The Hubbard model within the equations of motion approach

The Hubbard model has a special role in Condensed Matter Theory as it is considered as the simplest Hamiltonian model one can write in order to describe anomalous physical properties of some class of real materials. Unfortunately, this model is not exactly solved except for some limits and therefore one should resort to analytical methods, like the Equations of Motion Approach, or to numerical techniques in order to attain a description of its relevant features in the whole range of physical parameters (interaction, filling and temperature). In this manuscript, the Composite Operator Method, which exploits the above mentioned analytical technique, is presented and systematically applied in order to get information about the behavior of all relevant properties of the model (local, thermodynamic, single- and two- particle ones) in comparison with many other analytical techniques, the above cited known limits and numerical simulations. Within this approach, the Hubbard model is shown to be also capable to describe some anomalous behaviors of the cuprate superconductors.Comment: 232 pages, more than 300 figures, more than 500 reference

Structure of HgBa2CuO4+delta (0.06

The results of a neutron-diffraction study of the HgBa2CuO4+δ structure at ambient pressure and under external pressure at different extra oxygen concentrations are presented. The results have been analyzed together with the data of previous investigations. It is shown that in the cation-stoichiometric samples the Oδ oxygen is only present in the center of the mercury layer, Tc is parabolically dependent on δ, and Tc,max is obtained at δopt=0.13±0.01. The influence of pressure on the structure strongly depends on the doping level. At low oxygen content (δ≈0.06), the compression of the structure is practically uniform. An increase of the extra oxygen content to 0.19 (overdoped state) results in the larger compression of the apical Cu-O(2) and Ba-Oδdistances, while the HgO2 dumb-bell as well as the distance between Ba and O belonging to the (CuO2) layer become practically pressure independent. These results are in agreement with models, in which the effect of the charge transfer from the reservoir to the (CuO2) layers does not play a dominant role in the Tc increase with pressure at low and optimal δ values, while in the overdoped state the charge transfer is enhanced under pressure, thus inducing the Tc decrease. © 1999 The American Physical Society

Neuronal expression of splice variants of "Glial" glutamate transporters in brains afflicted by alzheimer's disease: unmasking an intrinsic neuronal property

Anomalies in glutamate homeostasis may contribute to the pathological processes involved in Alzheimer's disease (AD). Glutamate released from neurons or glial cells is normally rapidly cleared by glutamate transporters, most of which are expressed at the protein level by glial cells. However, in some patho-physiological situations, expression of glutamate transporters that are normally considered to be glial types, appears to be evoked in populations of distressed neurons. This study analysed the expression of exon-skipping forms of the three predominant excitatory amino acid (glutamate) transporters (EAATs1-3) in brains afflicted with AD. We demonstrate by immunocytochemistry in temporal cortex, the expression of these proteins particularly in limited subsets of neurons, some of which appeared to be dys-morphic. Whilst the neuronal expression of the "glial" glutamate transporters EAAT1 and EAAT2 is frequently considered to represent the abnormal and ectopic expression of such transporters, we suggest this may be a misinterpretation, since neurons such as cortical pyramidal cells normally express abundant mRNA for these EAATs (but little if any EAAT protein expression). We hypothesize instead that distressed neurons in the AD brain can turn on the translation of pre-existent mRNA pools, or suppress the degradation of alternately spliced glutamate transporter protein, leading to the "unmasking" of, rather than evoked expression of "glial" glutamate transporters in stressed neurons

Ediacara biota flourished in oligotrophic and bacterially dominated marine environments across Baltica

The environments and food sources that sustained Ediacara biota 575-541 million years ago remain unclear. Here, the authors perform lipid biomarker and isotopic analyses on biota fossil-containing Ediacaran strata from Baltica and propose the presence of a microbial loop bolstered by bacteria