Comparative study between two quantum spin systems KCuCl3_{3} and TlCuCl3_{3}

We have performed an {\it ab initio} study of the electronic structure of two isostructural quantum spin systems, KCuCl$_{3}$ and TlCuCl$_{3}$, which have recently attracted much attention due to their unconventional magnetic properties. Our first-principles analysis shows unambiguously the role of Tl, as opposed to structural differences between the two compounds, in making TlCuCl$_{3}$ a {\it strongly} coupled s=1/2 dimer system compared to KCuCl$_{3}$ which shows a {\it weakly} coupled s=1/2 dimer behavior. Good agreement with the existing analysis of inelastic neutron scattering results has been observed.Comment: minor changes in text with additional references; in EPL forma

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.

Role of Te in the low dimensional multiferroic material FeTe2O5Br

Using first principles density functional calculations, we study the electronic structure of the low-dimensional multiferroic compound FeTe2O5Br to investigate the origin of the magnetoelectric (ME) effect and the role of Te ions in this system. We find that without magnetism even in the presence of Te-5s lone pairs, the system remains centrosymmetric due to the antipolar orientation of the lone pairs. Our study shows that the exchange striction within the Fe tetramers as well as between them is responsible for the ME effect in FeTe2O5Br. We also find that the Te^4+ ions play an important role in the inter-tetramer exchange striction as well as contribute to the electric polarization in FeTe2O5Br, once the polarization is triggered by the magnetic ordering.Comment: 8 pages, 8 figures, Journal version: http://link.aps.org/doi/10.1103/PhysRevB.88.09440

Third-generation muffin-tin orbitals

By the example of sp^3-bonded semiconductors, we illustrate what 3rd-generation muffin-tin orbitals (MTOs) are. We demonstrate that they can be downfolded to smaller and smaller basis sets: sp^3d^10,sp^3, and bond orbitals. For isolated bands, it is possible to generate Wannier functions a priori. Also for bands, which overlap other bands, Wannier-like MTOs can be generated a priori. Hence, MTOs have a unique capability for providing chemical understanding.Comment: 13 pages, 8 eps figure