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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. However, materials with such peculiar electronic states are rare and often rely on subtle properties 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.
Observation of the flat band in the VSe2/Bi2Se3 heterostructures a Experimental electronic structures of a 12 QL Bi2Se3 sample. b–e Electronic structure of 0.3, 1, 2, and ML VSe2/12 QL Bi2Se3 heterostructures, respectively. f Electronic structure of 1 ML VSe2 grown on a HOPG substrate. Spectra in a–f were collected with 110 eV linear horizontal polarized lights along the Γ¯−M¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar {\Gamma} - \bar M$$\end{document} direction in the BZ. BVB represents the bulk valance band of Bi2Se3. g Corresponding EDCs obtained along the kx = −0.25 Å. h Photon energy-dependent EDCs of 3 ML VSe2/12 QL Bi2Se3 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, j Constant energy counters at the EF and the EFB for 1 ML VSe2/12 QL Bi2Se3 sample, respectively. The momentum maps were recorded with 169 eV photons and the corresponding ARPES map can be found in Supplementary Fig. 4. k Wide momentum window ARPES map of the 1 ML VSe2/12 QL Bi2Se3 sample obtained with 220 eV photon energy along the Γ¯−M¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar {\Gamma} - \bar M$$\end{document} direction of the BZ. The EDC on the top of k is obtained at the EF. Blue and red hexagons in i, j correspond to the BZ of VSe2 and Bi2Se3, respectively. In-plane lattice parameters of 4.14 Å for Bi2Se3 and 3.356 Å for VSe2 were employed to compute the BZs27,31.
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ARTICLE
Emergent at 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 at band in a VSe
2
/Bi
2
Se
3
heterostructure. Our photoemission study
demonstrates that the at 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 at
band even though they overlap in proximity of the Dirac point. These features make this at
band distinguishable from the ones previously found. Thereby, the observation of a at 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
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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 at band, which is predicted to
host high-temperature superconductivity14, fractional quantum
Hall effect5,6, and ferromagnetism79. In superconductors, a at
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 graphene1214. Other examples of at band materials are
Kagome lattices in which the at band stems from destructive
quantum interference due to the frustrated lattice geometry7,8.
The Kagome-type at 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)810.
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 at band transition metal
dichalcogenides (TMDs) through the formation of the Moiré
superlattices, which could support strongly correlated physics at
higher temperatures1517. All these previous works conclude that
the at 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 at band
in the electronic states. This at 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 at band reverses the sign at several points within
the BZ. Another notable observation is that the VSe
2
overlayer
and the emergence of the at 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 at 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-eld (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 mist between the in-plane lattice parameters of the lm
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
fourve-unit cells of Bi
2
Se
3
(4a
BS
=16.56 Å or 5a
BS
=20.7 Å)
and ve-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 ndings24.
Observation of the at 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. 2bd 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
,aat
band at 0.47 eV binding energy and with a ~0.18 eV bandwidth
emerges in the surface electronic structure (Fig. 2b). The atness
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 at 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 at 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 at 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 at 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 Å1from each ARPES
maps. The peak stemming from the at band can be seen up to 2
ML VSe
2
, while it is not visible for thicker lms. 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 conrmed by taking advantage of energy
dependency of the electron mean free path. In Fig. 2h, the EDCs
at k
x
=0.3 Å1for 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 at band
argument.
Another interesting observation is that the at band overlaps with
the lower branch of the Dirac cone in the vicinity of k
x
=
1
without inducing any prominent change in its spectral shape. This
canbeevenbetterseeninthelms with the thicker VSe
2
coverage
conrming that the at band, DSSs, and the dispersive V 3dstate of
VSe
2
coexist in the surface electronic structure (Fig. 2bdand
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 at band, a k
x
k
y
intensity plots at E
F
and at the binding energy of the at 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 ower-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 at band lls 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 xed k
x
and k
y
momentum
all show the existence of the at band. Such electronic state spread
over a large momentum area can signicantly 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 mist 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 at 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 Å1yielding 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 signi-
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
=
1
originates from the bottom of the Dirac cone of Bi
2
Se
3
. Away
from the k
y
=
1, the plot has non-vanishing spectral intensity
contributed by the at 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 nite density of
states along the k
y
momentum direction. This implies the dis-
persionless nature of the at band along the k
z
momentum
direction. To further validate this observation, we present the
binding energyk
z
plots along the k
y
=±0.25 Å1in Fig. 3d, e,
respectively. The plots clearly show that the at band at 0.47 eV
binding energy is k
z
independent conrming 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 at band, Fig. 3f depicts the EDCs taken
at different k
z
points. One can see that the EDC of the at 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 spinorbit
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
ðÞ½where 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. 4ac. 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 at band in the VSe
2
/Bi
2
Se
3
heterostructures. a Experimental electronic structures of a 12 QL Bi
2
Se
3
sample. beElectronic
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
afwere 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
=
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 at 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
=
1and k
y
=±0.5 Å1, and the maxima at
k
y
=±0.25 Å1. This shows that similar to the DSSs, the CD in
the at 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
=
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 nal state effect in the
photoemission process29. This was discussed in ref. 30 with details
where they propose the non-trivial connection between the
spinorbit texture and the CD signal. Thereby, the helical CD
texture and the nodal line band suggest that the at band could be
topologically non-trivial.
Temperature-dependent electronic structure. To further study
the nature of the at 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 Å1, respectively. Dashed
cyan colored lines in dand erepresent the dispersion of the at band along the k
z
direction. fEDCs at various k
z
points to study the spectral shape of the
at 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. 5ad
and 0.3 ML VSe
2
/12 QL Bi
2
Se
3
in Fig. 5fi 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 at. This can be seen in the EDCs obtained along the k
y
=
0.25 Å1from the ARPES maps (Fig. 5ej). It is clear from the
data that temperature does not affect the line shape or the binding
energy of the at band. At any given temperature, the at 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 at 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 at 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 at 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 signicantly 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 at 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 at 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 at 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 at band on the surface
electronic structure on this heterostructure. One of the rst scenario
to consider as the origin of the at 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 at 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 ffiffi
3
p×ffiffi
3
p
silicene superstructure by STS where the local density of states
forms the electronic Kagome lattice36. Interface dislocation or strain
can also atten the original bands by introducing pseudo-magnetic
eld term to the Hamiltonian in Moiré superstructures4.Further-
more, a pronounced band attening 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 at 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 at 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 at band can be
resolved with higher photon energies on thicker VSe
2
lms. As a
rst example of the dispersionless electronic excitation in a
topologically non-trivial band structure, our results could open a
new pathway in the critical eld 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. adARPES 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 Å1from each spectrum. fjSame as aebut 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 rst degassed at 550 oC for three hours and ashed
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 lm 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 × 1010 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 lms 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 nal 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 × 1010 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 ndings 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) Ofce of Science User Facility operated for
the DOE Ofce 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, Ofce of Basic Energy Sciences, under Contract number DE-SC0012704.
ARPES experiments in Hiroshima were performed with the approval of program
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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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... In van der Waals heterostructures containing 1T -VSe 2 , signatures of band hybridization and strong correlations have been observed in the form of flat bands [17] and spectral kinks [18]. ...
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A fascinating transition-metal dichalcogenide (TMDC) compound, MoSe2, has attracted a lot of interest in electrochemical, photocatalytic, and optoelectronic systems. However, detailed studies on the structural stability of the various MoSe2 polymorphs are still lacking. For the first time, the relative stability of 11 different MoSe2 polymorphs (1H, 2H, 3Ha, 3Hb, 2T, 4T, 2R1, 1T1, 1T2, 3T, and 2R2) is proposed, and a detailed analysis of these polymorphs is carried out by employing the first-principles calculations based on density functional theory (DFT). We computed the physical properties of the polymorphs such as band structure, phonon, and elastic constants to examine the viability for real-world applications. The electronic properties of the involved polymorphs were calculated by employing the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06). The energy band gap of the polymorphs (1H, 2H, 3Ha, 3Hb, 2T, 4T, and 2R1) is in the range of 1.6-1.8 eV, coinciding with the experimental value for the polymorph 2H. The covalent bonding nature of MoSe2 is analyzed from the charge density, charge transfer, and electron localization function. Among the 11 polymorphs, 1H, 2H, 2T, and 3Hb polymorphs are predicted as stable polymorphs based on the calculation of the mechanical and dynamical properties. Even though the 4T and 3Ha polymorphs' phonons are stable, they are mechanically unstable; hence, they are considered to be under a metastable condition. Additionally, we computed the direction-dependent elastic moduli and isotropic factors for both mechanically and dynamically stable polymorphs. Stable polymorphs are analyzed spectroscopically using IR and Raman spectra. The thermal stability of the polymorphs is also studied.
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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.
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A kagome lattice of 3d transition metal ions is a versatile platform for correlated topological phases hosting symmetry-protected electronic excitations and magnetic ground states. However, the paradigmatic states of the idealized two-dimensional kagome lattice—Dirac fermions and flat bands—have not been simultaneously observed. Here, we use angle-resolved photoemission spectroscopy and de Haas–van Alphen quantum oscillations to reveal coexisting surface and bulk Dirac fermions as well as flat bands in the antiferromagnetic kagome metal FeSn, which has spatially decoupled kagome planes. Our band structure calculations and matrix element simulations demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals, and evidence that the coexisting Dirac surface state realizes a rare example of fully spin-polarized two-dimensional Dirac fermions due to spin-layer locking in FeSn. The prospect to harness these prototypical excitations in a kagome lattice is a frontier of great promise at the confluence of topology, magnetism and strongly correlated physics. A prototypical kagome metal with magnetic and topological properties is identified.
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Electronic systems with flat bands are predicted to be a fertile ground for hosting emergent phenomena including unconventional magnetism and superconductivity 1–15 , but materials that manifest this feature are rare. Here, we use scanning tunnelling microscopy to elucidate the atomically resolved electronic states and their magnetic response in the kagome magnet Co 3 Sn 2 S 2 (refs. 16–20 ). We observe a pronounced peak at the Fermi level, which we identify as arising from the kinetically frustrated kagome flat band. On increasing the magnetic field up to ±8 T, this state exhibits an anomalous magnetization-polarized many-body Zeeman shift, dominated by an orbital moment that is opposite to the field direction. Such negative magnetism is induced by spin–orbit-coupling quantum phase effects 21–25 tied to non-trivial flat band systems. We image the flat band peak, resolve the associated negative magnetism and provide its connection to the Berry curvature field, showing that Co 3 Sn 2 S 2 is a rare example of a kagome magnet where the low-energy physics can be dominated by the spin–orbit-coupled flat band. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.
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We propose a novel mechanism of flat band formation based on the relative biasing of only one sublattice against other sublattices in a honeycomb lattice bilayer. The mechanism allows modification of the band dispersion from parabolic to "Mexican hat"-like through the formation of a flattened band. The mechanism is well applicable for bilayer graphene-both doped and undoped. By angle-resolved photoemission from bilayer graphene on SiC, we demonstrate the possibility of realizing this extremely flattened band (< 2-meV dispersion), which extends two-dimensionally in a k-space area around the K ¯ point and results in a disk-like constant energy cut. We argue that our two-dimensional flat band model and the experimental results have the potential to contribute to achieving superconductivity of graphene- or graphite-based systems at elevated temperatures.
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A flatband representing a highly degenerate and dispersionless manifold state of electrons may offer unique opportunities for the emergence of exotic quantum phases. To date, definitive experimental demonstrations of flatbands remain to be accomplished in realistic materials. Here, we present the first experimental observation of a striking flatband near the Fermi level in the layered Fe3Sn2 crystal consisting of two Fe kagome lattices separated by a Sn spacing layer. The band flatness is attributed to the local destructive interferences of Bloch wave functions within the kagome lattices, as confirmed through theoretical calculations and modelings. We also establish high-temperature ferromagnetic ordering in the system and interpret the observed collective phenomenon as a consequence of the synergetic effect of electron correlation and the peculiar lattice geometry. Specifically, local spin moments formed by intramolecular exchange interaction are ferromagnetically coupled through a unique network of the hexagonal units in the kagome lattice. Our findings have important implications to exploit emergent flat-band physics in special lattice geometries.
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Emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed. However, the role of these phenomena in shaping the electronic properties in van der Waals heterointerfaces has hitherto not been established. By reducing the material thickness and forming a heterointerface, we find two types of charge-ordering transitions in monolayer VSe2 on graphene substrates. Angle-resolved photoemission spectroscopy (ARPES) uncovers that Fermi-surface nesting becomes perfect in ML VSe2. Renormalization group analysis confirms that imperfect nesting in three dimensions universally flows into perfect nesting in two dimensions. As a result, the charge density wave transition temperature is dramatically enhanced to a value of 350 K compared to the 105 K in bulk VSe2. More interestingly, ARPES and scanning tunneling microscopy measurements confirm an unexpected metal-insulator transition at 135 K, driven by lattice distortions. The heterointerface plays an important role in driving this novel metal-insulator transition in the family of monolayered transition metal dichalcogenides.
Book
Physics at Surfaces is a unique graduate-level introduction to the physics and chemical physics of solid surfaces, and atoms and molecules that interact with solid surfaces. A subject of keen scientific inquiry since the last century, surface physics emerged as an independent discipline only in the late 1960s as a result of the development of ultra-high vacuum technology and high speed digital computers. With these tools, reliable experimental measurements and theoretical calculations could at last be compared. Progress in the last decade has been truly striking. This volume provides a synthesis of the entire field of surface physics from the perspective of a modern condensed matter physicist with a healthy interest in chemical physics. The exposition intertwines experiment and theory whenever possible, although there is little detailed discussion of technique. This much-needed text will be invaluable to graduate students and researchers in condensed matter physics, physical chemistry and materials science working in, or taking graduate courses in, surface science.
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In this article, we report a comparative study of the electronic structure of Cr-doped and pristine Bi2Se3. Circular dichroism and photon-energy-dependent angle-resolved photoemission experiments were performed. Even though the surface states seen on the Cr-doped samples are gapped, they exhibit strong circular dichroism, for which we provide its origin in accordance with the nontrivial band structure of the bulk. The surface electronic structure measurements with linear and circular polarized light show signatures that the orbital composition of the surface states is changed with Cr doping. Our observations not only provide further spectroscopic information about topological materials, but also promotes an alternative experimental tool to control their spin-orbital texture.
Article
We study the effects of heterostrain on moiré bands in twisted bilayer graphene and bilayer transition metal dichalcogenide (TMD) systems. For bilayer graphene with a twist angle near 1∘, we show that heterostrain significantly increases the energy separation between conduction and valence bands as well as the Dirac velocity at charge neutrality, which resolves several puzzles in scanning tunneling spectroscopy and quantum oscillation experiments at once. For bilayer TMD, we show that applying small heterostrain generally leads to flat moiré bands that are highly tunable.
Article
Single layers of transition metal dichalcogenides (TMDCs) are excellent candidates for electronic applications beyond the graphene platform; many of them exhibit novel properties including charge density waves (CDWs) and magnetic ordering. CDWs in these single layers are generally a planar projection of the corresponding bulk CDWs because of the quasi-two-dimensional nature of TMDCs; a different CDW symmetry is unexpected. We report herein the successful creation of pristine single-layer VSe2, which shows a (7×3) CDW in contrast to the (4×4) CDW for the layers in bulk VSe2. Angle-resolved photoemission spectroscopy from the single layer shows a sizable (7×3) CDW gap of ∼100 meV at the zone boundary, a 220 K CDW transition temperature twice the bulk value, and no ferromagnetic exchange splitting as predicted by theory. This robust CDW with an exotic broken symmetry as the ground state is explained via a first-principles analysis. The results illustrate a unique CDW phenomenon in the two-dimensional limit.