬂat ﬁlm formation, which was believed to be aheterostructure of MnSe/Bi2Se3.
ARPES and SARPES measurements were performed in situ
after the sample preparation. The ARPES measurements were
performed at BL-5U and 7U of UVSOR-III using p-polarized
photons in the energy range of 30−65 and 7.5−28 eV,
respectively.36 All the data shown were taken at 30 K unlessotherwise indicated. The energy and angular resolutions were
15 meV and 0.25°, respectively. The SARPES measurements
were performed at BL-9B of HiSOR using p-polarized photons
in the energy of 18−21 eV at 30 K.37 In some of themeasurements, a magnetic ﬁeld as large as ∼0.1 T was applied
to the sample. The energy and angular resolutions were 30 meV
and 0.75°, respectively.
LEED measurements were also performed in situ after the
sample formation in another UHV chamber. The LEED
patterns with incident energy from 80 to 400 eV were recorded
in steps of 1 eV by a digital CCD camera at 100 K. IV curves of

14 inequivalent diﬀraction spots were obtained. In order todetermine the surface structure, we calculated the IV curves inthe tensor LEED to ﬁt the experimental IV curves using the
SATLEED package of Barbieri/Van Hove38 and minimized
Pendry’s R-factor (Rp). As shown in Figure 2b, each atomic
layer was treated diﬀerently according to their environments.
The in-plane lattice constant of the heterostructure wasdetermined from positions of the LEED spots as well as the
RHEED spots and was the same with that of Bi2Se3. Angularmomentum up to 17 (lmax = 17) was taken into accountbecause of the strong scattering of the heavy Bi atom (Z = 83).
Considering the mean penetration depth of the incident
electrons of ∼10 Å, only the topmost seven surface layers wereallowed to relax, and we used the bulk Bi2Se3 parameters for thelayers beneath. In search of the optimal structure, the Debye
temperature of each atom was changed in steps of 10 K from 50
K up to 300 K. If MnSe grew on Bi2Se3, it should have thep3m1 symmetry. However, the symmetrically inequivalent
spots, such as (1,0) and (0,1), exhibited almost the same IV
curves. Since this was also seen for the pristine Bi2Se3 LEED
patterns, it probably comes from the fact that there are twin
domains on the surface that are related by a 180° rotation. The
superposition of the two domains should lead to the apparent
2-fold symmetry. Taking this double-domain surface into
account, we took the average of the IV curves both in thecalculation and in the experimental data such that {h,k} is theaverage of (h,k) and (k,h) spots (see Figure 2a). Note thatspots having the same mirror indices of h and k do not needaveraging.
For the SQUID measurements, the fabricated samples werecapped with ∼15 nm of Se before taking it out of the UHVchamber, and a commercial MPMS-52 system (Quantum
Design) was used. The diamagnetic contribution of the Si
substrate was derived from the ﬁeld-dependent magnetization
curves at room temperature and subtracted from all data.
XMCD measurements were performed at BL-4B of UVSORIII with circularly polarized X-ray radiation (positive helicity
with the polarization of 0.6) at 7.2 K and a magnetic ﬁeld ashigh as ±5 T was applied to the sample.39 Instead of changing
the photon polarization, we reversed the direction of themagnetic ﬁeld to derive the XMCD spectra. As shown in Figure
4c, + (−) magnetic ﬁeld corresponds to the direction
antiparallel (parallel) to the direction of the photon incidence.
The measurements were ﬁrst performed for the samples with
Se-capping. After measuring, the Se-capped samples wereannealed at ∼240 °C to remove the capping layers. LEED
observations showed a clear recovery of the 1 × 1 surface
periodicity for the cap-removed samples.
For structural optimization and electronic band calculations
we used the Vienna Ab Initio Simulation Package40,41 withgeneralized gradient approximation (GGA-PBE)42 to theexchange-correlation potential. The interaction between theion cores and valence electrons was described by the projector
augmented-wave method.43,44 The Hamiltonian contains scalar
relativistic corrections, and the spin−orbit interaction (SOI) is
taken into account by the second variation method.45 To
correctly describe the highly correlated Mn-d electrons we
include the correlation eﬀects within the GGA+U method.46
The in-plane lattice constant of the heterostructure was ﬁxed tothat of Bi2Se3. DFT-D3 van der Waals corrections47 wereapplied for accurate structure optimization. The Chern number
was calculated for a symmetric MnBi2Se4/5QL-Bi2Se3/
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Letter

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MnBi2Se4 slab by using the method based on tracking theevolution of hybrid Wannier functions realized in Z2 Pack.48■

ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.nanolett.7b00560.
Additional details on ARPES, SARPES, ab inito
calculations, SQUID, and XMCD measurements (PDF)■

AUTHOR INFORMATION

Corresponding Author*E-mail: hirahara@phys.titech.ac.jp. Phone: +81 (0)3 5734
2365. Fax: +81 (0)3 5734 2365.
ORCID

Toru Hirahara: 0000-0002-2574-1708
Sergey V. Eremeev: 0000-0002-9477-3017
Tetsuroh Shirasawa: 0000-0001-5519-6977
Present Addresses(T.S.) National Institute of Advanced Industrial Science andTechnology, Ibaraki 305-8560, Japan.
(T.H.) Department of Crystalline Materials Science, Nagoya
University, Nagoya 464-8603, Japan.
(M.M.) Energy Materials Laboratory, Toyota Technological
Institute, Nagoya 468-8511, Japan.
(S.K.) Graduate School of Frontier Biosciences and Department of Physics, Osaka University, Suita 565-0871, Japan.
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
The authors thank A. V. Matetskiy, A. A. Saranin, O. Rader, andA. Kimura for discussions. This work has been supported by
Grants-In-Aid from Japan Society for the Promotion of Science(Nos. 15H05453, 16K13683, 19340078, and 23244066), theToray Science Foundation, the Basque Country Government,
Departamento de Educación, Universidades e Investigación(Grant No. IT-756-13), the Spanish Ministry of Science andInnovation (Grant Nos. FIS2010-19609-C02-01, FIS201348286-C02-02-P, and FIS2013-48286-C02-01-P), the Tomsk
State University Academic D.I. Mendeleev Fund Program(Grant No. 8.1.05.2015), and Saint Petersburg State University(project 15.61.202.2015). The ARPES experiments were
performed under the UVSOR Proposal Nos. 25-808, 26-531,
27-533, 28-526, and S-15-MS-0034, and the SARPES experiments were performed under the HiSOR Proposal No. 15-A14. The XMCD measurements were performed under the
UVSOR proposal number S-16-MS-2017. The LEED measurements were performed under the ISSP Proposal number H17A250. The SQUID measurements were performed using
facilities of the Cryogenic Research Center, the University ofTokyo. Calculations were performed on the SKIF-Cyberia
supercomputer at the National Research Tomsk State
University.

■

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