We report on the magnetic properties of thoroughly-characterized Zn1−xCoxO epitaxial thin films, with low Co concentration, x = 0.003 − 0.005. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co 2+ ions in ZnO possess a strong single ion anisotropy which leads to an "easy plane" ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co materials.PACS numbers: 71.20. Be, 75.30.Gw, 76.30.Fc Spintronics, an emerging branch of micro-and nanoelectronics which manipulates the electron spin rather than its charge, has need for spin polarization components. In most spintronic devices, metallic ferromagnetic (FM) materials are used to this end. However the physics of metal-semiconductor injection is incompatible with the concept of semiconductor devices, preventing their application [1]. A suitable solution would be a FM semiconductor at room-temperature.The magnetic properties of diluted magnetic semiconductors are due to the substitution of cations by transition-metal (TM) ions, and have been extensively studied for at least five decades [2]. Co-doped ZnO -a possible candidate for high-T c FM semiconductors -has attracted much interest from both theoretical and experimental points of view. Yet, there is an ongoing debate about its magnetic properties. Early theoretical studies using the local spin density approximation (LSDA) for Zn 1−x Co x O found it to be a FM semimetal [3]. Contrary to this, more recent LSDA calculations [4,5] on large supercells detected a competition between FM and antiferromagnetic (AFM) interactions, i.e. an AFM or spinglass groundstate.Experimentally, high-T c FM phases in Zn 1−x Co x O (x = 0.1-0.25) were found in thin films produced by pulsed laser deposition [6], by the sol-gel method [7], and by rf magnetron co-sputtering [8]. They were also found in bulk single crystals prepared by implantation [9]. Controversially, AFM correlations between TM ions and the absence of any FM bulk phases were observed in Zn 1−x Co x O (x = 0.005-0.15, 0.2) samples fabricated by precursor decomposition [4], in polycrystalline powder samples [10] as well as in thin films [11].In this rather contradictory situation a major question which arises is whether a reliable identification of an intrinsic FM phase of ZnO doped by Co is possible at all.Here we address this question on both experimental and theoretical grounds. We argue that such an identification requires a thorough examination of the magnetic properties of Co 2+ ions in the ZnO lattice, and in particular the magnetic anisotropy of cobalt. By EPR and magnetic measurements, we first prove that Co 2+ , which has a spin S = 3/2, shows a huge single ion anisotropy of DS z 2 type, with D = 2.76 cm −1 . We then validate this result theoretically by combining crystal field theory with an estimate of the crystal field parameters. Theory and experiment clearly d...

A detailed study of electronic phase transitions in the ionic Hubbard model at half filling is presented. Within the dynamical mean field approximation a series of transitions from the band insulator via a metallic state to a Mott-Hubbard insulating phase is found at intermediate values of the one-body potential ∆ with increasing the Coulomb interaction U . We obtain a critical region in which the metallic phase disappears and a novel coexistence phase between the band and the Mott insulating state sets in. Our results are consistent with those obtained at low dimensions, thus they provide a concrete description for the charge degrees of freedom of the ionic Hubbard model. PACS numbers: 71.10.Hf, 71.27.+a, 71.30.+h One important problem in the field of electronic structure theory is the understanding of the Mott metalinsulator transition (MIT).1,2 Many details have already been clarified using the canonical model for this transition -the one-band Hubbard model. At half filling (i.e., having in average one electron at each lattice site) this transition from the metallic to the Mott-Hubbard insulating (MI) phase occurs with increasing the on-site repulsion U . Great progress in understanding the MIT in Hubbard-like models has been achieved in the last decade with the development of the dynamical mean field theory (DMFT). Although many aspects of the coherent (Fermi liquid) metallic phase and the incoherent MI phase are now quite well understood, other questions remain open, especially the relationship between the MI regime and band insulators (BI), where one sub-band is almost completely filled by two electrons such that an excitation gap is formed.To study the interplay between band and MottHubbard insulators, various extended versions of the oneband Hubbard model have been proposed. It is remarkable that different models show separate kinds of behavior. Some of them have a crossover from the BI to the MI regime, 3 whereas others have clear transition points with a metallic phase in between.4 A widely studied model for the second class is the ionic Hubbard model (IHM). 5,6,7In the one-dimensional (1D) IHM, however, a ferroelectric (or bond-ordered) phase is realized which separates BI and MI, and the metallic phase shrinks to only one point.10,11 Already in 1D, a finite metallic region can be recovered by introducing an intra-sublattice hopping t ′ into the IHM.12 In 2D 7 or at larger dimensions 5 a correlation induced metallic phase was reported between BI and MI, but it is still under debate whether this metallic phase shrinks to a line 5 or if it ends up at certain particular point by increasing the ionicity ∆. In this work we show that the metallic phase disappears at a certain value of ∆ above which we found a coexistence region composed of distinct insulating phases.It might be not very surprising to find quite different behavior in various models at different dimensions. But even if we restrict our attention to the IHM, the phase diagram is highly disputed. The 2D case was studied by a cluster extension...