Optical spectroscopic study of the interplay of spin and charge in α′-NaV2O5

We investigate the temperature-dependent optical properties of alpha'-NaV2O5, in the energy range 4 meV-4 eV. The symmetry of the system is discussed on the basis of infrared phonon spectra. By analyzing the optically allowed phonons at temperatures below and above the phase transition, we conclude that a second-order change to a larger unit cell takes place below 34 K, with a fluctuation regime extending over a broad temperature range. In the high-temperature undistorted phase, we find good agreement with the recently proposed centrosymmetric space group Pmmn. On the other hand, the detailed analysis of the electronic excitations detected in the optical conductivity provides direct evidence for a charge disproportionated electronic ground state, at least on a local scale: A consistent interpretation of both structural and optical conductivity data requires an asymmetrical charge distribution on each rung, without any long-range order. We show that, because of the locally broken symmetry, spin-flip excitations carry a finite electric dipole moment, which is responsible for the detection of direct two-magnon optical absorption processes for E parallel to a. The charged-magnon model, developed to interpret the optical conductivity of alpha'-NaV2O5, is described in detail, and its relevance to other strongly correlated electron systems, where the interplay of spin and charge plays a crucial role in determining the low-energy electrodynamics,is discussed.