The constituent particles of matter can arrange themselves in various ways, giving rise to emergent phenomena that can be surprisingly rich and often cannot be understood by studying only the individual constituents. Discovering and understanding the emergence of such phenomena in quantum materials—especially those in which multiple degrees of freedom or energy scales are delicately balanced—is of fundamental interest to condensed-matter research1,2. Here we report on the surprising observation of emergent ferroelectricity in graphene-based moiré heterostructures. Ferroelectric materials show electrically switchable electric dipoles, which are usually formed by spatial separation between the average centres of positive and negative charge within the unit cell. On this basis, it is difficult to imagine graphene—a material composed of only carbon atoms—exhibiting ferroelectricity3. However, in this work we realize switchable ferroelectricity in Bernal-stacked bilayer graphene sandwiched between two hexagonal boron nitride layers. By introducing a moiré superlattice potential (via aligning bilayer graphene with the top and/or bottom boron nitride crystals), we observe prominent and robust hysteretic behaviour of the graphene resistance with an externally applied out-of-plane displacement field. Our systematic transport measurements reveal a rich and striking response as a function of displacement field and electron filling, and beyond the framework of conventional ferroelectrics. We further directly probe the ferroelectric polarization through a non-local monolayer graphene sensor. Our results suggest an unconventional, odd-parity electronic ordering in the bilayer graphene/boron nitride moiré system. This emergent moiré ferroelectricity may enable ultrafast, programmable and atomically thin carbon-based memory devices.
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The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.
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We thank D. Bandurin, V. Fatemi, L. Levitov, Y. Lin, J. Mundy, R. Ramesh, J. Sanchez-Yamagishi, H. Shen, J. Song, S. Todadri, A. Vishwanath and N. Yuan for discussions; and T. Dinh for initial efforts on this project. Work in the P.J.-H. group was supported by the US DOE, BES Office, Division of Materials Sciences and Engineering under award DE-SC0001819 (device fabrication and transport measurements), the Center for the Advancement of Topological Semimetals, an Energy Frontier Research Center funded by the US Department of Energy Office of Science, through the Ames Laboratory under contract DE-AC02-07CH11358 (data analysis), and the Gordon and Betty Moore Foundation’s EPiQS Initiative through grant GBMF9643 to P.J.-H. The development of new nanofabrication and characterization techniques enabling this work has been supported by the US DOE Office of Science, BES, under award DE-SC0019300. Partial support for measurement and characterization training was through AFOSR grant FA9550-16-1-0382. This work made use of the Materials Research Science and Engineering Center Shared Experimental Facilities supported by the National Science Foundation (NSF) (grant number DMR-0819762). N.G. and S.-Y.X. acknowledge support from DOE, BES DMSE (data taking and analysis), and National Science Foundation under grant number DMR-1809815 (manuscript writing). K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, grant number JPMXP0112101001, JSPS KAKENHI grant numbers JP20H00354 and the CREST(JPMJCR15F3), JST. R.A. (capacitance measurements), Z.B., Y.Z. and L.F. (theory) acknowledge support from NSF Science and Technology Center for Integrated Quantum Materials grant DMR-1231319. M.-H.L. was supported by Taiwan Ministry of Science and Technology (MOST) under grant numbers 109-2112-M-006-020-MY3 and 108-2638-M-006-002-MY2. N.M. and J.K. acknowledge the support by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under award DE-SC0020042.
The authors declare no competing interests.
Peer review information Nature thanks Kayoung Lee and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
Device schematic (a, d, g, j, n), dual-gate resistance map (b, e, h, k, o), n–D map (forward and backward) (l, p), and resistance as a function of externally applied displacement field at zero doping (c, f, i, m, q) for representative devices M1 (a–c), N0 (d–f), T1 (g–i), H1 (j–m) and H4 (n–q). The line traces along the electric-field direction are marked by black dashed lines. The superlattice resistance peaks are marked by cyan dashed lines. Note that the horizontal resistance line in h stems from a region of the sample that is only controlled by the top gate. Note that line traces in q are taken at next = 0 from the n–D map (p), whereas line traces in Fig. 1f in the main text are taken at VTG = 0 from the dual-gate map (Supplementary Fig. 25e, f), hence the difference in resistance magnitude.
a–c, Forward (a) and backward (b) scans of the four-probe longitudinal resistance as a function of VTG and VBG and their difference (c). d–f, g–i, Same measurements as in a–c, except that VBG is swept between 0 V and 50 V (d–f) and between −40 V and 0 V (g–i). We present a phenomenological model to simulate the resistance maps in Supplementary Section V.3.2.
a–d, The four-probe resistance as a function of VBG (range, −10 V to 10 V) and VTG with ranges from 0 V to 5 V (a),from 5 V to −5 V (b), from −5 V to 10 V (c), and from 10 V to −5 V (d). The scan sequences are specified in the insets.
a–c, Four-probe longitudinal resistance as a function of VTG and VBG for the forward (a) and backward (b) scans and their difference (c). d–i, Same measurements as in a–c, except that VTG is measured within 0 V to 5 V (d–f) and within −5 V to 0 V (g–i).
a, b, The resistance measured while sweeping the externally applied displacement field Dext in the forward (a) and backward (b) direction at each fixed carrier density next. The carrier density scan direction is from the negative to positive values. c–e, Carrier density extracted from Hall measurements along the lines L1 (c), L2 (d) and L3 (e) denoted in a. Red and blue curves were taken during the forward and backward scan of Dext, respectively.
Extended Data Fig. 6 Probing the out-of-plane electrical polarization using the top MLG sensor in device H4.
a, Experimental configuration for measurements of the conductance of the BLG. b, Measured conductance of the BLG as a function of VBG (top gate is grounded). The red and blue curves correspond to the forward and backward VBG scans, respectively. The vertical dashed lines denote the VBG values that correspond to the charge-neutrality point of the BLG for forward and backward scans. c, Experimental configuration for the measurements of the conductance of the tp-MLG with gate voltage VBG applied to bottom metal gate and the BLG grounded. d, Measured conductance of the tp-MLG as a function of VBG with the experimental configuration in c. The gate voltages are the same as in b (Methods). The red and blue dots denote the conductance of the tp-MLG when the BLG is charge neutral. e, Experimental configuration for the measurements of the conductance of the tp-MLG with gate voltage VBL applied to the BLG. f, Measurements of the conductance of the tp-MLG as a function of VBL with the experimental configuration in e. The conductance of the monolayer at the red and blue dots in d can be inversely mapped to two different VBL values, which corresponds to the difference of electrostatic potentials on the top surface of BLG induced by ferroelectric switching (Methods). The in-plane bias voltage VSD was kept below 1 mV for all the measurements.
a, b, Bottom capacitance Cb between the bottom gate and BLG as a function of the externally applied field, Dext as the fast-scan axis, and gate-defined carrier density, next as the slow-scan axis. The white arrows indicate the sweep direction of Dext in each panel. Deviations of the capacitance from the geometric value reflect modulations in the electronic compressibility, ∂n/∂μ, from the total area of BLG overlapping the bottom gate. Data were collected by sweeping the displacement field at each fixed carrier density, as in Fig. 2h, i. Dark features indicate regions of incompressibility resulting from the opening of a gap in the BLG. The gapless point, a compressible state with high Cb, is achieved at a finite Dext that depends on the sweep direction. c, Forward and backward traces from a and b at a fixed next. d, Resistance traces at the same density showing resistance peaks corresponding to the incompressible features in c. e, Circuit schematic of the bottom gate capacitance measurement, including a two-stage cryogenic amplifier (enclosed in dashed box). Capacitance is measured by applying a small a.c. excitation voltage to the bottom gate, δVBG, while also applying a nearly 180° out-of-phase signal, δVref, to a reference capacitor, Cref to null the voltage at the bridge balance point, (B). Deviations in the balanced signal caused by variations in compressibility are amplified by two high electron-mobility transistors and measured at the drain of the second stage, δVout. Carrier density next and external field Dext are controlled by top- and bottom-gate d.c. voltages VTG and VBG, in the same way as in the transport measurements.
a–c, Forward (red) and backward (blue) sweeps of the bottom-gate capacitance, Cb, from device H2 at fixed carrier density next with sweep rates of 2.2 mV nm−1 s−1 (a), 4.5 mV nm−1 s−1 (b), and 9.8 mV nm−1 s−1 (c). Sweep rates shown in each panel denote the rate at which the externally applied displacement field Dext/ε0 was ramped in the BN dielectric layers. No noticeable variation was observed in the capacitance features for the large range of sweep rates.
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Zheng, Z., Ma, Q., Bi, Z. et al. Unconventional ferroelectricity in moiré heterostructures. Nature 588, 71–76 (2020). https://doi.org/10.1038/s41586-020-2970-9
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