Sign-Reversing Hall Effect in Atomically Thin High-Temperature Bi2.1Sr1.9CaCu2.0O8+δ Superconductors

We developed novel techniques to fabricate atomically thin ${\mathrm{Bi}}_{2.1}{\mathrm{Sr}}_{1.9}{\mathrm{CaCu}}_{2.0}{\mathrm{O}}_{8+\ensuremath{\delta}}$ van der Waals heterostructures down to two unit cells while maintaining a transition temperature ${T}_{c}$ close to the bulk, and carry out magnetotransport measurements on these van der Waals devices. We find a double sign change of the Hall resistance ${R}_{xy}$ as in the bulk system, spanning both below and above ${T}_{c}$. Further, we observe a drastic enlargement of the region of sign reversal in the temperature-magnetic field phase diagram with decreasing thickness of the device. We obtain quantitative agreement between experimental ${R}_{xy}(T,B)$ and the predictions of the vortex dynamics-based description of Hall effect in high-temperature superconductors both above and below ${T}_{c}$.