  Sn adatoms on a Si(111) substrate with 1/3 monolayer coverage form a
two-dimensional triangular adatom lattice with one unpaired electron per site
and an antiferromagnetic Mott insulating state. The Sn layers can be modulation
hole-doped and metallized using heavily-doped $p$-type Si(111) substrates, and
become superconducting at low temperatures. While the pairing symmetry of the
superconducting state is currently unknown, the combination of repulsive
interactions and frustration inherent to the triangular adatom lattice opens up
the possibility for a chiral order parameter. Here, we study the
superconducting state of Sn/Si(111) using scanning tunneling
microscopy/spectroscopy and quasi-particle interference imaging. We find
evidence for a doping-dependent $T_c$ with a fully gapped order parameter, the
presence of time-reversal symmetry breaking, and a strong enhancement of the
zero-bias conductance near the edges of the superconducting domains. While each
individual piece of evidence could have a more mundane interpretation, our
combined results suggest the tantalizing possibility that Sn/Si(111) is an
unconventional chiral d-wave superconductor.
