We investigate the importance of quantum orbital fluctuations in the orthorhombic and monoclinic phases of the Mott insulators LaVO3 and YVO3. First, we construct ab-initio material-specific t2g Hubbard models. Then, by using dynamical mean-field theory, we calculate the spectral matrix as a function of temperature. Our Hubbard bands and Mott gaps are in very good agreement with spectroscopy. We show that in orthorhombic LaVO3, quantum orbital fluctuations are strong and that they are suppressed only in the monoclinic 140 K phase. In YVO3 the suppression happens already at 300 K. We show that Jahn-Teller and GdFeO3-type distortions are both crucial in determining the type of orbital and magnetic order in the low temperature phases. The Mott insulating t 2 2g perovskites LaVO 3 and YVO 3 exhibit an unusual series of structural and magnetic phase transitions ( Fig. 1) with temperature-induced magnetization reversal phenomena [1] and other exotic properties [2,3]. While it is now recognized that the V-t 2g orbital degrees of freedom and the strong Coulomb repulsion are the key ingredients, it is still controversial whether classical (orbital order) [1,4,5,6,7,8] or quantum (orbital fluctuations) [2, 9] effects are responsible for the rich physics of these vanadates.At 300 K, LaVO 3 and YVO 3 are orthorhombic paramagnetic Mott insulators. Their structure (Fig. 2) can be derived from the cubic perovskite ABO 3 , with A=La,Y and B=V, by tilting the VO 6 octahedra in alternating directions around the b-axis and rotating them around the c-axis. This GdFeO 3 -type distortion is driven by AO covalency which pulls a given O atom closer to one of its four nearest A-neighbors [10,11]. Since the Y 4d level is closer to the O 2p level than the La 5d level, the AO covalency increases when going from LaVO 3 to YVO 3 and, hence, the shortest AO distance decreases from being 14 to being 20 % shorter than the average, while the angle of tilt increases from 12 to 18 0 , and that of rotation from 7 to 13 0 [12,13]. Finally, the A-cube is deformed such that one or two of the ABA body-diagonals is smaller than the average by, respectively, 4 and 10 % in LaVO 3 and YVO 3 . These 300 K structures are determined mainly by the strong covalent interactions between O 2p and the empty B e g and A d orbitals, hardly by the weak interactions involving B t 2g orbitals, and are thus very similar to the structures of the t 1 2g La and Y titanates [10,11].

We explore the origin of ferromagnetism in CuCr 2 S 4 and CuCr 2 Se 4 by 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. Cr t 2gCr t 2gCr t 2g 2g Cr t UP UP UP UP DN DN DN DN

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 −0.25×106deg∕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.

PACS 72.15.Eb -Electrical and thermal conduction in crystalline metals and alloys PACS 71.20.Eh -Electron density of states and band structure of crystalline solids: Rare earth metals and alloys PACS 61.05.cp -X-ray diffraction Abstract -Resistivity measurements and temperature-dependent X-ray structural analyses are reported for the crystalline compounds GdPd3BxC1−x. 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.