Charge-neutral behavior of monolayer graphene placed on a quantum paraelectric substrate

The transport property of the charge neutral point (CNP) has attracted attention in graphene as it is strongly affected by various perturbations. Helical edge states, where spin-up and -down electrons counter-propagate with each other, have been proposed to appear in graphene at CNP when placed on SrTiO₃ under relatively weak perpendicular magnetic fields (~2 T).

Here, we report the detailed transport properties of the hexagonal boron nitride(hBN)/graphene placed on a SrTiO₃ (001) substrate. Outside of the charge neutral point, the quantum Hall effect is observed, indicating the reasonable quality of the graphene. In contrast, two-terminal resistance shows a nearly constant value of 7/2(h/e²) around CNP. This value suggests the existence of the helical edge states when considering the Hall-bar geometry. However, the resistance values at CNP vary significantly depending on the electrode combinations and magnetic field polarity, indicating nontrivial inhomogeneous distributions of edge states. Our study shows that, while graphene on SrTiO₃ may be a promising candidate to realize helical edge states in a moderate magnetic field, the inhomogeneity of the graphene/SrTiO₃ interface should be solved to fabricate quantum devices.