478 research outputs found
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
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
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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
TREATMENT OF SHIZOPHRENIC PATIENTS AND rTMS
Introduction: Schizophrenia (SCH) is a heterogeneous syndrome characterized by positive and negative symptoms. Despite
appropriate medication, about 1/4 of patients suffer for refractory positive and/or negative symptoms, which are associated with
functional handicap, increase of duration and of the number of hospitalizations. Numerous studies have suggested that the
pathophysiology of auditory hallucinations (AH) is related to a hyper activity of the left temporoparietal cortex (TPC). On the other
hand, negative symptoms are associated with a prefrontal hypoactivity and the efficiency of pharmacological treatments is frequently
partial. Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive brain stimulation tool with excellent tolerability and
safety. Given its hypothesized mechanisms of action and the clinical beneficial effects obtained in several types of pathology (Aleman
et al. 2007), the efficacy of rTMS has been investigated for drug-resistant SCH symptoms.
Objective: Our objective is to expose the knowledge concerning the rTMS use in the treatment of these symptoms and to purpose
a critical analysis of these data.
Method: a systematic review of the literature has been conducted using NIH Pubmed. The following search terms were used:
TMS - rTMS - Schizophrenia - negative symptoms - hallucinations.
Results: Concerning the treatment of AH, 16 publications and 4 meta analyses were selected. For the negative symptoms, we
retained 16 studies and 3 meta analyses. The most extensively investigated application for rTMS in SCH is the use of low-frequency
stimulation to the left TPC with the aim to improve AH symptomatology. When compared to sham, this type of acute course of rTMS
has been proven to induce a substantial and significant reduction in AH. But this effect does not seem long-lasting and maintenance
protocols must be developed. Concerning negative symptoms, the results are less solid but we find some works which demonstrate an
improvement of these symptoms while various stimulation parameters were used. Recently, new parameters of stimulation in
particular the theta burst stimulation have permitted us to obtain larger effects with longer duration. The interest of these new
parameters will be discussed here.
Conclusion: Overall, rTMS studies have demonstrated some promise in the treatment of SCH. However, more research is
required to enhance rTMS efficacy and increase its beneficial effect duration and to test new therapeutic strategies in this topic
Electronic structure of the incommensurate compound
We extracted, from strongly-correlated ab-initio calculations, a complete
model for the chain subsystem of the
incommensurate compound. A second neighbor model has been determined as
a function of the fourth crystallographic parameter , 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 . 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
TREATMENT OF SHIZOPHRENIC PATIENTS AND rTMS
Introduction: Schizophrenia (SCH) is a heterogeneous syndrome characterized by positive and negative symptoms. Despite
appropriate medication, about 1/4 of patients suffer for refractory positive and/or negative symptoms, which are associated with
functional handicap, increase of duration and of the number of hospitalizations. Numerous studies have suggested that the
pathophysiology of auditory hallucinations (AH) is related to a hyper activity of the left temporoparietal cortex (TPC). On the other
hand, negative symptoms are associated with a prefrontal hypoactivity and the efficiency of pharmacological treatments is frequently
partial. Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive brain stimulation tool with excellent tolerability and
safety. Given its hypothesized mechanisms of action and the clinical beneficial effects obtained in several types of pathology (Aleman
et al. 2007), the efficacy of rTMS has been investigated for drug-resistant SCH symptoms.
Objective: Our objective is to expose the knowledge concerning the rTMS use in the treatment of these symptoms and to purpose
a critical analysis of these data.
Method: a systematic review of the literature has been conducted using NIH Pubmed. The following search terms were used:
TMS - rTMS - Schizophrenia - negative symptoms - hallucinations.
Results: Concerning the treatment of AH, 16 publications and 4 meta analyses were selected. For the negative symptoms, we
retained 16 studies and 3 meta analyses. The most extensively investigated application for rTMS in SCH is the use of low-frequency
stimulation to the left TPC with the aim to improve AH symptomatology. When compared to sham, this type of acute course of rTMS
has been proven to induce a substantial and significant reduction in AH. But this effect does not seem long-lasting and maintenance
protocols must be developed. Concerning negative symptoms, the results are less solid but we find some works which demonstrate an
improvement of these symptoms while various stimulation parameters were used. Recently, new parameters of stimulation in
particular the theta burst stimulation have permitted us to obtain larger effects with longer duration. The interest of these new
parameters will be discussed here.
Conclusion: Overall, rTMS studies have demonstrated some promise in the treatment of SCH. However, more research is
required to enhance rTMS efficacy and increase its beneficial effect duration and to test new therapeutic strategies in this topic
Ab initio evaluation of the charge-ordering in
We report {\it ab initio} calculations of the charge ordering in
using large configurations interaction methods on
embedded fragments. Our major result is that the 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
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