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## Creator

S. Akatsuka, M. Sakano, T. Yamamoto, T. Nomoto, R. Arita, R. Murata, T. Sasagawa, [K. Watanabe](https://orcid.org/0000-0003-3701-8119), [T. Taniguchi](https://orcid.org/0000-0002-1467-3105), N. Mitsuishi, M. Kitamura, K. Horiba, K. Sugawara, S. Souma, T. Sato, H. Kumigashira, K. Shinokita, H. Wang, K. Matsuda, S. Masubuchi, T. Machida, K. Ishizaka

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[180 °-twisted bilayer <math>  <msub>    <mi>ReSe</mi>    <mn>2</mn>  </msub></math> as an artificial noncentrosymmetric semiconductor](https://mdr.nims.go.jp/datasets/b8b71147-8b2a-4933-82df-52a38757385b)

## Fulltext

180&thinsp;&deg;-twisted bilayer ${\rm ReSe}_{2}$ as an artificial noncentrosymmetric semiconductorPHYSICAL REVIEW RESEARCH 6, L022048 (2024)Letter180 °-twisted bilayer ReSe2 as an artificial noncentrosymmetric semiconductorS. Akatsuka,1 M. Sakano ,1 T. Yamamoto ,1 T. Nomoto ,2 R. Arita ,2,3 R. Murata,4 T. Sasagawa,4 K. Watanabe ,5T. Taniguchi ,6 N. Mitsuishi ,3 M. Kitamura,7,8 K. Horiba,8 K. Sugawara,9,10 S. Souma,9 T. Sato ,9 H. Kumigashira ,11K. Shinokita ,12 H. Wang,12 K. Matsuda ,12 S. Masubuchi ,13 T. Machida,13 and K. Ishizaka1,31Quantum-Phase Electronics Center and Department of Applied Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan2Research Center for Advanced Science and Technology, The University of Tokyo, Meguro-ku, Tokyo 153-8904, Japan3RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan4Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan5Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan6Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan7Photon Factory, Institute of Materials Structure Science, High energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan8Institute for Advanced Synchrotron Light Source, National Institute for Quantum Science and Technology (QST), Sendai 980-8579, Japan9Department of Physics, Graduate School of Science, and Advanced Institute for Materials Research (WPI-AIMR),Tohoku University, Sendai 980-8578, Japan10Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST),Tokyo 102-0076, Japan11Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980-8577, Japan12Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan13Institute of Industrial Science, The University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan(Received 24 September 2023; revised 4 February 2024; accepted 25 April 2024; published 3 June 2024)We have fabricated a 180 °-twisted bilayer ReSe2 by stacking two centrosymmetric monolayer ReSe2 flakes inopposite directions, which is expected to cause the loss of spatial inversion symmetry. We successfully observedspatial inversion-symmetry breaking, in contrast to the monolayer and natural bilayer ReSe2 by the secondharmonic generation. ARPES measurements further revealed emergent band dispersions in the 180 °-twistedbilayer ReSe2, distinct from those of the monolayer ReSe2 used in its fabrication. The band calculation showsthe finite lifting of spin degeneracy (∼50 meV) distinct from natural monolayer and bilayer ReSe2, whichdemonstrates that the spin-momentum locked state leading to Berry curvature related phenomena can be realizedeven with the stacking of centrosymmetric monolayers.DOI: 10.1103/PhysRevResearch.6.L022048Advances in the fabrication techniques for exfoliated two-dimensional flakes and their van der Waals heterostructureshave provided a platform for manipulating material sym-metries through stacking order [1–4]. Among the varioussymmetries, spatial inversion symmetry is important in de-termining electronic structure and physical properties. One ofthe most representative examples is group VI transition metaldichalcogenide (TMD) semiconductors such as MoS2. Mono-layer MoS2 has a noncentrosymmetric crystal structure withthreefold rotational symmetry, which lifts the spin degeneracyat the Brillouin zone corners by spin-orbit interactions [5–8],leading to Berry curvature related phenomena appearing innonlinear transport properties and optoelectronic properties[5–12]. The bilayer system has two distinct stacking orders:2H and 3R type, in which the adjacent layers are stacked with180 ° and 0 ° twisting [13]. The former recovers the spatialPublished by the American Physical Society under the terms of theCreative Commons Attribution 4.0 International license. Furtherdistribution of this work must maintain attribution to the author(s)and the published article’s title, journal citation, and DOI.inversion symmetry, and the net spin polarization cancels out[13,14]. The latter maintains the breaking of spatial inversionsymmetry [13] and, recently, emergent ferroelectricities in0 °-stacked bilayer systems have been reported [3,4].On the other hand, there are examples where the stackingorder, regardless of the stacking of centrosymmetric monolay-ers, breaks the spatial inversion symmetry in a bilayer system.Td -WTe2 [15–22] is a well-known striking material. The crys-tal structure of the monolayer WTe2 is centrosymmetric andis classified as a distorted 1T type (CdI2 type) in the TMDfamily, which is defined by a network of edge-sharing WTe6octahedra [Fig. 1(a)]. The distinctive feature of the crystalsymmetry of bulk Td -WTe2 is that adjacent layers stack inopposite directions in nature, in contrast to the usual 1T -typebulk TMD families (e.g., HfSe2 [23–25], TiSe2 [23,24,26],and ReSe2 [27]) which have spatial inversion symmetry inde-pendent of the number of layers. This 180 °-twisted stackingbreaks the spatial inversion symmetry in bilayer WTe2, wherethe spin splitting of approximately 0.1 eV has been observedby microfocused angle-resolved photoemission spectroscopy(μ-ARPES) [19,22]. Inspired by these previous studies, wecan expect that, in principle, the magnitude of spin splitting2643-1564/2024/6(2)/L022048(6) L022048-1 Published by the American Physical Societyhttps://orcid.org/0000-0002-2099-5506https://orcid.org/0000-0002-8148-2616https://orcid.org/0000-0002-4333-6773https://orcid.org/0000-0001-5725-072Xhttps://orcid.org/0000-0003-3701-8119https://orcid.org/0000-0002-1467-3105https://orcid.org/0000-0002-1120-9577https://orcid.org/0000-0002-4544-5463https://orcid.org/0000-0003-4668-2695https://orcid.org/0000-0002-7752-3251https://orcid.org/0000-0002-3990-8484https://orcid.org/0000-0001-7039-6694https://ror.org/057zh3y96https://ror.org/057zh3y96https://ror.org/03gv2xk61https://ror.org/0112mx960https://ror.org/026v1ze26https://ror.org/026v1ze26https://ror.org/01g5y5k24https://ror.org/020rbyg91https://ror.org/01dq60k83https://ror.org/00097mb19https://ror.org/01dq60k83https://ror.org/02kpeqv85https://ror.org/057zh3y96https://crossmark.crossref.org/dialog/?doi=10.1103/PhysRevResearch.6.L022048&domain=pdf&date_stamp=2024-06-03https://doi.org/10.1103/PhysRevResearch.6.L022048https://creativecommons.org/licenses/by/4.0/S. AKATSUKA et al. PHYSICAL REVIEW RESEARCH 6, L022048 (2024)FIG. 1. (a) The typical nondistorted octahedral coordination forthe T-type TMD. The yellow (orange) triangle indicates the ori-entation of the top (bottom) Se triangular networks. (b), (c) Sideview (b) and top view (c) of the distorted T-type crystal structure ofReSe2. The black frames in (c) indicate a unit cell for the monolayer(1L) ReSe2. The light blue segments represent the Re zigzag-chain-like structure. (d) Schematic views of the crystal symmetries for1L, bilayer (2L), and 180 °-twisted 2L ReSe2. The black rectanglerepresents each unit of the ReSe2 layer. The black circles representthe inversion centers. The dotted circle depicted for the antiparallel2L ReSe2 represents a lack of the inversion center by their stackingorder. (e) Two-dimensional Brillouin zone for the 1L, 2L, and 180 °-twisted 2L ReSe2. The Brillouin zone is slightly shear deformedfrom a regular hexagon. However, in this study, we represent �-K(�-M) as the direction parallel (perpendicular) to the Re zigzag chainbecause the actual high-symmetrical K point (M point) locates only0.008 (0.013) Å−1 off the kx (ky) axis.could be controlled by artificially changing the stacking angle,even when stacking with centrosymmetric 1T -type TMDmonolayers.In this study, we focus on the layered semiconductorReSe2. Reflecting the strong spin-orbit interaction of theRe 5d orbitals, the effect of inversion-symmetry breaking isexpected to appear in the nonlinear optical properties andelectronic band dispersions. The direct and indirect band gapsof bulk ReSe2 are 1.40 eV [28] and 1.18–1.19 eV [29,30],respectively. ReSe2 is known to have a weak interlayer cou-pling, making it possible to fabricate down to a monolayerby mechanical exfoliation [31–34] with an exitonic directband gap of 1.50 eV [32]. The overall electronic structuredoes not change significantly regardless of the number oflayers, indicating weak van der Waals interactions betweenthe layers [32,34,35]. ReSe2 has a distorted 1T -type tricliniccrystal structure (space group P1̄), as shown in Figs. 1(a)–1(c). The opposing Se-triangle networks (shown in yellowand orange) sandwich the distorted Re layer characterizedby Re zigzag chains (indicated by the light blue lines). Asshown in Fig. 1(d), we schematically illustrate the concept ofthis study for the artificial fabrication of noncentrosymmetricbilayer ReSe2. The essence extracted from the crystal struc-ture of monolayer (1L) ReSe2 is represented by two opposingSe-triangular networks (yellow and orange triangles) and theinversion center (black circle). In the natural bilayer case, aninversion center appeared between the layers. However, in theFIG. 2. (a)–(c) Optical microscope images of the samples forARPES and SHG measurement. The inset in (a) shows a schematicof the sample. The black segments represent 10 µm. The monolayerReSe2 flakes used are outlined with orange and green broken lines.Orange circles indicate the SHG measurement position. (d)–(f) Polarplot of the SH intensity from 1L, 2L, and 180 °-twisted ReSe2.Linear-polarized component of the SHG parallel to the linear polar-ization of the incident light is detected. For (f), the data between 0 °and 180 ° are symmetrized and displayed in the region between 180 °and 360 °. (g)–(i) The calculated second order electrical susceptibil-ity χxxx and χyyy for 1L, 2L, and 180 °-twisted ReSe2.180 °-twisted 2L ReSe2, the triangles face the same directionacross the interlayer, such that the inversion center does notappear anywhere between the layers, shown as a dotted circle.Although there are translational degrees of freedom uponstacking in the x and y directions, the fabricated structurecannot have an inversion center in any cases. In 180 °-twisted2L ReSe2, spin degeneracy is expected to be lifted in recip-rocal space owing to spin-orbit interaction, which has thepotential to give rise to physical properties related to the Berrycurvature, such as the shift current [11], nonlinear Hall effect[12], and ferroelectric switching [18].The 180 °-twisted 2L ReSe2 sample was fabricated usingan all-dry pick-up [36,37], tear and stack [38], and flip method[39] using an Elvacite 2552C copolymer inside a glove boxchamber [40]. In addition, we also prepared the monolayerand natural bilayer ReSe2 samples for control experiments.During fabrication, hexagonal boron nitride (hBN), graphite,and ReSe2 flakes were sequentially picked using a polymerstamp. The assembled heterostructure was transferred to an-other polymer stamp at room temperature to turn it over.Then it was dropped onto a SiO2/Si substrate with a prepat-terned metal electrode, as schematically shown in the insetof Fig. 2(a). Optical microscope images for 1L, 2L, and180 °-twisted 2L ReSe2 are shown in Figs. 2(a)–2(c), respec-tively. The ReSe2 flakes were approximately 10 µm in size,L022048-2180 °-TWISTED BILAYER ReSe2 AS … PHYSICAL REVIEW RESEARCH 6, L022048 (2024)outlined with orange and green broken lines. In this study, weperformed optical second harmonic generation (SHG) mea-surements at room temperature [40] to examine the breakingof the spatial inversion symmetry. In addition, to observethe emergent electronic band dispersions in 180 °-twisted 2LReSe2 we performed μ-ARPES measurements using a photonenergy of 100 eV and a spot size of 12×15 µm2 at BL28 in thePhoton Factory, KEK [41]. The total energy resolution was setat 35 meV. During the measurement, a sample manipulatortemperature was kept below 20 K. The Fermi levels (EF) weredetermined using polycrystalline gold electrically connectedto the respective samples. The band structure calculations [40]were performed using the Vienna Ab initio Simulation Pack-age (VASP) [42,43]. The crystal structures were determinedby structure optimization taking into account the van derWaals correction with the density functional theory (DFT)-D3 method [44]. The second-order susceptibility calculationswere performed based on the tight-binding model constructedfrom the DFT electronic structures through the WANNIER90code [45].Figures 2(d)–2(f) show the polar-angle dependences ofthe normalized SHG signals from the 1L, 2L, and 180 °-twisted 2L ReSe2 samples. The orange circles indicate themeasurement areas on the topmost ReSe2 flakes (depictedwith the orange frames). The SHG signals are detected onlyfrom 180 °-twisted 2L; however, they are negligible from 1Land 2L reflecting the spatial inversion symmetry of natu-ral ReSe2 crystal independent of the number of layers. (SeeSupplemental Material [40] for the detailed SHG simulationsand measurements.) These experimental results are consistentwith the description using simple structure models shown inFig. 1(d). To compare with the experimental observation, wecalculated second-order susceptibility χxxx and χyyy, whichrepresent the SHG response parallel to the incident light po-larization [along x and y, respectively, as shown in Fig. 1(c)][40]. The energy dependences of χxxx and χyyy for 1L, 2L,and 180 °-twisted 2L are shown in Fig. 2(g). In contrast tothe negligible χxxx and χyyy for 1L and 2L, the finite χxxxand χyyy were demonstrated for 180 °-twisted 2L, indicatingthat the observed SHG signal was consistent with the designof the breaking of spatial inversion symmetry. The crystalstructure of 180 °-twisted 2L used for the calculations and thesimulated polar pattern of the SH intensity are shown in theSupplemental Material [40].We performed ARPES measurements on 1L, 2L, and 180 °-twisted 2L ReSe2 to observe their two-dimensional electronicstructures. Figures 3(a)–3(c) show the ARPES intensity map-ping at the constant energies of E − EF = −1.3 eV. Theconstant energy contours for the highest valence band (HVB)observed in Figs. 3(a)–3(c) clearly show anisotropic contoursof the isoenergetic surface that are not closed along the ky di-rection, reflecting the quasi-one-dimensional Re zigzag chainalong x as shown in Fig. 1(c). Figures 3(d)–3(f) show theARPES images along the ky (�-M, left side) and kx (�-K-M,right side) directions [Fig. 1(e)] for 1L, 2L, and 180 °-twisted2L ReSe2, respectively. Considering that the band dispersionsat ±kx (±ky) are linked via time-reversal symmetry and shouldbe the same when ignoring the spin degrees of freedom,the ARPES intensities in Figs. 3(d)–3(f) display the sums ofthem for better visibility. In the energy region from the FermiFIG. 3. (a)–(c) ARPES mapping images at a constant energy ofE − EF = −1.3 eV of 1L, 2L, and 180 °-twisted 2L ReSe2. Whitebroken lines represent the Brillouin zones for 1L, 2L, and 180 °-twisted 2L. (d)–(f) Combined ARPES images along the �-M (ky)and �-K-M (kx) directions. Those ARPES images along the kx (ky)direction are symmetrized with respect to kx (ky) = 0. Black circlemarkers represent the peak positions of the ARPES spectra for therespective highest valence bands. Red segments in (e), (f) representenergy cuts for the energy distribution curves shown in Figs. 4(c) and4(d), respectively. (g)–(i) Images obtained by the curvature analysis[46] for the ARPES images in (d)–(f). White broken lines in (g)represent the band dispersions of monolayer ReSe2 obtained from theARPES image, which are inevitably observed from the 180 °-twisted2L ReSe2 samples. (j)–(l) Calculated band dispersions of 1L, 2L,and 180 °-twisted 2L ReSe2. The origins of energy axes are set asthe maximum of the valence band (VBM). The black rectangles in(k), (l) indicate the area of the magnified view in Figs. 4(a) and 4(b),respectively.energy to E = EF − 0.8 eV, which is omitted in Fig. 3, nobands originating from ReSe2 are observed, whereas there arecertain signals from graphite. This is consistent with previousstudies of 1L and 2L ReSe2 [33,34]. For better visualizationof the band dispersions, curvature plots [46] for the respectiveARPES images are shown in Figs. 3(g)–3(i). Comparing 1L,2L, and 180 °-twisted 2L, a clear difference became apparentwhen focusing on the HVB. A recent precise ARPES studyrevealed that the valence band maximum (VBM) of bulkReSe2 is located off the high symmetry point along the kydirection (ky ∼ 0.15 Å−1) at kz = π/c with upward convex-shaped dispersions [47,48]. As discussed below, the ARPESL022048-3S. AKATSUKA et al. PHYSICAL REVIEW RESEARCH 6, L022048 (2024)results demonstrate that the two-dimensional confinement ofthe electronic structure causes a shift in the position of theVBM. The detailed analysis of our result on 1L ReSe2 showsthat the HVB is almost flat with 40 meV dispersion withinthe k = ±0.15 Å−1 range around the � point, as indicatedby the markers in Fig. 3(d) that represent the positions ofthe intensity peaks. In contrast to 1L ReSe2, 2L and 180 °-twisted 2L ReSe2 exhibited upward convex-shaped HVBswith the VBM at the � point as shown in Figs. 3(e) and 3(f).Figures 3(j)–3(l) show the band dispersions obtained fromfirst-principles band calculations with optimizing the crystalstructure [40] for 1L, 2L, and 180 °-twisted 2L ReSe2, re-spectively. Compared to the calculation results using the fixedatomic coordinates of bulk crystal [27,40], the calculationwith structural optimization tends to form flatter HVBs aroundthe � point, which reproduces the observed band dispersionsfor 1L and 2L ReSe2. Although the precise crystal structureof the 180 °-twisted 2L ReSe2 remains undetermined, thecalculation result assuming one of the possible crystal struc-tures [40] appears plausible, as it also reproduces the ARPESresults well.Subsequently, we discuss the effect of 180 °-twisted stack-ing on band dispersions in ReSe2. In Figs. 3(j)–3(l), we canobserve that the number of bands in the band calculationsdoubles from 1L to 2L, and from 2L to 180 °-twisted 2LReSe2, respectively. The former was owing to the bilayer split-ting caused by doubling the number of atoms in a unit cell.However, the latter doubling was due to the breaking of spatialinversion symmetry at 180 °-twisted 2L induced by staggeredstacking [Fig. 1(d)], leading to the lifting of spin degener-acy. The maximum spin-splitting energy was estimated to beapproximately 70 meV in this calculation. To evaluate the pos-sible spin-split band dispersions appearing in the experimentalresults, we reviewed the ARPES images of the 180 °-twisted2L ReSe2 in Figs. 3(f) and 3(i). Note that the ARPES in-tensities from the nonoverlapped and/or nonhybridized 1LReSe2 flakes were inevitable because the beam-spot size wascomparable to the sample size. In Fig. 3(f), we show a guidefor the eyes depicted with broken white curves representingthe band dispersions of 1L ReSe2 extracted from the ARPESimage in Fig. 3(d). In addition to the HVB, a band dispersionappearing in 180 °-twisted 2L ReSe2 as indicated by a whitearrow in Fig. 3(i) is observed around E − EF = −1.9 eV atthe � point where the band dispersion does not exist inthe 1L ReSe2 [Fig. 3(f)]. In a comparison of the calculatedband dispersions between 2L and 180 °-twisted 2L ReSe2 inFigs. 3(k) and 3(l), the corresponding band dispersions arewell isolated from the other band dispersions. As expectedfrom the symmetry requirement shown in Fig. 1(d), only180 °-twisted 2L ReSe2 [Fig. 3(l)] exhibited spin-split banddispersion.Finally, we focus on the experimental results of the banddispersion at 180 °-twisted 2L ReSe2 to examine whetherthere is a footprint of spin-split band dispersion. Magnifiedviews of the calculated band dispersions of 2L and 180 °-twisted 2L ReSe2, corresponding to the area indicated by theblack rectangles in Figs. 3(k) and 3(l), are shown in Figs. 4(a)and 4(b), respectively. Here, we can see band splitting withlifting of the spin degeneracy only for 180 °-twisted 2L ReSe2.Spin splitting is larger in the ky direction, with a maximum ofFIG. 4. (a), (b) Enlarged views of the calculational band disper-sion in 2L and 180 °-twisted 2L ReSe2. The corresponding regionsare indicated by the black rectangle in Figs. 3(k) and 3(l). (c), (d)EDCs extracted from the ARPES images of 2L and 180 °-twisted2L ReSe2 as denoted by the red segments in Figs. 3(e) and 3(f),respectively. Black solid circle markers represent the peak positions.Open circle markers in (c) represent peak positions from the areawith different possible chemical potentials. The blue curve shows theEDC at k = 0 is replaced to indicate the broadening of the spectrumof kx = 0.12 as a representative. The thinner blue curve representsthe EDC at k = 0, shifted by 27 meV and reused to suggest thebroadening of the spectrum at kx = 0.08 Å−1 as a representativeexample.approximately 50 meV. We performed calculations for threedifferent x, y shifts on stacking and identified that the energiesof spin splitting were almost similar [40]. The energy distribu-tion curves (EDCs) extracted from the ARPES images at 2Land 180 °-twisted 2L ReSe2 in Figs. 3(e) and 3(f) are shownin Figs. 4(c) and 4(d), respectively. The corresponding energycuts from k = −0.12 to 0.12 Å−1 are indicated by the red linesin Figs. 3(e) and 3(f). Regarding the EDCs of 180 °-twisted 2LReSe2, when comparing the EDC spectra at the � point andaway from the � point, it appears that the latter exhibited asomewhat broader shape. (For comparison, the EDC at kx =0.08 Å−1 is overlaid on the EDC at kx = 0, depicted in blue, asa representative example.) In contrast, the EDCs of 2L ReSe2also exhibited broad two-peak-like structures but rather withparallel dispersions, as indicated by the black solid and opencircle markers. The weak peaks indicated by open circles donot appear in the calculation and their origin is not clear, butconsidering a rather broad and hazy ARPES image obtainedfrom 2L ReSe2, they may be attributed to ARPES intensitiesoriginating from inhomogeneous areas, e.g., with differentchemical potentials. To conclusively determine the presenceor absence of the approximately 50 meV band splitting, asL022048-4180 °-TWISTED BILAYER ReSe2 AS … PHYSICAL REVIEW RESEARCH 6, L022048 (2024)expected by combining SHG experiments and first-principlesband calculations, future challenges for this study include di-rect observations utilizing a nanofocused ARPES experimentwith a smaller spot size and/or a microfocused spin-resolvedARPES experiment.In summary, we fabricated a 180 °-twisted 2L ReSe2 withthe breaking of spatial inversion symmetry, whereas 1L andnatural 2L ReSe2 possessed symmetry. We have detected theSHG signal only in the 180 °-twisted 2L ReSe2, in goodagreement with the calculation, indicating the breaking of thespatial inversion symmetry. Our ARPES study also indicatedthe stacking-dependent emergent electronic band dispersionsin these ReSe2 thin flakes. This study successfully pavesthe way for the artificial creation of noncentrosymmetrictwo-dimensional systems by stacking inversion-symmetrictwo-dimensional crystals.This research was partly supported by a CREST project(Grants No. JPMJCR18T1 and No. JPMJCR20B4) fromthe Japan Science and Technology Agency (JST), JapanSociety for the Promotion of Science KAKENHI (Grants-in-Aid for Scientific Research) (Grants No. JP20H01834,No. JP20H05664, No. JP21H01012, No. JP21H01757,No. JP21H04652, No. JP21H05232, No. JP21H05233,No. JP21H05234, No. JP21H05235, No. JP21H05236, No.JP21K18181, No. JP22K18986, No. JP23H02052, and No.JP23H05469), JST SPRING (Grant No. JPMJSP2106), andJST PRESTO (Grant No. JPMJPR20A8). K.W. and T.T.acknowledge support from World Premier International Re-search Center Initiative (WPI), MEXT, Japan. This work waspartly performed under the approval of the Photon FactoryProgram Advisory Committee (Proposal No. 2021G141, No.2023G088, No. 2018S2-001, and No. 2021S2-001).[1] A. K. Geim and I. V. Grigorieva, Van der Waals heterostruc-tures, Nature (London) 499, 419 (2013).[2] T. Akamatsu, T. Ideue, L. Zhou, Y. Dong, S. Kitamura, M.Yoshii, D. Yang, M. Onga, Y. Nakagawa, K. Watanabe, T.Taniguchi, J. Laurienzo, J. Huang, Z. Ye, T. Morimoto, H. Yuan,and Y. Iwasa, A van der Waals interface that creates in-planepolarization and a spontaneous photovoltaic effect, Science 372,68 (2021).[3] K. Yasuda, X. Wang, K. Watanabe, T. Taniguchi, and P. Jarillo-Herrero, Stacking-engineered ferroelectricity in bilayer boronnitride, Science 372, 1458 (2021).[4] X. Wang, K. Yasuda, Y. Zhang, S. Liu, K. Watanabe, T.Taniguchi, J. Hone, L. Fu, and P. 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