  Understanding the mechanism of high temperature (high Tc) superconductivity
is a central problem in condensed matter physics. It is often speculated that
high Tc superconductivity arises from a doped Mott insulator as described by
the Hubbard model. An exact solution of the Hubbard model, however, is
extremely challenging due to the strong electron-electron correlation.
Therefore, it is highly desirable to experimentally study a model Hubbard
system in which the unconventional superconductivity can be continuously tuned
by varying the Hubbard parameters. Here we report signatures of tunable
superconductivity in ABC-trilayer graphene (TLG) / boron nitride (hBN) moir\'e
superlattice. Unlike "magic angle" twisted bilayer graphene, theoretical
calculations show that under a vertical displacement field the ABC-TLG/hBN
heterostructure features an isolated flat valence miniband associated with a
Hubbard model on a triangular superlattice. Upon applying such a displacement
field we find experimentally that the ABC-TLG/hBN superlattice displays Mott
insulating states below 20 Kelvin at 1/4 and 1/2 fillings, corresponding to 1
and 2 holes per unit cell, respectively. Upon further cooling, signatures of
superconducting domes emerge below 1 kelvin for the electron- and hole-doped
sides of the 1/4 filling Mott state. The electronic behavior in the TLG/hBN
superlattice is expected to depend sensitively on the interplay between the
electron-electron interaction and the miniband bandwidth, which can be tuned
continuously with the displacement field D. By simply varying the D field, we
demonstrate transitions from the candidate superconductor to Mott insulator and
metallic phases. Our study shows that TLG/hBN heterostructures offer an
attractive model system to explore rich correlated behavior emerging in the
tunable triangular Hubbard model.
