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 phases^{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20}. 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 field^11,12,13,14. 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.

某些扭曲的范德华异质结构中出现的平坦带提供了一种环境，其中电子之间的强相互作用导致各种相关的相^{1,2,3,4,5,6,7,8,9,10,11， 12,13,14,15,16,17,18,19,20}。特别是，双螺旋双层石墨烯主体的异质结构关联了绝缘状态，可以通过扭转角和外部电场来调节绝缘状态^11,12,13,14。在这里，我们报告了扭曲的双层双层石墨烯的电输运测量，我们在相图中检查了自发对称破坏的基本作用。随着温度的降低，每个相关绝缘体附近的金属态的电阻率都会突然下降，并具有相关的非线性电流-电压特性。尽管在质量上与超导性相似，但霍尔系数的符号同时发生逆转，而是随着突然电阻率的下降而自发对称性破坏，而焦耳热似乎是非线性传输的基础。我们的结果表明，类似的机制可能与更广泛的半导体平带范德华异质结构有关。