Environment effects on effective magnetic exchange integrals and local spectroscopy of extended strongly correlated systems

The present work analyzes the importance of the different components of the environment effects on the local spectroscopy of extended strongly correlated systems. It has been found that the usual formal charge definition of the charge transfer and Madelung potential are far too crude for an accurate determination of the local excitation energies in embedded fragment calculations. A criterion for the validation of the embedding against the infinite system density of states has been proposed.Comment: 8 pages, 9 figure

A surprising relation between double exchange and Heisenberg model spectra: Application to half-doped manganites

The Zener polarons recently found in half-doped manganites are usually seen as mixed valence entities ruled by a double exchange Hamiltonian involving only correlated electrons of the metals. They can however be considered as ferrimagnetic local units if the holes are localized on the bridging oxygen atoms as implicitely suggested by recent mean-field it ab initio calculations. In the latter case, the physics is ruled by a Heisenberg Hamiltonian involving magnetic oxygen bridges. This paper shows that the spectra resulting from the resolution of both models are analytically identical. This single resulting model spectrum accurately reproduces the spectrum of Zener polarons in Pr0.6Ca0.4MnO3 manganite studied by means of explicitely correlated ab initio calculations. Since the physics supported by each model are different, the analysis of the exact Hamiltonian ground state wave function should a priori enables one to determine the most appropriate model. It will be shown that neither the spectrum nor the wavefunction analysis bring any decisive arguments to settle the question. Such undecidability would probably be encountered in experimental information.Comment: 4 pages, 2 figure

Abnormal Striatal Dopamine Transmission in Schizophrenia

Despite numerous revisions and reformulations, dopamine (DA) hypothesis of schizophrenia remains a pivotal neurochemical hypothesis of this illness. The aim of this review is to expose and discuss findings from positron emission tomography (PET) or single-photon-emission computed tomography (SPECT) studies investigating DA function in the striatum of medicated, drug-naïve or drug-free patients with schizophrenia and in individuals at risk compared with healthy volunteers. DA function was studied at several levels: i) at a presynaptic level where neuroimaging studies investigating DOPA uptake capacity clearly show an increase of DA synthesis in patients with schizophrenia; ii) at a synaptic level where neuroimaging studies investigating dopamine transporter availability (DAT) does not bring any evidence of dysfunction; iii) and finally, neuroimaging studies investigating DA receptor density show a mild increase of D2 receptor density in basic condition and, an hyperreactivity of DA system in dynamic condition. These results are discussed regarding laterality, sub-regions of striatum and implications for the at-risk population. Striatal DA abnormalities are now clearly demonstrated in patients with schizophrenia and at risk population and could constitute an endophenotype of schizophrenia. Subtle sub-clinical striatal DA abnormalities in at risk population could be a biomarker of transition from a vulnerability state to the expression of frank psychosis

The Spin-SAF transition in NaV2O5 induced by spin-pseudospin coupling

We present microscopic estimates for the spin-spin and spin-speudospin interactions of the quarter-filled ladder compound NaV2O5, obtained by exactly diagonalizing appropriate clusters of the underlying generalized Hubbard Hamiltonian. We present evidence for a substantial interladder spin-pseudospin interaction term which would allow simultaneously for the superantiferroelectric (SAF) charge (pseudospin) ordering and spin dimerization. We discuss the values of the coupling constants appropriate for NaV2O5 and deduce the absence of a soft antiferroelectric mode

Ab initio evaluation of the charge-ordering in α′NaV2O5\alpha^\prime NaV_2O_5

We report {\it ab initio} calculations of the charge ordering in $\alpha^\prime NaV_2O_5$ using large configurations interaction methods on embedded fragments. Our major result is that the $2p_y$ electrons of the bridging oxygen of the rungs present a very strong magnetic character and should thus be explicitly considered in any relevant effective model. The most striking consequence of this result is that the spin and charge ordering differ substantially, as differ the experimental results depending on whether they are sensitive to the spin or charge density.Comment: 4 page

Electronic structure of the Sr0.4Ca13.6Cu24O41Sr_{0.4}Ca_{13.6}Cu_{24}O_{41} incommensurate compound

We extracted, from strongly-correlated ab-initio calculations, a complete model for the chain subsystem of the $Sr_{0.4}Ca_{13.6}Cu_{24}O_{41}$ incommensurate compound. A second neighbor $t-J+V$ model has been determined as a function of the fourth crystallographic parameter $\tau$, for both low and room temperature crystallographic structures. The analysis of the obtained model shows the crucial importance of the structural modulations on the electronic structure through the on-site energies and the magnetic interactions. The structural distortions are characterized by their long range effect on the cited parameters that hinder the reliability of analyses such as BVS. One of the most striking results is the existence of antiferromagnetic nearest-neighbor interactions for metal-ligand-metal angles of $90^\circ$. A detailed analysis of the electron localization and spin arrangement is presented as a function of the chain to ladder hole transfer and of the temperature. The obtained spin arrangement is in agreement with antiferromagnetic correlations in the chain direction at low temperature

Ferromagnetic Polarons in La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3

Unrestricted Hartree-Fock calculations on La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3 in the full magnetic unit cell show that the magnetic ground states of these compounds consist of 'ferromagnetic molecules' or polarons ordered in herring-bone patterns. Each polaron consists of either three or five Mn ions separated by O- ions with a magnetic moment opposed to those of the Mn ions. Ferromagnetic coupling within the polarons is strong while coupling between them is relatively weak. Magnetic moments on the Mn ions range between 3.8 and 3.9 Bohr magnetons in La0.5Ca0.5MnO3 and moments on the O- ions are -0.7 Bohr magnetons. Each polaron has a net magnetic moment of 7.0 Bohr magnetons, in good agreement with recently reported magnetisation measurements from electron microscopy. The polaronic nature of the electronic structure reported here is obviously related to the Zener polaron model recently proposed for Pr0.6Ca0.4MnO3 on the basis of neutron scattering data.Comment: 4 pages 5 figure

Chronic administration of atypical antipsychotics improves behavioral and synaptic defects of STOP null mice.

International audienceINTRODUCTION: Recent studies have suggested that schizophrenia is associated with alterations in the synaptic connectivity involving cytoskeletal proteins. The microtubule-associated protein stable tubule only polypeptide (STOP) plays a key role in neuronal architecture and synaptic plasticity, and it has been demonstrated that STOP gene deletion in mice leads to a phenotype mimicking aspects of positive and negative symptoms and cognitive deficits classically observed in schizophrenic patients. In STOP null mice, behavioral defects are associated with synaptic plasticity abnormalities including defects in long-term potentiation. In these mice, long-term administration of typical antipsychotics has been shown to partially alleviate behavioral defects but, as in humans, such a treatment was poorly active on deficits related to negative symptoms and cognitive impairments. Here, we assessed the effects of risperidone and clozapine, two atypical antipsychotics, on STOP null mice behavior and synaptic plasticity. RESULTS: Long-term administration of either drug results in alleviation of behavioral alterations mimicking some negative symptoms and partial amelioration of some cognitive defects in STOP null mice. Interestingly, clozapine treatment also improves synaptic plasticity of the STOP null animals by restoring long-term potentiation in the hippocampus. DISCUSSION: All together, the pharmacological reactivity of STOP null mice to antipsychotics evokes the pharmacological response of humans to such drugs. Totally, our study suggests that STOP null mice may provide a useful preclinical model to evaluate pharmacological properties of antipsychotic drugs