Magnetic Properties of a-Si films doped with rare-earth elements

Amorphous silicon films doped with Y, La, Gd, Er, and Lu rare-earth elements (a-Si:RE) have been prepared by co-sputtering and studied by means of electron spin resonance (ESR), dc-magnetization, ion beam analysis, optical transmission, and Raman spectroscopy. For comparison the magnetic properties of laser-crystallized and hydrogenated a-Si:RE films were also studied. It was found that the rare-earth species are incorporated in the a-Si:RE films in the RE3+ form and that the RE-doping depletes the neutral dangling bonds (D0) density. The reduction of D0 density is significantly larger for the magnetic REs (Gd3+ and Er3+) than for the non-magnetic ones (Y3+, La3+, Lu3+). These results are interpreted in terms of a strong exchange-like interaction, J RE-DB SRE SDB, between the spin of the magnetic REs and that of the D0. All our Gd-doped Si films showed basically the same broad ESR Gd3+ resonance (DHpp ~ 850 Oe) at g ~ 2.01, suggesting the formation of a rather stable RE-Si complex in these films.Comment: 15 pages, 7 figure

Transesterification of vegetable oils: a review

The transesterification of vegetable oils with methanol as well as the main uses of the fatty acid methyl esters are reviewed. The general aspects of this process and the applicability of different types of catalysts (acids, alkaline metal hydroxides, alkoxides and carbonates, enzymes and non-ionic bases, such as amines, amidines, guanidines and triamino(imino)phosphoranes) are described. Special attention is given to guanidines, which can be easily heterogenized on organic polymers. However, the anchored catalysts show leaching problems. New strategies to obtain non-leaching guanidine-containing catalysts are proposed. Finally, several applications of fatty acid esters, obtained by transesterification of vegetable oils, are described.A transesterificação de óleos vegetais com metanol, bem como as principais aplicações de ésteres metílicos de ácidos graxos, são revisadas. São descritos os aspectos gerais desse processo e a aplicabilidade de diferentes tipos de catalisadores (ácidos, hidróxidos, alcóxidos e carbonatos de metais alcalinos, enzimas e bases não-iônicas, como aminas, amidinas, guanidinas e triamino(imino)fosforanos). Enfoque especial é dado às guanidinas, que podem ser facilmente heterogeneizadas em polímeros orgânicos. No entanto, as bases ancoradas lixiviam dos polímeros. Novas estratégias para a obtenção de guanidinas heterogeneizadas mais resistentes à lixiviação são propostas. Finalmente, são descritas várias aplicações para ésteres de ácidos graxos, obtidos por transesterificação de óleos vegetais.199210Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Propriedades magnéticas de semicondutores amorfos dopados com terras-raras a-SiRE(:H) e de grafites pirolíticos altamente orientados HOPG

Orientadores: Carlos Rettori, Iris C. Linares de TorrianiTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Esta tese de doutorado resume-se no estudo de dois sistemas distintos: Semicondutores amorfos de silício dopados com terras-raras "a-SiRE(:H)", e grafites pirolíticos altamente orientados "HOPG ". Os semicondutores amorfos estudados são filmes finos de silício dopados com terras-raras (RE), crescidos por rf-sputtering (processo de deposição de filmes a partir de uma atmosfera de plasma criada por rádio-freqüência), hidrogenados ou não. Terras-raras magnéticas (Gd e Er) e não-magnéticas (Y, La e Lu) foram utilizadas com o intuito de estudar o Er incorporado na matriz de Si, devido às suas características especiais para a tecnologia fotônica. Por ser uma sonda mais simples de se estudar do ponto de vista magnético (por apresentar estado fundamental S, e portanto efeitos de campo cristalino em 2ª ordem), o íon de Gd3+ foi usado para determinar o comportamento dessa classe de lantanídeos dentro dos filmes de silício. Apesar das principais técnicas espectroscópicas utilizadas requererem sistemas paramagnéticos, também foram crescidos filmes com espécies dopantes não-magnéticas, ou diamagnéticas, a fim de subtraírem-se efeitos não magnéticos e, dessa maneira, simplificar a análise do sistema abordado. Os resultados de ressonância paramagnética eletrônica (RPE) revelam que a dopagem do filme com RE diminui a quantidade de defeitos magnéticos na matriz de uma a três ordens de grandeza, sendo que essa diminuição está relacionada com o spin de cada RE dopante. Este comportamento é regido ou pelo fator de spin S(S + 1), ou pelo fator de de Gennes (gJ - 1)2J(J + 1), como no caso de supercondutores. Nota-se também a criação de uma ligação química Si-RE extremamente estável, já que a linha de ressonância do Gd3+ não se modifica com alterações nas concentrações de RE ou H, com variação na temperatura, ou no tipo de vizinhança química (matriz amorfa ou nanocristalina). Os grafites pirolíticos altamente orientados, HOPG, assim como grafites cristalinos sintéticos (kish) para comparação, foram analisados em três bandas distintas de RPE. Foram utilizadas as bandas Q, X e S, para que se atingissem campos de ressonância com valores compreendidos entre 12.100 Oe e 1000 Oe, já que temos a verificação de uma transição metal-isolante (MIT) nos compostos citados, assistida por campo magnético. Dessa forma foi estudada, ineditamente, a variação do valor-g das amostras de grafites em função da freqüência de microonda. No final, conclui-se a formação de um campo magnético interno no grafite, devido à criação de um momento magnético, que ocorre pela abertura de um gap no ponto K do espectro de dispersão linear de Dirac. Esse campo interno está relacionado com a MIT verificada nas amostrasAbstract: The work on this PhD thesis was accomplished by the study of two distinct systems: amorphous semiconductors doped with rare-earth metals "a-SiRE(:H)", and highly oriented pirolytic graphites "HOPG". The amorphous semiconductors in interest are silicon thin films, hydrogenated or not, doped with rare-earth metals (RE), grown by rf-sputtering (film deposition process using a plasma atmosphere formed by radio frequency waves). Magnetic (Gd and Er) and non-magnetic (Y, La and Lu) rare-earth metals were used as dopants to evaluate the behaviour of Er incorporated on the Si matrix, due to its special features shown on fotonic technology. Gd3+ ions were chosen to explore the behaviour of the lanthanide group in silicon films, because it shows an S type fundamental state, which gives second order crystalline electrical field effects. Despite of the need of paramagnetic systems for the main spectroscopic technics employed, films with non-magnetic and diamagnetic dopant species were also grown to be used as reference for non-magnetic effects, simplifying analysis of the systems of interest. The results of electron paramagnetic ressonance (EPR) show a one to three fold decrease on the amount of magnetic deffects on the Si matrix for films dopped with RE, which varies accordingly to the spin of every RE dopant species. This behaviour scales through the RE series based on a spin factor, S(S + 1), or de Gennes factor, (gJ - 1)2J(J + 1), as for superconductors. It is also shown that a highly stable Si-RE bond is formed, as the Gd3+ line of ressonance is not shifted upon changes on RE or H concentration, temperature and framework environment (amorphous or nanocrystalline matrix). Highly oriented pyrolitic graphites, HOPG, as does synthetic crystalline graphites (kish), used for comparison purposes, were analysed by three distinct EPR bands. Ressonance fields as high as 12,100 Oe and 1,000 Oe were employed using Q, X and S bands, in order to explore a magnetic field-induced metal-insulator transition (MIT) observed in this system. This multiple-Bands EPR experiment allow us to report the first study of the variations of g-value of graphite samples as a function of microwave frequency. This work revealed a microscopic evidence for an internal magnetic field on graphite, which arises from magnetic moments formed presumably by a gap oppening at the K point on the linear dispersion spectrum of Dirac driven by an applied external magnetic field. This internal field is based on the MIT observed in these samplesDoutoradoFísicaDoutor em Ciência

Estudo de semicondutores amorfos dopados com terras raras (Gd e Er) e de polímeros condutores através das técnicas de RPE e susceptibilidade magnética

Orientador: Carlos RettoriDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Este trabalho de tese aborda dois sistemas de estudo: semicondutores amorfos e polímeros condutores. Ambos os sistemas foram estudados através da técnica de RPE, sendo ainda utilizadas as técnicas de susceptibilidade magnética, Raman e RBS (estas duas últimas como técnicas complementares), para o estudo de semicondutores amorfos. No estudo de semicondutores, foram utilizados filmes finos de silício amorfo dopados com terras raras, essencialmente érbio e gadolíneo, para o estudo de suas propriedades magnéticas. Estes estudos permitiram determinar o estado de valência da terra rara, bem como suas concentrações na matriz de silício do filme fino. De acordo com nossos resultados, a valência dos íons de terra rara é 3+, permitindo-nos concluir que a camada eletrônica 4f, por não contribuir com elétrons para a camada de condução, não poderia fazer parte do cálculo de bandas nesse tipo de semicondutor. E ainda, através da análise dos dados de susceptibilidade magnética do íon de Er 3+ em campo cristalino de simetria cúbica, pudemos estimar, de forma inédita, a separação máxima (overall splitting) de seus estados eletrônicos. No capítulo de polímeros condutores, foram estudadas amostras de poli (3-metiltiofeno) dopadas com ClO4 - , onde pudemos evidenciar uma transformação de fase na faixa de temperatura de 230 K até 130 K. Através da técnica de RPE, foram mostradas informações a respeito da cristalização, nível de dopagem, e presença de polarons ou bipolarons nesse polímero condutorAbstract: This thesis involves the study of amorphous semiconductors and conducting polymers, which have been characterized by EPR and magnetic susceptibility measurements, and to a lesser extent by Raman spectroscopy and RBS. The semiconductors were studied using thin films of silicon dopped with rare earth metals, e.g. erbium and gadolinium, which had their magnetic properties studied. Using these studies we could determine the state of valence of the rare earths as well as their concentrations in the silicon matrix. According to our results, the valence of the rare earth metal ions is 3+, and we were able to conclude that 4f electronic shells could not be used for the calculation of the conducting band in this system. Furthermore, the analysis of the data on the magnetic susceptibility of the Er3+ ion with cubic crystalline acting field, gave us the opportunity to estimate the overall splitting of their electronic states for the first time. The conducting polymers were studied using samples of poly(3-methylthiophene) dopped with ClO4- , which show a phase transition in the range of 230 K to 130 K. The electron paramagnetic resonance also gives important information on the crystalization, dopping level and the presence of polarons or bipolarons in conducting polymersMestradoFísicaMestre em Físic

Unconventional Metallic Magnetism in LaCrSb{3}

Neutron-diffraction measurements in LaCrSb{3} show a coexistence of ferromagnetic and antiferromagnetic sublattices below Tc=126 K, with ordered moments of 1.65(4) and 0.49(4) Bohr magnetons per formula unit, respectively (T=10 K), and a spin reorientation transition at ~95 K. No clear peak or step was observed in the specific heat at Tc. Coexisting localized and itinerant spins are suggested.Comment: PRL, in pres

Absence of Metal-Insulator-Transition and Coherent Interlayer Transport in oriented graphite in parallel magnetic fields

Measurements of the magnetoresistivity of graphite with a high degree of control of the angle between the sample and magnetic field indicate that the metal-insulator transition (MIT), shown to be induced by a magnetic field applied perpendicular to the layers, does not appear in parallel field orientation. Furthermore, we show that interlayer transport is coherent in less ordered samples and high magnetic fields, whereas appears to be incoherent in less disordered samples. Our results demonstrate the two-dimensionality of the electron system in ideal graphite samples.Comment: 4 figures, details adde

Copper Containing Silicates as Catalysts for Liquid Phase Cyclohexane Oxidation

Copper containing silicates have been prepared by an acid-catalyzed sol-gel process. The materials were characterized by X-ray diffraction and fluorescence, EPR spectroscopy, elemental analysis, N2-physisorption, thermogravimetry, differential scanning calorimetry, temperature-programmed reduction, FTIR and UV/VIS spectroscopy. The silicates were shown to be efficient catalysts for the oxidation of cyclohexane with tert-butyl hydroperoxide as oxidant. Cyclohexanol and cyclohexanone were obtained as the main products. The metal was shown to be weakly bound to the silicate matrix and metal leaching was observed. Leaching was quantified by X-ray fluorescence and leaching tests showed that the catalytic activity is due to supported copper species. Leached copper showed no activity in the homogeneous phase.Silicatos contendo cobre foram preparados pelo processo sol-gel catalisado por ácido. Os materiais foram caracterizados por difração e fluorescência de raios X, espectroscopia EPR, análise elementar, fisissorção de N2, termogravimetria, calorimetria diferencial de varredura, redução à temperatura programada, espectroscopia FTIR e UV/VIS. Os silicatos mostram-se catalisadores eficientes para a oxidação do cicloexano com hidroperóxido de tert-butila como oxidante e obtiveram-se cicloexanona e cicloexanol como produtos principais da reação. O metal mostrou-se fracamente ligado à matriz de sílica e foi observada lixiviação do metal para a fase líquida. A lixiviação foi quantificada por fluorescência de raios X. Testes de lixiviação mostraram que a atividade catalítica é devida às espécies de cobre suportadas. O cobre lixiviado não mostrou nenhuma atividade catalítica em fase homogênea.170176Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Fluctuating field model for conduction electron spin resonance in graphite

We outline a theory for conduction electron-spin resonance (CESR) in highly oriented pyrolytic graphite. The fundamental approximation is to treat the spin-orbit interaction as an effective field. In this approach, the shift in the g factor, which is associated with the mean value of the field, is related to the orbital susceptibility of the electrons. The linewidth comes from fluctuations in the effective field caused by the scattering of the electrons. The theory is used to interpret our CESR measurements.701

Evidence for internal field in graphite: A conduction electron spin resonance study

We report conduction electron spin resonance measurements performed on highly oriented pyrolitic graphite samples between 10 K and 300 K using S (f = 4 GHz), X (f = 9.4 GHz), and Q (f = 34.4 GHz) microwave bands for the external dc-magnetic field applied parallel (H || c) and perpendicular (H perp c) to the sample hexagonal c-axis. The results obtained in the H || c geometry are interpreted in terms of the presence of an effective internal ferromagnetic-like field Heff-int(T,H) that increases as the temperature decreases and the applied dc-magnetic field increases. We associate the occurrence of the Heff-int(T,H) with the field-induced metal-insulator transition in graphite and discuss its origin in the light of relevant theoretical models.Comment: 10 pages (tex), 5 figures (ps

Asymmetrical hippocampal connectivity in mesial temporal lobe epilepsy: evidence from resting state fMRI

<p>Abstract</p> <p>Background</p> <p>Mesial temporal lobe epilepsy (MTLE), the most common type of focal epilepsy in adults, is often caused by hippocampal sclerosis (HS). Patients with HS usually present memory dysfunction, which is material-specific according to the hemisphere involved and has been correlated to the degree of HS as measured by postoperative histopathology as well as by the degree of hippocampal atrophy on magnetic resonance imaging (MRI). Verbal memory is mostly affected by left-sided HS, whereas visuo-spatial memory is more affected by right HS. Some of these impairments may be related to abnormalities of the network in which individual hippocampus takes part. Functional connectivity can play an important role to understand how the hippocampi interact with other brain areas. It can be estimated via functional Magnetic Resonance Imaging (fMRI) resting state experiments by evaluating patterns of functional networks. In this study, we investigated the functional connectivity patterns of 9 control subjects, 9 patients with right MTLE and 9 patients with left MTLE.</p> <p>Results</p> <p>We detected differences in functional connectivity within and between hippocampi in patients with unilateral MTLE associated with ipsilateral HS by resting state fMRI. Functional connectivity resulted to be more impaired ipsilateral to the seizure focus in both patient groups when compared to control subjects. This effect was even more pronounced for the left MTLE group.</p> <p>Conclusions</p> <p>The findings presented here suggest that left HS causes more reduction of functional connectivity than right HS in subjects with left hemisphere dominance for language.</p