  With angle-resolved photoemission spectroscopy, we studied the electronic
structure of TaFe$_{1.23}$Te$_3$, which is a two-leg spin ladder compound with
a novel antiferromagnetic ground state. Quasi-two-dimensional Fermi surface is
observed, indicating sizable inter-ladder hopping, which would facilitate the
in-plane ferromagnetic ordering through double exchange interactions. Moreover,
an energy gap is not observed at the Fermi surface in the antiferromagnetic
state. Instead, the shifts of various bands have been observed. Combining these
observations with density-functional-theory calculations, we propose that the
large scale reconstruction of the electronic structure, caused by the
interactions between the coexisting itinerant electrons and local moments, is
most likely the driving force behind the magnetic transition.
TaFe$_{1.23}$Te$_3$ thus provides a simpler system that contains similar
ingredients as the parent compounds of iron-based superconductors, which yet
could be readily modeled and understood.
