A simple ``negative" route to pair density wave order

Pair density waves (PDW) are novel forms of superconducting states that exhibit periodically modulated superconducting order parameter. An outstanding challenge in condensed matter physics is to elucidate how intrinsic PDW order may emerge in strongly correlated electron systems. In this study, we propose that PDW is prone to form in multiband superconductors simply with interband Cooper pairing taking place between electrons from oppositely dispersing bands. This scenario is heuristically motivated by the observation that uniform interband pairing in such systems would exhibit negative superfluid weight—a signature of an instability towards phase modulation, making PDW a natural candidate for the ground state. Using mean-field theory and large-scale DMRG calculations, we demonstrate this mechanism in a minimal model with interband attraction. DMRG reveals power-law superconducting correlations with exponents $K_{sc}$ decreasing systematically with ladder width, signaling off-diagonal long-range order in the 2D limit. The PDW additionally induces subleading charge order, the modulation wavevectors of which are governed by those of the PDW. Our calculation thus establishes PDW as the primary order, offering a simple route to PDW states in multiband systems.