  We study the emergence of bosonic pairs in a system of two coupled
one-dimensional fermionic chains subject to a gauge flux (two-leg flux ladder),
with both attractive and repulsive interaction. In the presence of strong
attractive nearest-neighbor interaction and repulsive next-to-nearest-neighbor
interaction, the system crosses into a regime in which fermions form tightly
bound pairs, which behave as bosonic entities. By means of numerical
simulations based on the density-matrix-renormalization-group (DMRG) method, we
show in particular that in the strongly paired regime, the gauge flux induces a
quantum phase transition of the Ising type from vortex density wave (VDW) to a
charge density wave (CDW), characteristic of bosonic systems.
