  The creation of polarized electron-positron pairs by the nonlinear
Breit-Wheeler process in short laser pulses is investigated using the
Baier-Katkov semiclassical method beyond local-constant-field approximation
(LCFA), which allows for identifying the interferences effects in the positron
polarization. When the laser intensity is in the intermediate %multiphoton
regime, the interferences of pair production in different formation lengths
induce an enhancement of pair production probability for spin-down positrons,
which significantly affects the polarization of created positrons. The
polarization features are distinct from that obtained with LCFA, revealing the
invalidity of LCFA in this regime. Meanwhile, the angular distribution for
different spin states varies, resulting in an angular-dependent polarization of
positrons. The average polarization of positrons at beam center is highly
sensitive to the laser's carrier-envelope phase (CEP), which provides a
potential alternative way of determining the CEP of strong lasers. The
verification of the observed interference phenomenon is possible for the
upcoming experiments.
