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ARTICLE
Emergent flat band electronic structure in a
VSe
2
/Bi
2
Se
3
heterostructure
Turgut Yilmaz 1✉, Xiao Tong2, Zhongwei Dai 2, Jerzy T. Sadowski 2, Eike F. Schwier3, Kenya Shimada 3,
Sooyeon Hwang2, Kim Kisslinger2, Konstantine Kaznatcheev1, Elio Vescovo1& Boris Sinkovic4
Flat band electronic states are proposed to be a fundamental tool to achieve various quantum
states of matter at higher temperatures due to the enhanced electronic correlations. How-
ever, materials with such peculiar electronic states are rare and often rely on subtle prop-
erties of the band structures. Here, by using angle-resolved photoemission spectroscopy, we
show the emergent flat band in a VSe
2
/Bi
2
Se
3
heterostructure. Our photoemission study
demonstrates that the flat band covers the entire Brillouin zone and exhibits 2D nature with a
complex circular dichroism. In addition, the Dirac cone of Bi
2
Se
3
is not reshaped by the flat
band even though they overlap in proximity of the Dirac point. These features make this flat
band distinguishable from the ones previously found. Thereby, the observation of a flat band
in the VSe
2
/Bi
2
Se
3
heterostructure opens a promising pathway to realize strongly correlated
quantum effects in topological materials.
https://doi.org/10.1038/s43246-020-00115-w OPEN
1National Synchrotron Light Source II, Brookhaven National Lab, Upton, New York 11973, USA. 2Center for Functional Nanomaterials, Brookhaven National
Lab, Upton, New York 11973, USA. 3Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi Hiroshima 739-0046,
Japan. 4Department of Physics, University of Connecticut, Storrs, Connecticut 06269, USA. ✉email: tyilmaz@bnl.gov
COMMUNICATIONS MATERIALS| (2021) 2:11 | https://doi.org/10.1038/s43246-020-00115-w | www.nature.com/commsmat 1
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The physics of solids is largely determined by their energy
band structures. Therefore, the investigation and control of
distinct electronic band dispersions assume a great length
to understand and discover new states of the matter. One of the
exotic electronic states is a type of flat band, which is predicted to
host high-temperature superconductivity1–4, fractional quantum
Hall effect5,6, and ferromagnetism7–9. In superconductors, a flat
band can boost the coupling constant and the transition tem-
perature (T
c
) as a result of an enhanced density of states at the
Fermi level (E
F
)10,11. This mechanism was utilized to explain the
unexpected superconductivity in rhombohedral graphite and
twisted graphene12–14. Other examples of flat band materials are
Kagome lattices in which the flat band stems from destructive
quantum interference due to the frustrated lattice geometry7,8.
The Kagome-type flat bands have been observed in FeSn and
Fe
3
Sn
2
by angle-resolved photoemission spectroscopy (ARPES)
and in Co
3
Sn
2
S
2
by scanning tunneling spectroscopy (STS)8–10.
However, the complexity of the electronic structure in the pho-
toemission data and the lack of momentum resolution in STS
make the observations elusive. Computational efforts have also
been made focusing on designing flat band transition metal
dichalcogenides (TMDs) through the formation of the Moiré
superlattices, which could support strongly correlated physics at
higher temperatures15–17. All these previous works conclude that
the flat band media could be fertile to many novel states of the
matter. However, the limited number of the materials with such
non-trivial bands hinders future studies.
Motivated by earlier studies, we investigate the surface elec-
tronic structure of VSe
2
TMD grown on the surface of Bi
2
Se
3
topological insulator (TI) and show the emergence of a flat band
in the electronic states. This flat band covers the entire k
x
–k
y
plane of the Brillouin zone (BZ) and displays dispersionless
behavior along the k
z
direction as well. Furthermore, circular
dichroism ARPES (CD-ARPES) measurements reveal that the CD
signal of the flat band reverses the sign at several points within
the BZ. Another notable observation is that the VSe
2
overlayer
and the emergence of the flat band do not reshape the Dirac cone
of Bi
2
Se
3
in the vicinity of the Dirac point (DP) unlike the case of
transition metal doping which opens a large gap at the DP18.We
also observe Moiré patterns in VSe
2
domains of monolayer (ML)
thickness and stripe-type patterns in bare Bi
2
Se
3
through scan-
ning tunneling microscopy (STM). Further elucidations on the
crystalline and chemical properties of the system are provided by
scanning transmission electron microscopy (STEM), and micro-
spot low-energy electron diffraction (µLEED). Our results
demonstrate a rich physics in this system and suggest a large
family of materials as possible emergent flat bands and thus will
motivate future studies based on heterostructure formed by other
quantum materials such as superconductors.
Results
Structural properties.Bi
2
Se
3
and VSe
2
are layered materials with
their atomic stacking geometry shown in Fig. 1a. The layers in
each compound are separated by van der Waals (vdW) gaps with
weak covalent out-of-plane bonds connecting the layers. These
properties allow for the formation of well-ordered VSe
2
/Bi
2
Se
3
heterostructure despite the large in-plane lattice mismatch of
around 20% between the two materials19,20. Figure 1b, c depict
the relevant core-levels of such structures 0 ML, 0.3 ML, 2 ML,
and 3 ML VSe
2
on 12 quintuple layer (QL) Bi
2
Se
3
. Upon
deposition of the VSe
2
, the Bi 5dpeaks of Bi
2
Se
3
located at 25.1
and 28 eV remain at the same binding energies, indicating the
absence of V metals at the interface and/or in the bulk (Fig. 1b)21.
This also shows that VSe
2
surface deposition does not modify the
chemical potential of Bi
2
Se
3
seen as the absence of an electron or
hole doping effect. Compared with pristine Bi
2
Se
3
, the Se 3dpeak,
however, appears at 0.1 eV higher binding energy for VSe
2
grown
sample. The difference in binding energy is possibly related to the
charge density wave (CDW) phase of VSe
2
19. Furthermore, V 2p
1/
2
and 2p
3/2
peaks of VSe
2
shown in Fig. 1c are located at 513 and
520.6 eV binding energies corresponding to +4 oxidation states
being in agreement with the recent report20.
To further explore the system, we show a high-angle annular
dark-field (HAADF)-STEM cross-section image of a 3 ML VSe
2
/
12 QL Bi
2
Se
3
heterostructure in Fig. 1d. Bi
2
Se
3
and VSe
2
exhibit
regular atomic layers with smooth interfaces and vdW gaps
marked with red arrows in Fig. 1d. On the other hand, the
interface spacing between the Bi
2
Se
3
and VSe
2
layers comparably
smaller than the vdW gaps which could strongly modify the local
electronic structure. Furthermore, the STEM energy dispersive X-
ray spectroscopy elemental maps presented in Supplementary
Fig. 1 show the atomic distribution of Bi in the Bi
2
Se
3
layers, V in
VSe
2
layers, and Se across the heterostructure as expected.
To study the local crystal structure, Fig. 1e, f show the STM
image of Bi
2
Se
3
and VSe
2
regions of a 0.3 ML VSe
2
/12 QL Bi
2
Se
3
sample, respectively. The Bi
2
Se
3
surface has a stripe-like pattern
similar to Cs and Fe doped Bi
2
Se
3
22. The STM image of the VSe
2
domains presented in Fig. 1f exhibits a Moiré pattern with ~2
nm × 2 nm superstructure. This differs from the previous studies
conducted on VSe
2
/graphene19. Moiré pattern can be formed by a
small misfit between the in-plane lattice parameters of the film
and the underlying material or the relative rotation of two layers
to each other, or both. By contrast, the lattice mismatch between
the VSe
2
and Bi
2
Se
3
is quite large (about 20%). Unfortunately, we
cannot make a quantitative analysis for precise determination of
the in-plane lattice parameters or the atomic displacement due to
the limitation in our STM data taken at room-temperature
experiment. However, similar Moiré pattern formation is also
observed on ML MoSe
2
grown on a graphene substrate whose
origin is attributed to the lattice mismatch between the multiple
unit cells of the two materials23. Thereby, the Moiré pattern in
VSe
2
could be formed due to the small mismatch between
four–five-unit cells of Bi
2
Se
3
(4a
BS
=16.56 Å or 5a
BS
=20.7 Å)
and five-unit cells of VSe
2
(5a
VS
=16.8 Å or 6a
VS
=20.16 Å) for
the rotationally aligned lattice geometry. Alternatively, the Moiré
pattern could form be formed by the rotational misalignments of
Bi
2
Se
3
and VSe
2
atomic lattices. Moreover, the details of the STM
data reveal that the layer height is 6.8 Å for VSe
2
(Supplementary
Fig. 2) being in the line with the recent findings24.
Observation of the flat band in a VSe
2
/Bi
2
Se
3
heterostructure.
To examine the band structure, the binding energy vs. k
x
plots are
given in Fig. 2a for 12 QL Bi
2
Se
3
and in Fig. 2b–d for various
thickness of VSe
2
on 12 QL Bi
2
Se
3
.Bi
2
Se
3
exhibits the typical
band structure with the linear Dirac surface states (DSSs) forming
the Dirac cone with the Dirac point (DP) at 0.36 eV below E
F
25.
Upon deposition of 0.3 ML VSe
2
on the surface of Bi
2
Se
3
,aflat
band at 0.47 eV binding energy and with a ~0.18 eV bandwidth
emerges in the surface electronic structure (Fig. 2b). The flatness
of the band is well distinguished in the ARPES maps where bulk
bands and the DSS of Bi
2
Se
3
strongly disperse as a function of k
x
,
while the flat band retains dispersionless across the
Γ
Mhigh-
symmetry lines. VSe
2
growth also induces the well-known M-
state quantization26 of the bulk valance band of Bi
2
Se
3
shown in
Fig. 2b. The flat band can be resolved in 1 ML and, less intense, in
2 ML VSe
2
(Fig. 2c, d) and further increasing the thickness of
VSe
2
to 3 ML leads to disappearance of the flat band from the
ARPES map (Fig. 2e). On the other side, the thicker VSe
2
grown
on a Bi
2
Se
3
exhibits the same electronic feature with the one
grown on a highly ordered pyrolytic graphite (HOPG) substrate
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(Fig. 2f), indicating that the Bi
2
Se
3
sublayer plays a crucial role in
the formation of the new electronic states. It is also noticeable
that the nature of the flat band stays unchanged with increasing
VSe
2
thickness as its binding energy and dispersionless character
remain the same. The only observable evolution is in the spectral
intensity which gradually decreases with increasing VSe
2
thick-
ness. This emphasized in Fig. 2g displaying the energy distribu-
tion curves (EDCs) taken at k
x
=‒0.25 Å−1from each ARPES
maps. The peak stemming from the flat band can be seen up to 2
ML VSe
2
, while it is not visible for thicker films. This is likely to
be correlated with the electron mean free path and suggests that
this state is localized at the interface between the two materials.
This can be further confirmed by taking advantage of energy
dependency of the electron mean free path. In Fig. 2h, the EDCs
at k
x
=‒0.3 Å−1for a 3 ML VSe
2
/12 QL Bi
2
Se
3
heterostructure
are shown for three photon energies. As expected, due to the
enhenced electron mean free path27, a shoulder on the high
binding energy side of V 3ddevelops with increasing photon
energy from 50 to 220 eV, supporting interface-induced flat band
argument.
Another interesting observation is that the flat band overlaps with
the lower branch of the Dirac cone in the vicinity of k
x
=0Å
−1
without inducing any prominent change in its spectral shape. This
canbeevenbetterseeninthefilms with the thicker VSe
2
coverage
confirming that the flat band, DSSs, and the dispersive V 3dstate of
VSe
2
coexist in the surface electronic structure (Fig. 2b–dand
Supplementary Fig. 5). It is also found that the DP of Bi
2
Se
3
does not
experience an energy shift upon surface deposition of VSe
2
.This
indicates the absence of any band bending effect which is consistent
with the behavior of Bi 5dcore levels presented in Fig. 1b.
To further investigate the flat band, a k
x
–k
y
intensity plots at E
F
and at the binding energy of the flat band (E
FB
) for a 1 ML VSe
2
/12
QL Bi
2
Se
3
sample are shown in Fig. 2i, j, respectively. The Fermi
surface is dominated by a flower-like electron pocket formed by the
V3dorbitals of VSe
2
centered at
Γpoint as similar to the earlier
observation for VSe
2
grown on different substrates28. The constant
energy cut at E
FB
is instead quite featureless. Besides, a weak
residual of the start-like features of VSe
2
, no new, distinct
dispersion is seen along the any direction of the BZ. This indicates
that the flat band fills the entire BZs of Bi
2
Se
3
and VSe
2
as depicted
by blue and red hexagons in Fig. 2i, j, respectively. This can be also
seen in Supplementary Fig. 4 where the spectra taken along the
different directions in the BZ for fixed k
x
and k
y
momentum
all show the existence of the flat band. Such electronic state spread
over a large momentum area can significantly enhance the
electronic correlation yielding quantum effects at very high-
temperatures. It is also worth noting that the LEED pattern of the
sample shows stretched diffraction spots along the rotational
direction indicating the presence of the rotationally misaligned
VSe
2
domains (Supplementary Fig. 2a) with respect to each other
and to the Bi
2
Se
3
substrate. The rotational misfit of ±3° estimated
from µLEED pattern, however, is too small for a band to span
whole BZ and to induce a fully occupied constant energy counter
in the momentum space.
In Fig. 2k, we also present an ARPES map covering two BZ
centers. The flat band connects two
Γpoints to each other.
Another intriguing realization in this spectra is that the Bi
2
Se
3
and VSe
2
shares the same in-plane lattice constants. This is
consistent with our LEED measurements, which does not show
distinct diffraction patterns arising from VSe
2
and Bi
2
Se
3
(Supplementary Fig. 2a). This is supported with the EDC taken
at the Fermi level and given on the top of the spectra in Fig. 2k.
Two zone centers are separated with k
x
=1.9 Å−1yielding a 4.03
Å in-plane lattice parameter. This is smaller than in-lane lattice
Fig. 1 Structural characterization of the VSe
2
/Bi
2
Se
3
heterostructures. a Schematic representations of the top and side views of Bi
2
Se
3
and VSe
2
crystal
structures. Hexagonal BZs with the high-symmetry points are given in the lower part of a.bCore-level photoemission spectra of Bi 5dand Se 3dof 0, 0.3, 1,
2, and 3 ML VSe
2
/12 QL Bi
2
Se
3
heterostructures. cV2pcore levels for 0.3, 1, 2, and 3 ML VSe
2
/12 QL Bi
2
Se
3
heterostructures. The atomic stoichiometry of
Se to V is computed to be 2 by using the peak areas and photoionization cross-sections. Bi 5dand Se 3dpeaks were recorded with 110 eV synchrotron
radiation, whereas V 2ppeaks were conducted by using 700 eV synchrotron radiation. dHAADF-STEM cross-section image of 3 ML VSe
2
/12 QL Bi
2
Se
3
heterostructure. The color contrast in dis correlated with the atomic number (Z-contrast). e,fRoom-temperature STM images of Bi
2
Se
3
(at sample bias
100 mV, set point 1 nA) and VSe
2
(at sample bias 80 mV, set point 1 nA) surfaces, respectively, obtained from 0.3 ML VSe
2
/12 QL Bi
2
Se
3
. Yellow
parallelogram in frepresents the unit cell of the Moiré pattern.
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parameter of Bi
2
Se
3
(a=4.545 Å)19, while larger than the one of
VSe
2
(a=3.356 Å)23. This strong lattice relaxation can signifi-
cantly modify the electronic structure of both material in the
vicinity of the interface and induce new electronic states.
Photon energy-dependent electronic structure. In ARPES
experiments, by recording the electronic structure with a wide
photon energy range, a k
‖
vs. k
z
or binding energy vs. k
z
dis-
persions can be also extracted. This method allows studying the
dispersion of the energy bands along the k
z
(out-of-plane)
direction to distinguish the two-dimensional (2D) bands from the
dispersive bulk bands. Such spectra acquired at varying photon
energies (from 45 to 120 eV with 5 eV steps) for a 1 ML VSe
2
/12
QL Bi
2
Se
3
heterostructure are given in Fig. 3. In the plot of k
z
vs.
k
y
dispersion at E
F
, DSSs marked with dashed black lines exhibit
no k
z
dependence (Fig. 3a). Similar spectrum at E
FB
given in
Fig. 3b shows that a high spectral intensity along the k
y
=0Å
−1
originates from the bottom of the Dirac cone of Bi
2
Se
3
. Away
from the k
y
=0Å
−1, the plot has non-vanishing spectral intensity
contributed by the flat band. This can be better seen in the
momentum distribution curves obtained at various k
z
points
(Fig. 3c) in which each spectrum exhibits always finite density of
states along the k
y
momentum direction. This implies the dis-
persionless nature of the flat band along the k
z
momentum
direction. To further validate this observation, we present the
binding energy–k
z
plots along the k
y
=±0.25 Å−1in Fig. 3d, e,
respectively. The plots clearly show that the flat band at 0.47 eV
binding energy is k
z
independent confirming its non-bulk derived
nature. We should also note that the M-shape bulk band located
in the vicinity of 1 eV binding energy exhibits a nearly non-
dispersive feature along the k
z
as shown in Fig. 3d, e. To further
reveal the details of the flat band, Fig. 3f depicts the EDCs taken
at different k
z
points. One can see that the EDC of the flat band
does not exhibit a k
z
-dependent evolution in the binding energy
and bandwidth, providing a signature that it has 2D nature and
originates from single type of atomic orbital.
Circular dichroism ARPES. CD-ARPES has gained great atten-
tion due to its feasibility to investigate the helical spin–orbit
texture in topological surface states29. The principle of the
method is the spectral weight differences in ARPES arising from
the opposite helicity of the circularly polarized lights. CD-ARPES
is then obtained from IRCP ILCP
ðÞ½=IRCP þILCP
ðÞ½where I
RHP
and I
LHP
are photoemission intensities for right hand circular
polarized (RCP) and left hand circular polarized (LCP) lights,
respectively. Thus, we have recorded the band structure of 0.3 ML
VSe
2
/12 QL Bi
2
Se
3
sample with RCP and LCP, shown in Fig. 4a,
b, respectively. The corresponding CD-ARPES is presented in a
binary color map in Fig. 4c (red: negative-CD and blue: positive-
CD). The bulk bands of Bi
2
Se
3
dispersing below 0.8 eV binding
energy show a strong CD signal as seen in Fig. 4a–c. CD signal of
the DSSs exhibits a spectral weight switching from the −k
y
to +k
y
regions when changing the excitation energy from RCP to LCP.
For clarity, the DC signal vs. k
y
is plotted in Fig. 4d at 0.1 eV
Fig. 2 Observation of the flat band in the VSe
2
/Bi
2
Se
3
heterostructures. a Experimental electronic structures of a 12 QL Bi
2
Se
3
sample. b–eElectronic
structure of 0.3, 1, 2, and ML VSe
2
/12 QL Bi
2
Se
3
heterostructures, respectively. fElectronic structure of 1 ML VSe
2
grown on a HOPG substrate. Spectra in
a–fwere collected with 110 eV linear horizontal polarized lights along the
Γ
Mdirection in the BZ. BVB represents the bulk valance band of Bi
2
Se
3
.
gCorresponding EDCs obtained along the k
x
=−0.25 Å. hPhoton energy-dependent EDCs of 3 ML VSe
2
/1
2
QL Bi
2
Se
3
obtained with 50, 110, and 220 eV
photons. A Tougaard background is subtracted from each spectrum. The corresponding ARPES maps are presented in Supplementary Fig. 3. i,jConstant
energy counters at the E
F
and the E
FB
for 1 ML VSe
2
/1
2
QL Bi
2
Se
3
sample, respectively. The momentum maps were recorded with 169 eV photons and the
corresponding ARPES map can be found in Supplementary Fig. 4. kWide momentum window ARPES map of the 1 ML VSe
2
/1
2
QL Bi
2
Se
3
sample obtained
with 220 eV photon energy along the
Γ
Mdirection of the BZ. The EDC on the top of kis obtained at the E
F
. Blue and red hexagons in i,jcorrespond to
the BZ of VSe
2
and Bi
2
Se
3
, respectively. In-plane lattice parameters of 4.14 Å for Bi
2
Se
3
and 3.356 Å for VSe
2
were employed to compute the BZs27,31.
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binding energy where the CD is positive for left and negative for
the right side of the Dirac cone, marked with vertical arrows.
Further away from the k
y
=0Å
−1, the plot in Fig. 4d still shows
non-zero CD. This is likely originating from the V 3dorbitals,
which dominate the density of states at the E
F
for VSe
2
19.
To investigate the dichroism effect in the flat band, the CD at
E
FB
is also plotted as a function of k
y
in Fig. 4e and it exhibits sign
inversions at k
y
=0Å
−1and k
y
=±0.5 Å−1, and the maxima at
k
y
=±0.25 Å−1. This shows that similar to the DSSs, the CD in
the flat band also exhibits helical texture where opposite k
y
momentums have opposite signs of the CD. Notably, zero CD
signal is also observed as white color in the CD-ARPES along the
k
y
=0Å
−1. This depicts the nodal line, which was proposed to be
the characteristic feature of the 2D electronic structure30.In
particular, the CD signal of the DSSs depends on the incident
photon energy assigning it to the final state effect in the
photoemission process29. This was discussed in ref. 30 with details
where they propose the non-trivial connection between the
spin–orbit texture and the CD signal. Thereby, the helical CD
texture and the nodal line band suggest that the flat band could be
topologically non-trivial.
Temperature-dependent electronic structure. To further study
the nature of the flat band in detail, we present the temperature-
Fig. 3 Photon energy-dependent electronic structure of a 1 ML VSe
2
/12 QL Bi
2
Se
3
heterostructure. a,bk
y
‒k
z
dispersions at the E
F
and the E
FB
,
respectively. Dashed red lines in amark the DSSs. cMDCs at different k
z
points. d,eBinding energy vs. k
z
maps at k
y
=±0.25 Å−1, respectively. Dashed
cyan colored lines in dand erepresent the dispersion of the flat band along the k
z
direction. fEDCs at various k
z
points to study the spectral shape of the
flat band. ARPES maps for the plots were conducted along the
Γ
K direction in the BZ. Data were conducted from the 1 ML VSe
2
/1
2
QL Bi
2
Se
3
heterostructure at 10 K.
Fig. 4 CD-ARPES. a,bARPES maps of 0.3 ML VSe
2
/12 QL Bi
2
Se
3
sample recorded with RCP and LCP lights, respectively. cComputed CD-ARPES. d,eCDs
as a function of k
y
at 0.1 eV binding energy and the E
FB
, respectively.
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dependent ARPES maps of 1 ML VSe
2
/12 QL Bi
2
Se
3
in Fig. 5a–d
and 0.3 ML VSe
2
/12 QL Bi
2
Se
3
in Fig. 5f–i for 10, 100, 200, and
300 K, respectively. For both samples, the temperature does not
induce a prominent change on the nondispersive nature of
the flat. This can be seen in the EDCs obtained along the k
y
=
−0.25 Å−1from the ARPES maps (Fig. 5e–j). It is clear from the
data that temperature does not affect the line shape or the binding
energy of the flat band. At any given temperature, the flat band is
located at 0.47 eV. Only noticeable change is the broadening of all
spectral features with increasing temperature as can be expected
from phonon contributions to the total lifetime. On the other
side, these data constitute a strong indication that the flat band is
not likely to originate from disorder or impurities. The disorder
usually shows a considerable dependence on the temperature as a
modulation in the density of defects. This refers that the binding
energy and the density of the states of the flat band should change
if it is formed by the impurities or disorders. This can be found in
a recent study where the disorder induced kink like states in
Bi
2
Se
3
can be diminished by lightly annealing the sample31. The
temperature-dependent ARPES data also show that the flat band
is not due to surface impurities. It is expected that the impurities
will segregate into the bulk of Bi
2
Se
3
and locate in sublattices with
increasing temperature32. This will significantly modify the che-
mical potential of the system. A number of ARPES studies have
been devoted to studying the effects of the adatoms on the band
structure of Bi
2
Se
3
and the only change found has been electron
or hole doping18,26,32. No experimental work has shown indica-
tion of any new states or flat like bands induced by surface
impurities on the surface electronic structure of a TI. Further-
more, resonant ARPES experiments also revealed that the tran-
sition metal impurity states evolve onto the surface states rather
than forming a flat band33,34. Finally, it is worth mentioning that
the ARPES maps taken at room temperature can be compared
with the room-temperature STM data presented in Fig. 1f, leading
to the conclusion that the Moiré pattern and the flat band elec-
tronic states coexist in our system.
Discussion
Here we present a detailed ARPES study on a VSe
2
/Bi
2
Se
3
system
and showed unexpected formation of a flat band on the surface
electronic structure on this heterostructure. One of the first scenario
to consider as the origin of the flat band is the surface V impurities.
Even though the theory predicts nearly nondispersive features
induced by impurities or disorders in TIs31,35, experimental studies
shows that these states evolve onto the surface states rather than
being flat in the momentum space33,34. Also, in contract to our
results, these states are expected to strongly modify the Dirac cone
and open an energy gap at the DP31,35. Another scenario to con-
sider would be the existence of superlattices as seen in ffiffiffi
3
p×ffiffiffi
3
p
silicene superstructure by STS where the local density of states
forms the electronic Kagome lattice36. Interface dislocation or strain
can also flatten the original bands by introducing pseudo-magnetic
field term to the Hamiltonian in Moiré superstructures4.Further-
more, a pronounced band flattening in this scenario requires
superstructure patterns with at least a few tens of nanometers
periodicity which is much larger than one observed in the present
case. However, in contrast to our observations, the flat band dis-
cussed within the superlattice frameworks is dispersionless only in
the BZ of the superstructure17,36.
On the other hand, our results show the emergent character of
our flat band, which is likely due to the formation of the interface
in the heterostructure between Bi
2
Se
3
sublayer and VSe
2
top layer.
This conclusion is supported by thickness and photon energy-
dependent photoemission data showing that the flat band can be
resolved with higher photon energies on thicker VSe
2
films. As a
first example of the dispersionless electronic excitation in a
topologically non-trivial band structure, our results could open a
new pathway in the critical field of experimental realization and
control of novel quantum effects.
Methods
Synthesis. Molecular beam epitaxial growth (MBE) technique was employed to
grow VSe
2
/Bi
2
Se
3
and Bi
2
Se
3
samples in a custom ultrahigh vacuum system located
at the ESM beamline of NSLS-II. Se and Bi sources (5 N) were evaporated from the
Fig. 5 Temperature-dependent electronic structure of the VSe
2
/Bi
2
Se
3
heterostructures. a–dARPES maps of 2 ML VSe
2
/12 QL Bi
2
Se
3
at 10, 100, 200,
and 300 K, respectively. eEDCs obtained along the k
y
=−0.25 Å−1from each spectrum. f–jSame as a–ebut for 0.3 ML VSe
2
/12 QL Bi
2
Se
3
sample. Spectra
were obtained with 110 eV photons.
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ceramic crucibles while the e-beam evaporation method was used for V (99.8%
purity) source. All samples were grown on Al
2
O
3
(0001) substrates at 255 °C. Before
the growth, the substrates were first degassed at 550 oC for three hours and flashed
at 850 °C for 5 min. An ML VSe
2
were grown on a freshly cleaved HOPG substrate
at 255 °C after annealing the substrate at 350 °C for 2 h. Sample thicknesses were
estimated within a 15% error bar by using a quartz thickness monitor and X-ray
photoemission spectroscopy. Samples for ARPES and µLEED experiments were
capped with 20 nm amorphous Se film before being removed from the MBE
chamber.
Core-level spectroscopy. Core-levels were recorded at 21-ID-1 ESM beamline of
National Synchrotron Light Source II (NSLS-II) by using a DA30 Scienta electron
spectrometer at 10 K sample temperature.
Angle-resolved photoemission spectroscopy. ARPES experiments were per-
formed at 21-ID-1 ESM beamline of NSLS-II by using a DA30 Scienta electron
spectrometer. The pressure in the photoemission chamber was 1 × 10−10 Torr and
samples were kept at 15 K during the experiment by a closed-cycle He cryostat. The
energy resolution in the ARPES experiments was better than 15 meV with a spot
size of ~20 µm. Before the ARPES experiment s, samples were annealed at 220 °C
for 30 min to remove the Se capping layer. The angle between the light and the
surface normal of the sample is 55° at the normal emission during the ARPES
experiments. The films were grounded with a tantalum clip. A part of the ARPES
experiments was conducted at the linear undulator beamline at the Hiroshima
Synchrotron Radiation Center BL-1 (Supplementary Fig. 5). Photon energy is
converted to k
z
momentum space by using the free electron final state approx-
imation hkz¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2meEkincos2θþVo
ðÞ
pwhere m
e
is the free electron mass, E
kin
is
the kinetic energy of a photoelectron, and V
o
is the inner potential taken as 11.8 eV
for Bi
2
Se
3
37.
TEM and STM microscopy. HAADF-STEM images were acquired with Hitachi
HD2700C dedicated STEM with a probe Cs corrector operating at 200 kV at room
temperature. Samples were prepared using the in-situ lift-out method on the FEI
Helios 600 Nanolab dual-beam FIB. Final milling was completed at 2 keV. STM
(Omicron VT- STM -XA 650) experiments were performed in an ultrahigh
vacuum (UHV) system with a base pressure of 2 × 10−10 Torr at room tempera-
ture. All the STM images were observed in constant current mode using Pt/Ir tips.
All bias values in the text refer to the bias applied to the sample. The STM images
were analyzed using Gwyddion-2.55 software package. HAADF-STEM and STM
experiments were conducted at the Center for Functional Nanomaterials, Broo-
khaven National Laboratory. Samples for STM were transferred with a vacuum
suitcase.
Low-energy electron diffraction. µLEED experiment was performed at X-ray
photoemission electron microscopy/low-energy electron microscopy (XPEEM/
LEEM) endstation of the ESM beamline (21-ID-2).
Data availability
The data that support the findings of this study are available from the corresponding
author upon request.
Received: 25 November 2020; Accepted: 12 December 2020;
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Acknowledgements
This research used ESM (21-ID-1, 21-ID-2) beamline of the National Synchrotron Light
Source II, a US Department of Energy (DOE) Office of Science User Facility operated for
the DOE Office of Science by Brookhaven National Laboratory under Contract number
DE-SC0012704. This work also used the resources of the Center for Functional Nano-
materials, Brookhaven National Laboratory, which is supported by the U.S. Department
of Energy, Office of Basic Energy Sciences, under Contract number DE-SC0012704.
ARPES experiments in Hiroshima were performed with the approval of program
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COMMUNICATIONS MATERIALS | (2021) 2:11 |https://doi.org/10.1038/s43246-020-00115-w | www.nature.com/commsmat 7
advisory committee of HISOR) Proposal number 19BG041). T.Y. thanks Professor A.V.
Balatsky for useful discussions.
Author contributions
T.Y. conceived and designed the experiments. T.Y. prepared the samples and performed
the photoemission experiments with the help from K. Kaznatcheev, E.V., and B.S. E.F.S.
and K.S. performed the ARPES experiments at HISOR. X.T. conducted the STM mea-
surements. Z.D. and J.T.S. performed µLEED measurements. S.H. and K. Kisslinger
performed HAADF-STEM experiments. T.Y. analyzed the experimental results and
wrote the manuscript with contribution from E.F.S., B.S., K. Kaznatcheev, and E.V.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/s43246-
020-00115-w.
Correspondence and requests for materials should be addressed to T.Y.
Peer review information Primary handling editor: John Plummer
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