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Register a new userThe symmetry problem in NaV2O5
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Authors
A. Damascelli
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We discuss the symmetry of NaV2O5 in the high temperature phase on the basis of optical conductivity data. Conclusive information cannot be obtained by studying the optically allowed lattice vibrations. However, intensity and polarization of the electronic excitations give a direct indication for a broken-parity electronic ground-state. This is responsible for the detection of charged bi-magnons in the optical spectrum.
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At room-temperature NaV2O5 was found to have the centrosymmetric space group Pmmn. This space group implies the presence of only one kind of V site in contrast with previous reports of the non-centrosymmetric counterpart P21mn. This indicates a non-integer valence state of vanadium. Furthermore, this symmetry has consequences for the interpretation of the transition at 34 K, which was ascribed to a spin-Peierls transition of one dimensional chains of V4+.
We investigated the temperature-dependent optical conductivity of NaV2O5 in the energy range 4 meV-4 eV. The intensities and the polarization dependence of the detected electronic excitations give a direct indication for a broken-parity electronic ground-state and for a non-centrosymmetric crystal structure of the system in the high-temperature phase. A direct two-magnon optical absorption process, proposed in this Letter, 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.
We present angular dependent EPR measurements in NaV2O5 at X-band frequencies in the temperature range 4.2 K < T < 670 K. A detailed analysis in terms of the antisymmetric Dzyaloshinski-Moriya and the anisotropic exchange interactions yields the following scheme of charge order: On decreasing temperature a quarter-filled ladder with strong charge disproportions, existing for T > 100 K, is followed by zig-zag charge-order fluctuations which become long-range and static below T_SP=34 K.
We demonstrate that angle-resolved soft x-ray spectroscopy can resolve absorption by inequivalent oxygen sites and by different orbitals belonging to the same site in NaV2O5. By rotating the polarization direction, we see a dramatic change in the absorption spectra at the oxygen K edge. Our theory identifies the detailed composition of the spectra and predicts a correct energy-ordering of the orbitals of three inequivalent oxygen atoms. Because different orbitals dominate absorption spectra at different energies and angles, one can excite at a specific site and ``orbital. In contrast, absorption at the vanadium L edge does not show large changes when varying the polarization direction. The reason for this is that different excitation channels (involving different initial states for the excited electron) overlap in energy and vary in compensating ways, obscuring each channels sensitive polarization dependence.
We present the temperature dependence of shear and longitudinal elastic constants in a-NaV2O5. For the longitudinal c22 and c33 modes we find anomalies at Tc in contrast to the Spin Peierls substance CuGeO3 where only the longitudinal mode along the chain shows a pronounced effect at TSP. The c66 shear mode (propagation along the chain in b-direction polarization in a-direction) shows strong softening of 12%. Such a large effect is absent for all shear modes in CuGeO3. We can interpret this softening with a coupling of the exy symmetry strain to the charge fluctuation of B1g symmetry. We give the possible low temperature charge distribution.
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