  The study of non-s-wave unconventional superconductivities in strongly
correlated-electron systems has been a central issue in condensed matter
physics for more than 30 years. In such unconventional superconductivities,
d-wave Cooper pairing with antiparallel spins has been often observed in
various quasi-two-dimensional (quasi-2D) bulk systems. Interestingly, many
theories predicted that the triangular lattice causes the d-wave pairing to be
unstable and may lead to more exotic pairing such as parallel spin
(spin-triplet) pairing. Here we focus on a bulk organic triangular-lattice
system in which superconductivity emerges near a nonmagnetic Mott insulating
phase. We demonstrate, by using low-power nuclear magnetic resonance (NMR)
measurements, that the spin susceptibility of the superconducting state retains
the normal state value even deep in the superconducting state. This result
indicates the possibility that the material exhibits spin-triplet
superconductivity. Our finding will bring insights also into understanding the
2D materials with triangular moire superlattices that are considered also to
show unconventional superconductivities near Mott-like insulating states.
