  A composite pairing structure of superconducting state is revealed by density
matrix renormalization group study in a two-leg $t$-$J$ model. The pairing
order parameter is composed of a pairing amplitude and a phase factor, in which
the latter explicitly depends on the spin background with an analytic form
identified in the anisotropic limit as the interchain hopping integral
$t_{\perp}\rightarrow 0$. Such a string-like phase factor is responsible for a
pair density wave (PDW) induced by spin polarization with a wavevector
$Q_{\mathrm {PDW}}=2\pi m$ ($m$ the magnetization). By contrast, the pairing
amplitude remains smooth, unchanged by the PDW. In particular, a local spin
polarization can give rise to a sign change of the order parameter across the
local defect. Unlike in an Fulde-Ferrell-Larkin-Ovchinnikov state, the nonlocal
phase factor here plays a role as the new order parameter characterizing the
PDW, whose origin can be traced back to the essential sign structure of the
doped Mott insulator.
