  We fabricate van der Waals heterostructure devices using few unit cell thick
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ for magnetotransport measurements. The
superconducting transition temperature and carrier density in atomically thin
samples can be maintained to close to that of the bulk samples. As in the bulk
sample, the sign of the Hall conductivity is found to be opposite to the normal
state near the transition temperature but with a drastic enlargement of the
region of Hall sign reversal in the temperature-magnetic field phase diagram as
the thickness of samples decreases. Quantitative analysis of the Hall sign
reversal based on the excess charge density in the vortex core and
superconducting fluctuations suggests a renormalized superconducting gap in
atomically thin samples at the 2-dimensional limit.
