We found direct experimental evidence for an orbital switching in the V 3d states across the metal-insulator transition in VO2. We have used soft-x-ray absorption spectroscopy at the V L2,3 edges as a sensitive local probe and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective bandwidths are reduced, that the system becomes more one dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.

Pyroxenes with the general formula AM Si 2 O 6 (A = mono-or divalent metal, M = di-or trivalent metal) are shown to be a new class of multiferroic materials. In particular, we have found so far that NaFeSi 2 O 6 becomes ferroelectric in a magnetically ordered state below 6 K. Similarly, magnetically driven ferroelectricity is also detected in the Li homologues, LiFeSi 2 O 6 (T C 18 K) and LiCrSi 2 O 6 (T C 11 K). In all these monoclinic systems the electric polarization can be strongly modified by magnetic fields. Measurements of magnetic susceptibility, pyroelectric current and dielectric constants (and their dependence on magnetic field) are performed using a natural crystal of aegirine (NaFeSi 2 O 6 ) and synthetic crystals of LiFeSi 2 O 6 and LiCrSi 2 O 6 grown from melt solution. For NaFeSi 2 O 6 a temperature versus magnetic field phase diagram for NaFeSi 2 O 6 is proposed. Exchange constants are computed on the basis of ab initio band structure calculations. The possibility of a spiral magnetic structure caused by frustration as origin of the multiferroic behaviour is discussed. We propose that other pyroxenes may also be multiferroic, and that the versatility of this family offers an exceptional opportunity to study general conditions for and mechanisms of magnetically driven ferroelectricity.

LDA+DMFT (Local Density Approximation combined with Dynamical Mean-Field Theory) computation scheme has been used to calculate spectral properties of LaFeAsO -the parent compound for new high-T c iron oxypnictides. Coulomb repulsion U and Hund's exchange J parameters for iron 3d electrons were calculated using first principles constrained density functional theory scheme in Wannier functions formalism. Resulting values strongly depend on the number of states taken into account in calculations: when full set of O-2p, As-4p, and Fe-3d orbitals with corresponding bands are included, computation results in U =3÷4 eV and J=0.8 eV. In contrast to that when the basis set is restricted to Fe-3d orbitals and bands only, computation gives much smaller parameter values F 0 =0.8 eV, J=0.5 eV. However, DMFT calculations with both parameter sets and corresponding to them choice of basis functions result in weakly correlated electronic structure that is in agreement with experimental X-ray and photoemission spectra.

Combining infrared reflectivity, transport, susceptibility and several diffraction techniques, we find compelling evidence that CaCrO3 is a rare case of a metallic and antiferromagnetic transitionmetal oxide with a three-dimensional electronic structure. LSDA calculations correctly describe the metallic behavior as well as the anisotropic magnetic ordering pattern of C type: The high Cr valence state induces via sizeable pd hybridization remarkably strong next-nearest neighbor interactions stabilizing this ordering. The subtle balance of magnetic interactions gives rise to magneto-elastic coupling, explaining pronounced structural anomalies observed at the magnetic ordering transition.Strongly correlated electron systems including the wide class of transition-metal oxides exhibit a quite general relation between magnetic order and electrical conductivity [1]: ferromagnetism typically coexists with metallic conductivity, whereas insulators usually exhibit antiferromagnetism. It is always a challenge to understand exceptions from this rule. The rare observations of ferromagnetism in insulating transition-metal oxides most often are due to a particular type of orbital ordering [2]. The few examples of antiferromagnetic (AFM) metals, e.g., (La/Sr) 3 Mn 2 O 7 [3] or Ca 3 Ru 2 O 7 [4], are characterized by reduced electronic and structural dimensionality, and the antiferromagnetic order corresponds to a stacking of ferromagnetic (FM) layers. Here we report the discovery of a three-dimensional transition-metal oxide with metallic conductivity, antiferromagnetic exchange interactions, and C-type antiferromagnetic order: the perovskite CaCrO 3 .Perovskites containing Cr 4+ (CaCrO 3 , SrCrO 3 , and PbCrO 3 ) were already studied previously [5,6,7,8,9,10], but neither the details of the crystal structure nor the nature of the magnetic ordering are known. Only very recently evidence for C-type AFM order was reported in multi-phase samples of SrCrO 3 [10]. Regarding the conductivity, the existing data are controversial. In Refs. [7,9] CaCrO 3 was claimed to be metallic, but more recently insulating behavior has been reported [5]. A similar controversy persists also for SrCrO 3 , which should definitely be more metallic than CaCrO 3 due to the less distorted crystal structure, but metallic behavior was observed in Ref.[5] only under pressure. These controversies most likely are connected with the difficulty to prepare high-quality stoichiometric materials and with the lack of large single crystals.CaCrO 3 exhibits an orthorhombic GdFeO 3 -type perovskite structure and early magnetization measurements indicate a magnetic transition at 90 K [8], which is confirmed in our samples. Two electrons occupy the Cr 3d shell (S=1), rendering the material electronically similar to insulating RVO 3 [11] (also 3d 2 ) and to metallic (Ca/Sr)RuO 3 (4d 2 ) [12]. CaCrO 3 shows an unusually high transition-metal valence, Cr 4+ , which may lead to a small or even negative charge-transfer gap [13,14], i.e., holes in the O band. In CrO 2 with ruti...

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