Seminars Archive

Optical Spectroscopy of Low Dimensional Spin Systems

Abstract
We investigated the temperature-dependent optical conductivity of CuGeO3 and aC-NaV2O5 in the energy range 4 meV- 4 eV. By analyzing the optically allowed phonons at various temperatures, we conclude that a second-order transition takes place below 14 K in CuGeO3, and below 34 K in aC-NaV2O5. In addition, we detected electronic bands on both materials. Whereas the optical data on pure and doped CuGeO3 can be fully understood within the spin-Peierls picture, the nature of the ground state and of the phase transition in aC-NaV2O5 is still controversial: in the high temperature undistorted phase, the infrared phonon spectrum is in good agreement with the recently proposed centrosymmetric space group Pmmn; on the other hand, intensity and polarization dependence of the electronic excitations provide direct evidence for a charge disproportionated electronic ground state. We discovered a peculiar property of spin-flip excitations if spins are located at parity-broken sites: a double spin-flip excitation acquires a finite electric dipole moment along the direction of the broken parity. As a consequence, charged bi-magnons can be excited in an optical experiment. In the case of aC-NaV2O5 we estimated an effective magnon charge of about 0.07 qe. A consistent interpretation of both structural and optical conductivity data on aC-NaV2O5 requires an asymmetrical charge distribution on each rung, without any long-range order for T>Tc=34 K.