Ab-initio Phonon Calculations for the layered compound TiOCl

We present first-principles frozen-phonon calculations for the three Raman-active $A_g$ modes in the spin-1/2 layered TiOCl system within two different well-known approaches: the local density approximation (LDA) and the so-called LDA+U approximation. We observe that the inclusion of electron correlation in a mean-field level as implemented in the LDA+U leads to a better overall agreement with experimental results. We also discuss the implications of the two approaches on the physics of TiOCl.Comment: Phys. Rev. B (Rapid Comm.) (in Press

Magnon splitting induced by charge ordering in NaV_2O_5

We consider the effects of charge ordering in NaV_2O_5 (below T_SP) on the exchange constants and on the magnon dispersion. We show that the experimentally observed splitting of the magnon branches along the a direction is induced by charge ordering. We find that one can distinguish between the proposed 'zig-zag' and 'in-line' patterns of charge ordering. Only the zig-zag ordering is consistent with the experimental results regarding (i) the unusual intensity modulation observed in magnetic neutron scattering, (ii) the reduction in the intra-ladder exchange constant below T_SP, and (iii) the magnon dispersion along a. We estimate the inter-ladder exchange constant to be 1.01meV=11.7K for T>T_SP.Comment: final version for PR

Electronic and magnetic structure of CsV2_2O5_5

We have studied the electronic structure of the spin-gapped system CsV$_2$O$_5$ by means of an ab initio calculation. Our analysis and a re-examination of the susceptibility data indicate that the behavior of this system is much closer to that of an alternating spin-1/2 antiferromagnetic chain with significant inter-dimer coupling and weaker inter-chain couplings than that of isolated dimers as was initially proposed. Comparison to the vanadate family members $\alpha '$-NaV$_2$O$_5$, $\gamma$-LiV$_2$O$_5$ and isostructural compounds like (VO)$_{2}$P$_{2}$O$_{7}$ (VOPO) is discussed.Comment: revised version. To appear in Phys. Rev.

Effects of Fe substitution on the electronic, transport, and magnetic properties of ZnGa2_2O4_4: A systematic ab-initio study

We present a density functional study of Fe doped into the tetrahedral and octahedral cation sites of the wide band gap spinel ZnGa$_2$O$_4$. We calculate the electronic structure for different substitutions and discuss the magnetic and transport properties for each case considering different approximations for the exchange-correlation potential. We show that for certain doped cases, significant differences in the predicted behavior are obtained depending on the exchange correlation potential adopted. Possible applications of the doped systems as magnetic semiconductors are outlined.Comment: 9 pages, 8 figures; to be published in Phys. Rev.