The flat bands that appear in some twisted van der Waals heterostructures provide a setting in which strong interactions between electrons lead to a variety of correlated phases1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20. In particular, heterostructures of twisted double bilayer graphene host correlated insulating states that can be tuned by both the twist angle and an external electric field11,12,13,14. Here, we report electrical transport measurements of twisted double bilayer graphene with which we examine the fundamental role of spontaneous symmetry breaking in its phase diagram. The metallic states near each of the correlated insulators exhibit abrupt drops in their resistivity as the temperature is lowered, along with associated nonlinear current–voltage characteristics. Despite qualitative similarities to superconductivity, the simultaneous reversals in the sign of the Hall coefficient point instead to spontaneous symmetry breaking as the origin of the abrupt resistivity drops, whereas Joule heating seems to underlie the nonlinear transport. Our results suggest that similar mechanisms are probably relevant across a broader class of semiconducting flat band van der Waals heterostructures.
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Source data are available for this paper. All other data that support the plots within this paper and other findings of this study are available from the corresponding authors on reasonable request.
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We thank C. Dean, A. Young, J.-H. Chu, D. Cobden, S. Chen, X. Liu, P. Kim, B. Lian, A. MacDonald, L. Levitov and L. Fu for helpful discussions. Technical support for the dilution refrigerator was provided by A. Manna and Z. Fei. This work was primarily supported by NSF MRSEC grant no. 1719797. M.Y. acknowledges partial support from the Army Research Office under grant no. W911NF-20-1-0211. The theoretical calculation was partially supported by DOE BES grant no. DE-SC0018171. X.X. acknowledges support from the Boeing Distinguished Professorship in Physics. X.X. and M.Y. acknowledge support from the State of Washington funded Clean Energy Institute. This work made use of a dilution refrigerator system that was provided by NSF grant no. DMR-1725221. Y. Li acknowledges the support of the China Scholarship Council. K.W. and T.T. were supported by the Elemental Strategy Initiative conducted by the MEXT, Japan, and the CREST (JPMJCR15F3), JST.
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The twist angle of each device is denoted at the top left corner of each image. All scale bars are 10 μm.
a-e, ρ as a function of n and D in all five devices acquired at B = 0 T. All are acquired at T = 2 K, except for c which is acquired at T = 20 mK. f-j, Corresponding RH for the same devices. The maps in f, h and i are antisymmetrized at ∣B∣ = 0.5 T, the map in g is antisymmetrized at ∣B∣ = 0.1 T, and the map in j is acquired at B = + 0.5 T but not antisymmetrized. k-o, Schematics of the phase diagram for each device. Blue corresponds to four-fold degeneracy, green corresponds to two-fold degeneracy, and orange indicates no remaining degeneracy. The dark gray lines denote CI states. The light gray coloring corresponds to experimentally inaccessible regions of the phase diagram.
a, ρ map surrounding the CI states in device D3 at T = 20 mK. b, ρ(T) acquired where the v = 1 CI state is most resistive. A weak metal-insulator transition is observed at T ≈ 150 mK. c, ρ(T) of the v = 3 CI state, exhibiting a metal-insulator transition at T ≈ 2.5 K.
a, ρ map in device D2 at T = 2 K, exhibiting a CI state at v = 2 and a halo feature. b, ρ(T) as a function of D at fixed v, acquired along the dashed red line in a. An arch-like feature is observed, similar to the behavior of device D3 shown in Fig. 2a. c, ρ map in device D1 at T = 2 K. A very weak CI state is observed at v = 2, as well as a distorted halo feature. d, ρ(T) acquired at the point marked by the red circle in c. Despite weaker correlations, an abrupt drop in ρ(T) is still observed to accompany the formation of the symmetry-broken halo region.
Maps of ρ at T = 2 K in devices a, D3 and b, D2, along with corresponding antisymmetrized Rxy in c and d. A small out-of-plane B component is also added in b-d in order to perform the (anti)-symmetrization.
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He, M., Li, Y., Cai, J. et al. Symmetry breaking in twisted double bilayer graphene. Nat. Phys. 17, 26–30 (2021). https://doi.org/10.1038/s41567-020-1030-6
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