  A new $\alpha$-emitting isotope $^{214}$U, produced by fusion-evaporation
reaction $^{182}$W($^{36}$Ar, 4n)$^{214}$U, was identified by employing the
gas-filled recoil separator SHANS and recoil-$\alpha$ correlation technique.
More precise $\alpha$-decay properties of even-even nuclei $^{216,218}$U were
also measured in reactions of $^{40}$Ar, $^{40}$Ca with $^{180, 182, 184}$W
targets. By combining the experimental data, improved $\alpha$-decay reduced
widths $\delta^2$ for the even-even Po--Pu nuclei in the vicinity of magic
neutron number $N=126$ were deduced. Their systematic trends are discussed in
terms of $N_{p}N_{n}$ scheme in order to study the influence of proton-neutron
interaction on $\alpha$ decay in this region of nuclei. It is strikingly found
that the reduced widths of $^{214,216}$U are significantly enhanced by a factor
of two as compared with the $N_{p}N_{n}$ systematics for the $84 \leq Z \leq
90$ and $N<126$ even-even nuclei. The abnormal enhancement is interpreted by
the strong monopole interaction between the valence protons and neutrons
occupying the $\pi 1f_{7/2}$ and $\nu 1f_{5/2}$ spin-orbit partner orbits,
which is supported by a large-scale shell model calculation.
