DIRECT 2-MAGNON OPTICAL-ABSORPTION IN ALPHA'-NAV2O5 - CHARGED MAGNONS

We investigate the temperature-dependent optical conductivity of alpha '-NaV2O5 in the energy range 4 meV-4 eV. The intensities and the polar ization dependence of the detected electronic excitations give a direc t indication for a broken-parity electronic ground state and for a non centrosymmetric crystal structure of the system in the high-temperatur e phase. A direct two-magnon optical absorption process, proposed here , is in quantitative agreement with the optical data. By analyzing the optically allowed phonons at various temperatures above and below the phase transition, we conclude that a second-order change to a larger unit cell takes place below 34 K.