We present a detailed description for the metal-insulator transition in paramagnetic VO 2 . Based on recent experimental data we show the importance of multiorbital electron-electron interactions along with firstprinciples band structure data for a consistent description of the metal-insulator transition in this system. Using the local-density approximation ͑LDA͒ϩdynamical mean field ͑DMFT͒ multiorbital iterated-perturbation theory scheme, which merges the LDA with DMFT, we show that the metal-insulator transition is accompanied by a large spectral weight transfer due to changes in the orbital occupations. Within this scenario we find good agreement with the one-electron spectral function in the metallic phase of VO 2 . We also compare our results for the total spectral density with other approaches which use the quantum Monte Carlo method to solve the impurity problem of DMFT.
We describe the correlated electronic structure of a prototype Fe-pnictide superconductor, SmO1−xFxFeAs, using LDA+DMFT. Strong, multi-orbital electronic correlations generate a lowenergy pseudogap in the undistorted phase, giving a bad, incoherent metal in qualitative agreement with observations. Very good semi-quantitative agreement with the experimental spectral functions is seen, and interpreted, within a correlated, multi-orbital picture. Our results show that Fe-pnictides should be understood as low-carrier density, incoherent metals, in resemblance to the underdoped cuprate superconductors. PACS numbers: 71.27.+a, 74.25.Jb, Discovery of high-T c superconductivity (HTSC) in the Fe-based pnictides [1] is the latest among a host of other, ill-understood phenomena in d-band oxides. HTSC in Fe-pnictides emerges upon doping a bad metal with spin density wave (SDW) order at q = (π, 0). Preliminary experiments indicate [2, 3] unconventional SC. Existent normal state data indicate a "bad metal" without Landau Fermi Liquid (FL) quasiparticles at low energy [1]. These observations in Fe-pnictides are reminiscent of cuprate SC. The small carrier density (giving rise to carrier pockets), along with Uemura scaling from µ-SR [4] similar to hole-doped cuprates strongly suggests a SC closer to the Bose condensed, rather than a BCS (ξ ≃ 1000a) limit.
We deduce a model relevant for the description of the ferromagnetic half-metal Chromium dioxide (CrO2), widely used in magnetic recording technology. The model describes the effect of dynamical, local orbital correlations arising from local quantum chemistry of the material. A finite temperature solution of the model in d = ∞ provides a natural explanation of the optical response, photoemission, resistivity and the large Woods-Saxon ratio observed in experiments. Our study confirms the important role of many body dynamical correlation effects for a proper understanding of the metallic phase of CrO2.PACS numbers: 75.30.Mb, 71.55.Jv Colossal Magnetoresistive (CMR) materials have been the focus of renewed theoretical and experimental investigations [1,2] in recent years, given their obvious technological potential. Interest has mainly been centered around the manganites, where the interesting ferromagnetic metallic regime is driven by the double-exchange (DE) mechanism, though a unified understanding of the correlated nature of the metallic state itself, as well as the high-T insulating paramagnet, requires consideration of orbital correlations and the Jahn-Teller (JT) distortion on an equal footing with the DE ferromagnetism.Some attention has also focused on CrO 2 , widely used in magnetic recording tapes. In contrast to the manganites, stoichiometric CrO 2 is already a ferromagnetic metal. Given the formal 4+ valence state of Cr, the two 3d electrons occupy the t 2g orbitals. One would intuitively expect to form S = 1 spin on each site, and an antiferromagnetic Mott insulator. Why CrO 2 is a ferromagnetic metal instead, has been answered by Korotin et al [3], who have carried out insightful (LDA + U) calculations for this material. Their main conclusions are: (i) the O 2p band(s) act, atleast partially, as electron (or hole) reservoirs resulting in Cr being mixed-valent (like M n in doped manganites), explaining metallicity, (ii) an almost dispersionless majority spin band of predominantly d character at about 1eV below E F over a large region of the Brillouin zone. This corresponds to strongly localized xy orbitals completely occupied by one majority spin electron. On the other hand, the d states of predominantly d yz+zx character hybridize with the O 2p band and disperse, crossing E F . The Hund's rule coupling between the localized d xy spin and the spin density of the band d yz+zx electrons polarizes the latter, giving a ferromagnetic state via the double-exchange mechanism. Thus, both the metallicity and ferromagnetism are correlated well with each other.A closer examination reveals that the metallic state of CrO 2 is strongly correlated, implying that many-body correlation effects beyond the local-density approximation (LDA) [4] (or its variants) need to be considered.A number of experimental observations tend to support such a picture:(1) Polarization dependent XPS measurements reveal substantial ligand orbital polarization. An exchange splitting energy of ∆ ex−spl ≃ 3.2 eV was deduced [5], implying s...
Recent experiments on 2H-TaSe(2) contradict the long-held view of the charge density wave arising from a nested band structure. An intrinsically strong coupling view, involving a charge density wave state arising as a Bose condensation of preformed excitons emerges as an attractive, albeit scantily investigated alternative. Using the local density approximation plus multiorbital dynamic mean field theory, we show that this scenario agrees with a variety of normal state data for 2H-TaSe(2). Based thereupon, the ordered states in a subset of dichalcogenides should be viewed as instabilities of a correlated, preformed excitonic liquid.
The precise nature of unconventional superconductivity (SC) in iron pnictides is presently a hotly debated issue. Here, using insights from normal state electronic structure and symmetry arguments, we show how an unconventional SC emerges from the bad metal "normal" state. Short-ranged, multiband spin and charge correlations generate nodeless SC in the active planar dxz,yz bands, and an interband proximity effect induces out-of-plane gap nodes in the passive d3z2-r2 band. While very good quantitative agreement with various key observations in the SC state and reconciliation with NMR and penetration depth data in the same picture are particularly attractive features of our proposal, clinching evidence would be an experimental confirmation of c-axis nodes in future work.
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