Origin of Ferromagnetism and its pressure and doping dependence in Tl2_{2}Mn2_{2}O7_{7}

Using NMTO-{\it downfolding} technique, we explore and establish the origin of ferromagnetism in the pyrochlore system, Tl$_{2}$Mn$_{2}$O$_{7}$. It is found to be driven by hybridization induced spin-polarization of the delocalized charge carriers derived from Tl-$s$ and O-$p$ states. The mean-field estimate of the ferromagnetic transition temperature, T$_c$, estimated using computed exchange integrals are found to be in good agreement with the measurements. We find an enhancement of T$_{c}$ for moderate doping with nonmagnetic Sb and a suppression of T$_{c}$ upon application of pressure, both in agreement with experimental findings.Comment: Accepted for publication in PR

Density-functional study of the electronic and optical properties of the spinel compound CuIr<SUB>2</SUB>S<SUB>4</SUB>

Using first-principles density-functional calculations we have computed the electronic and optical properties of spinel compound CuIr2S4, which undergoes a structural phase transition, accompanied by a metal-insulator transition at a temperature of about 230 K. The nature of this transition has been discussed in the literature in terms of both the correlated singlet formation picture as well as the orbitally driven Peierls transition picture. Our first-principles calculations find little role of correlation. Our calculated reflectivity and conductivity data for both the high-temperature and low-temperature phases are found to be in good agreement with measured data by Wang et al. Phys. Rev. B 69 153104 (2004)

Theoretical study of doped Tl2Mn2O7Tl_2Mn_2O_7 and Tl2Mn2O7Tl_2Mn_2O_7 under pressure

Using first-principles density-functional-based calculations, we study the effect of doping and pressure on the manganese-based pyrochlore compound, $Tl_2Mn_2O_7$, which exhibits colossal magnetoresistive behavior. The theoretical study is motivated by the counterintuitive experimental observation of suppression of the ferromagnetic transition temperature upon application of pressure and its enhancement upon substitution of Mn by a moderate amount of nonmagnetic Sb ion. We also attempt to resolve the issue related to crystal structure changes that may occur upon application of pressure

Moderate to large magneto-optical signals in high T<SUB>c</SUB> double perovskites

Using first-principles density functional calculations, we have computed the optical and magneto-optical properties of the Cr-based double perovskite compounds, Sr2CrB'O6 with B' = W,Re,Os. Our computed magneto-optic spectra show substantially large Kerr rotations of about -2° to -2.5° for Sr2CrWO6 and Sr2CrReO6 and a moderately large Faraday rotation of about -0.25×106 deg/cm in insulating Sr2CrOsO6, indicating possible industrial applications. Our study should motivate experimental investigations in this yet to be explored area of Sr2CrB'O6 compounds

Ab initio study of optical properties and magneto-optical Kerr effect in the pyrite compound CoS<SUB>2</SUB>

Using a combination of first-principles methods, we have studied the electronic and optical properties, and the magneto-optical Kerr effect in pyrite-structured cobalt sulphide compounds, which are of high technological importance due to their possible half-metallic character. Our study shows that pure CoS<SUB>2</SUB> can exhibit a large Kerr rotation of greater than 1°, which arises due to the intricate interplay of the band structure effect, the Drude peak, and the relatively small but nonzero spin-orbit effect. The computed Kerr rotation is found to be of the similar kind as that found for compounds containing atoms with strong spin-orbit coupling such as PtMnSb [ V. N. Antonov et al. Phys. Rev. B 56 13012 (1997)]. We have also studied the effect of Fe doping on CoS<SUB>2</SUB> compound. Our study shows that the Kerr rotation gets suppressed upon doping with Fe

Ferromagnetism in metallic chalcospinels CuCr<SUB>2</SUB>S<SUB>4</SUB> and CuCr<SUB>2</SUB>Se<SUB>4</SUB>

We explore the origin of ferromagnetism in CuCr2S4 and CuCr2Se4by analyzing the computed, ab initio electronic structure. Based on our analysis, we establish the kinetic-energy driven mechanism as operative in the case of double perovskites and pyrochlores to be also responsible in these compounds for the experimentally observed ferromagnetism with high Curie temperatures. We provide detailed microscopic understanding of the mechanism in terms of Nth order muffin-tin orbital based downfolding calculations, estimates of the magnetic exchange coupling, and the ferromagnetic transition temperature, computed in mean-field way

Theoretical study of doped Tl<SUB>2</SUB>Mn<SUB>2</SUB>O<SUB>7</SUB> and Tl<SUB>2</SUB>Mn<SUB>2</SUB>O<SUB>7</SUB> under pressure

Using first-principles density-functional-based calculations, we study the effect of doping and pressure on the manganese-based pyrochlore compound, Tl2Mn2O7, which exhibits colossal magnetoresistive behavior. The theoretical study is motivated by the counterintuitive experimental observation of suppression of the ferromagnetic transition temperature upon application of pressure and its enhancement upon substitution of Mn by a moderate amount of nonmagnetic Sb ion. We also attempt to resolve the issue related to crystal structure changes that may occur upon application of pressure

Electronic structure of FeCr<SUB>2</SUB>S<SUB>4</SUB>: evidence of Coulomb enhanced spin-orbit splitting

The electronic structure of the spinel compound, FeCr2S4, is studied using density-functional-theory-based calculations. Our calculations provide a microscopic understanding of the origin of the insulating behavior of this compound, which turn out to be driven by Coulomb enhanced spin-orbit coupling operative within the Fe-d manifold. We also investigate the possible role of the structural distortions and compare the calculated optical property data with that of the experimental one

Quantum Chemical Studies on Stability and Chemical Activities in Calcium Ion Bound Calmodulin Loops

Quantum chemical (QC) calculations for macromolecules require truncation of the molecule, highlighting the portion of interest due to heavy computation cost. As a result, an estimation of the effects of truncation is important to interpret the energy spectrum of such calculations. We perform density functional theory based QC calculations on calcium ion bound EF-hand loops of Calmodulin isolated from the crystal structure in an implicit solvent. We find that the terminal contributions of neutral capping are negligible across the entire ground-state energy spectrum. The coordination energy range and the nature of hybridization of the coordination state molecular orbitals remain qualitatively similar across these loops. While the HOMO and LUMO of loops in the N-terminal domain are dominated by the acidic aspartates, and the polar/hydrophobic residues, respectively, these levels of the C-terminal domain loops show strong localized electron density on the phenyl rings of the tyrosines. The Fukui index calculation identifies the hydroxyl oxygen in the phenyl ring of Y99 as a potent nucleophile. Our analysis indicates a general way of interpreting the electronic energy spectra to understand stability and functions of large biomolecules where the truncation of the molecule and, hence, the terminal capping effects are inevitable

Negative temperature coefficient of resistance in a crystalline compound

Resistivity measurements and temperature-dependent X-ray structural analyses are reported for the crystalline compounds GdPd<SUB>3</SUB>B<SUB>x</SUB>C<SUB>1- x</SUB>. We show that a controlled tuning of the temperature coefficient of resistance (TCR) can be done by modifying the structural parameters and chemical environment of the compounds. We have achieved the result of negative TCR in an ordered, non-Kondo crystalline compound. Electronic-structure calculations have been carried out to elucidate some of our observations