  We report the experimental observation of the transverse spin and associated
spin-momentum locking of surface plasmon polaritons (SPPs) excited in a
plasmonic single crystalline silver nanowire (AgNW). In contrast to the SPPs
excited in metal films, the electromagnetic field components of the evanescent
SPP mode propagating along the long axis ($x$ axis) of the NW can decay along
two longitudinal planes ($x$-$y$ and $x$-$z$ planes), resulting in two
orthogonal transverse spin components ($s_z$ and $s_y$). Analysis of the
opposite circular polarization components of the decaying SPP mode signal in
the longitudinal plane ($x$-$y$) reveals spin dependent biasing of the signal
and hence the existence of transverse spin component ($s_z$). The corresponding
transverse spin density ($s_3$) in the Fourier plane reveals spin-momentum
locking, where the helicity of the spin is dictated by the wave-vector
components of the SPP evanescent wave. Further, the results are corroborated
with three-dimensional numerical calculations. The presented results showcase
how a chemically prepared plasmonic AgNW can be harnessed to study optical
spins in evanescent waves, and can be extrapolated to explore sub-wavelength
effects including directional spin coupling and optical nano-manipulation.
