X-boson cumulant approach to the periodic Anderson model

The Periodic Anderson Model (PAM) can be studied in the infinite U limit by employing the Hubbard X operators to project out the unwanted states. We have already studied this problem employing the cumulant expansion with the hybridization as perturbation, but the probability conservation of the local states (completeness) is not usually satisfied when partial expansions like the Chain Approximation (CHA) are employed. Here we treat the problem by a technique inspired in the mean field approximation of Coleman's slave-bosons method, and we obtain a description that avoids the unwanted phase transition that appears in the mean-field slave-boson method both when the chemical potential is greater than the localized level Ef at low temperatures (T) and for all parameters at intermediate T.Comment: Submited to Physical Review B 14 pages, 17 eps figures inserted in the tex

Thermodynamic properties of the periodic Anderson model:X-boson treatment

We study the specific dependence of the periodic Anderson Model (PAM) in the limit of $U=\infty$ employing the X-boson treatment in two fifferent regimes of the PAM: the heavy fermion Kondo (HF-K) and the heavy fermion local magnetic regime (HF-LMM). We obtain a multiple peak structure for the specific heat in agreement with experimental results as well as the increase of the electronic effective mass at low temperatures associated with the HF-K regime. The entropy per site at low T tends to zero in the HF-K regime, corresponding to a singlet ground state, and it tends to $k_{B}ln(2)$ in the HF-LMM, corresponding to a doublet ground state at each site. The linear coefficient $\gamma(T)=C_{v}/T$ of the specific heat qualitatively agrees with the experimental results obtained for differents materials in the two regimes considered here.Comment: 9 pages, 14 figure

The periodic Anderson model from the atomic limit and FeSi

The exact Green's functions of the periodic Anderson model for $U\to \infty$ are formally expressed within the cumulant expansion in terms of an effective cumulant. Here we resort to a calculation in which this quantity is approximated by the value it takes for the exactly soluble atomic limit of the same model. In the Kondo region a spectral density is obtained that shows near the Fermi surface a structure with the properties of the Kondo peak. Approximate expressions are obtained for the static conductivity $$ and magnetic susceptibility $\chi (T)$ of the PAM, and they are employed to fit the experimental values of FeSi, a compound that behaves like a Kondo insulator with both quantities vanishing rapidly for $T\to 0$. Assuming that the system is in the intermediate valence region, it was possible to find good agreement between theory and experiment for these two properties by employing the same set of parameters. It is shown that in the present model the hybridization is responsible for the relaxation mechanism of the conduction electrons.Comment: 26 pages and 8 figure

Bulk Cr tips for scanning tunneling microscopy and spin-polarized scanning tunneling microscopy

A simple, reliable method for preparation of bulk Cr tips for Scanning Tunneling Microscopy (STM) is proposed and its potentialities in performing high-quality and high-resolution STM and Spin Polarized-STM (SP-STM) are investigated. Cr tips show atomic resolution on ordered surfaces. Contrary to what happens with conventional W tips, rest atoms of the Si(111)-7x7 reconstruction can be routinely observed, probably due to a different electronic structure of the tip apex. SP-STM measurements of the Cr(001) surface showing magnetic contrast are reported. Our results reveal that the peculiar properties of these tips can be suited in a number of STM experimental situations

Cumulant expansion of the periodic Anderson model in infinite dimension

The diagrammatic cumulant expansion for the periodic Anderson model with infinite Coulomb repulsion ($U=\infty$) is considered here for an hypercubic lattice of infinite dimension ($d=\infty$). The same type of simplifications obtained by Metzner for the cumulant expansion of the Hubbard model in the limit of $d=\infty$, are shown to be also valid for the periodic Anderson model.Comment: 13 pages, 7 figures.ps. To be published in J. Phys. A: Mathematical and General (1997

Compostos fenólicos de Hibiscus sabdariffa e influência de resíduos orgânicos em sua propriedade antitumoral e antioxidante

The aim of this study was to evaluate the phenolic and flavonoids contents and the antioxidant and antitumoral activity of leaf and calyx methanolic extracts from Hibiscus sabdariffa (roselle) cultivated with poultry litter and organosuper (R) under three modes of application. The total phenolic content in the each extract was determined using the Folin-Ciocalteu reagent and for aluminium chloride flavonoids. The antioxidant parameters were analyzed using a 2, 2-diphenyl-1-picrylhydrazyl ( DPPH.) free radical scavenging assay. An antitumor colorimetric assay using sulforhodamine B. The highest contents of phenolic and flavonoids were observed in leaf extracts (389.98 and 104.52 mg g(-1), respectively) and calyx extracts (474.09 and 148.35 mg g(-1), respectively) from plants cultivated with organosuper., although these values did not differ significantly from those observed for the other treatments. The average IC50 of leaves (43.48 mu g mL(-1)) and calyces (37.15 mu g mL(-1)) demonstrated that both have substances that may contribute to free radical scavenging action. The methanol extract from calyces showed significant selective activity against a leukemia line (K-562), with IC50 values of 0.12 mg mL(-1) (organosuper (R)) and 1.16 mg mL(-1) (poultry litter), with concentration-dependent, cytotoxic and cytocidal effects7516976CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFUNDAÇÃO DE APOIO AO DESENVOLVIMENTO DO ENSINO, CIÊNCIA E TECNOLOGIA DO ESTADO DE MATO GROSSO DO SUL - FUNDECTO objetivo deste estudo foi avaliar a atividade antioxidante, antitumoral e o conteúdo de fenólicos e flavonoides do extrato metanólico das folhas e cálices de Hibiscus sabdariffa (rosela) cultivada com cama de frango e organosuper ® em três modos de aplicação. O conteúdo fenólico total de cada extrato foi determinado utilizando o reagente de Folin-Ciocalteu e cloreto de alumínio para flavonoides. Os parâmetros antioxidantes foram avaliados pelo ensaio de seqüestro de radical livre 2, 2-difenil-1-picrilhidrazil (DPPH.). Na avaliação antitumoral utilizou-se o ensaio colorimétrico com sulforrodamina B. Observou-se o maior conteúdo de fenólicos e flavonoides em extratos de folhas (389.98 e 104.52 mg g–1, respectivamente) e extratos de cálice (474.09 e 148.35 mg g–1, respectivamente) de plantas cultivadas com organosuper ®, embora esses valores não diferiram significativamente daqueles observados para os outros tratamentos. O IC50 de folhas (43.48 µg mL–1) e cálices (37.15 µg mL–1) demonstrou que ambos têm substâncias que podem contribuir para a ação seqüestradora de radicais livres. O extrato metanólico dos cálices mostrou seletividade na linhagem de leucemia (K-562), com valores de IC50 de 0,12 mg mL–1 (organosuper®) e 1,16 mg mL–1 (cama-de-frango) com concentração-dependente e efeito citotóxico e citocidaUFD

In vitro biological screening of the anticholinesterase and antiproliferative activities of medicinal plants belonging to annonaceae

The aim of this research was to investigate the antiproliferative and anticholinesterase activities of 11 extracts from 5 Annonaceae species in vitro. Antiproliferative activity was assessed using 10 human cancer cell lines. Thin-layer chromatography and a microplate assay were used to screen the extracts for acetylcholinesterase (AchE) inhibitors using Ellman's reagent. The chemical compositions of the active extracts were investigated using high performance liquid chromatography. Eleven extracts obtained from five Annonaceae plant species were active and were particularly effective against the UA251, NCI-470 lung, HT-29, NCI/ADR, and K-562 cell lines with growth inhibition (GI(50)) values of 0.04-0.06, 0.02-0.50, 0.01-0.12, 0.10-0.27, and 0.02-0.04 mu g/mL, respectively. In addition, the Annona crassiflora and A. coriacea seed extracts were the most active among the tested extracts and the most effective against the tumor cell lines, with GI(50) values below 8.90 mu g/mL. The A. cacans extract displayed the lowest activity. Based on the microplate assay, the percent AchE inhibition of the extracts ranged from 12 to 52%, and the A. coriacea seed extract resulted in the greatest inhibition (52%). Caffeic acid, sinapic acid, and rutin were present at higher concentrations in the A. crassiflora seed samples. The A. coriacea seeds contained ferulic and sinapic acid. Overall, the results indicated that A. crassiflora and A. coriacea extracts have antiproliferative and anticholinesterase properties, which opens up new possibilities for alternative pharmacotherapy drugs48430831

In Vitro Biological Screening Of The Anticholinesterase And Antiproliferative Activities Of Medicinal Plants Belonging To Annonaceae.

The aim of this research was to investigate the antiproliferative and anticholinesterase activities of 11 extracts from 5 Annonaceae species in vitro. Antiproliferative activity was assessed using 10 human cancer cell lines. Thin-layer chromatography and a microplate assay were used to screen the extracts for acetylcholinesterase (AchE) inhibitors using Ellman's reagent. The chemical compositions of the active extracts were investigated using high performance liquid chromatography. Eleven extracts obtained from five Annonaceae plant species were active and were particularly effective against the UA251, NCI-470 lung, HT-29, NCI/ADR, and K-562 cell lines with growth inhibition (GI50) values of 0.04-0.06, 0.02-0.50, 0.01-0.12, 0.10-0.27, and 0.02-0.04 µg/mL, respectively. In addition, the Annona crassiflora and A. coriacea seed extracts were the most active among the tested extracts and the most effective against the tumor cell lines, with GI50 values below 8.90 µg/mL. The A. cacans extract displayed the lowest activity. Based on the microplate assay, the percent AchE inhibition of the extracts ranged from 12 to 52%, and the A. coriacea seed extract resulted in the greatest inhibition (52%). Caffeic acid, sinapic acid, and rutin were present at higher concentrations in the A. crassiflora seed samples. The A. coriacea seeds contained ferulic and sinapic acid. Overall, the results indicated that A. crassiflora and A. coriacea extracts have antiproliferative and anticholinesterase properties, which opens up new possibilities for alternative pharmacotherapy drugs.0