Lattice vibrations of α^′-NaV₂O₅ in the low-temperature phase: An alternative interpretation to magnetic bound states

We report high-resolution polarized infrared studies of the quarter-filled spin ladder compound α^′-NaV₂O₅ as a function of temperature (5 K<~T<~300 K). Numerous new modes were detected below the temperature T_c=34 K of the phase transition into a charge ordered nonmagnetic state accompanied by a lattice dimerization. We analyze the Brillouin-zone (BZ) folding due to lattice dimerization at T_c, and show that some peculiarities of the low-temperature vibrational spectrum come from quadruplets folded from the BZ point (1/2, 1/2, 1/4). We discuss an earlier interpretation of the 70-, 107-, and 133-cm^-1 modes as magnetic bound states, and propose the alternative interpretation of folded phonon modes strongly interacting with charge and spin excitations.