  We have performed transverse- and zero-field muon spin rotation/relaxation
experiments, as well as magnetometry measurements, on samples of Fe1-xVxSe and
their Li+NH3 intercalates Li0.6(NH2)0.2(NH3)0.8 Fe1-x Vx Se. We examine the low
vanadium substitution regime: x = 0.005, 0.01, and 0.02. The intercalation
reaction significantly increases the critical temperature (Tc) and the
superfluid stiffness for all x. The nonintercalated samples all exhibit Tc =
8.5 K while the intercalated samples all show an enhanced Tc > 40 K. Vanadium
substitution has a negligible effect on Tc, but seems to suppress the
superfluid stiffness for the nonintercalated samples and weakly enhance it for
the intercalated materials. The optimal substitution level for the intercalated
samples is found to be x = 0.01, with Tc = 41 K and {\lambda}_{ab}(0) = 0.18
{\mu}m. The nonintercalated samples can be modeled with either a single d-wave
superconducting gap or with an anisotropic gap function based on recent
quasiparticle imaging experiments, whereas the intercalates display multigap
nodal behavior which can be fitted using s + d- or d + d-wave models.
Magnetism, likely from iron impurities, appears after the intercalation
reaction and coexists and competes with the superconductivity. However, it
appears that the superconductivity is remarkably robust to the impurity phase,
providing an avenue to stably improve the superconducting properties of
transition metal substituted FeSe.
