30 research outputs found

    Doping phase diagrams of the heavy fermions Ce2MIn8 (M = Rh, IR)

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    Orientador: Pascoal Jos√© Giglio PagliusoTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Este trabalho apresenta estudos sistem√°ticos das propriedades magn√©ticas dos compostos Ce2MIn8 (M = Rh, Ir) dopados com Cd, Sn e Ga no s√≠tio cristalogr√°fico do In. Foram utilizadas as t√©cnicas de medidas macrosc√≥picas de susceptibilidade magn√©tica, calor espec√≠fico e resistividade el√©trica, al√©m de medidas de caracteriza√ß√£o estrutural pelo m√©todo de difra√ß√£o de p√≥ de raios-x e medidas microsc√≥picas da determina√ß√£o da estrutura magn√©tica usando a t√©cnica de espalhamento magn√©tico de raios-x e n√™utrons para os compostos dopados com Cd. Os compostos Ce2MIn8 (chamados de 218) s√£o tetragonais (grupo espacial P4/mmm) e s√£o a vers√£o bi-camada da fam√≠lia de f√©rmions pesados supercondutores Cem MnIn3m+2n(M = Co, Rh, Ir; m = 1,2; n = 0,1), na qual os compostos do tipo CeMIn5 (chamados de 115) foram muito estudados por apresentarem uma variedade de propriedades f√≠sicas interessantes, tais como efeito Kondo, supercondutividade n√£o convencional, ordenamento magn√©tico, comportamento non-Fermi-Liquid, e ocorr√™ncia de pontos cr√≠ticos qu√Ęnticos. Mais interessante ainda √© o fato de que estas propriedades podem ser controladas por dopagem, press√£o externa e/ou campo magn√©tico gerando diagramas de fases bastante interessantes. O objetivo de se estudar os parentes Ce2MIn8 √© entender o papel da dimensionalidade nas propriedades f√≠sicas destes compostos e comparar os resultados encontrados com aqueles observados para os 115. Foram constru√≠dos os diagramas de fase in√©ditos para as s√©ries de Ce2RhIn8-xCdx, Ce2IrIn8-xCdx, C e2Rh0.5Ir0.5In8-xC dx, Ce2RhIn8-xSnx e Ce2RhIn8-xGax a partir de dados obtidos utilizando as t√©cnicas mencionadas acima. Foi visto que a dopagem de Cd nos compostos Ce2MIn8 favorece o AFM atrav√©s do aumento da TN em fun√ß√£o da concentra√ß√£o de Cd. Os resultados sugerem que o Cd est√° agindo de modo a diminuir a densidade de carga local e isso est√° favorecendo o ordenamento magn√©tico. Por√©m, resultados da varia√ß√£o da temperatura do m√°ximo da resistividade el√©trica em fun√ß√£o da concentra√ß√£o de Cd, sugerem que efeitos de campo cristalino tamb√©m est√£o presentes nestes compostos. A estrutura magn√©tica dos compostos de Ce2RhIn8-xCdx e C e2IrIn8-xCdx dopados com x = 2.6 % de Cd foram estudados em baixas-T e encontrou-se um vetor de propaga√ß√£o do tipo (¬Ĺ ¬Ĺ 0) consistente com ordenamento AFM abaixo de TN. Verificou-se que a orienta√ß√£o do momento magn√©tico tende a ir na dire√ß√£o do plano-ab com dopagem de Cd nestes compostos. A substitui√ß√£o de In por Sn nos compostos de Ce2MIn8 mostrou que esta dopagem est√° desfavorecendo o ordenamento magn√©tico atrav√©s da diminui√ß√£o do valor da TN em fun√ß√£o da concentra√ß√£o de Sn. Os resultados sugerem que o Sn est√° favorecendo o efeito Kondo e que efeitos de campo cristalino s√£o menos observ√°veis nos compostos dopados com Sn. Foi visto que a dopagem com Ga no s√≠tio do In igualmente diminui a temperatura de ordenamento magn√©tico e favorece o efeito Kondo. Por√©m, os resultados sugerem que efeitos de campo cristalino est√£o presentes pela varia√ß√£o n√£o usual da temperatura do m√°ximo das curvas de resistividade el√©trica em fun√ß√£o da dopagem com Ga. Neste trabalho tamb√©m s√£o apresentados estudos da evolu√ß√£o do magnetismo dos el√©trons 4f, em compostos relacionados que n√£o s√£o f√©rmions pesados para R2CoGa8, para R = Gd, Tb, Dy, Ho, Er e Tm. Nestes compostos podemos observar a evolu√ß√£o do magnetismo e dos efeitos de campo cristalino sem a presen√ßa do efeito KondoAbstract: This thesis describes the investigation of the physical properties of the (Cd, Sn, Ga)-doped Ce2MIn8 (M = Rh, Ir) intermetallic compounds. Temperature dependent magnetic susceptibility, heat capacity and electrical resistivity measurements were performed to characterize the macroscopic properties of the synthesized compounds. X-ray powder diffraction were used to determine their phase purity and crystal structure and X-ray and neutron magnetic scattering techniques were used to study the microscopic magnetic structure in low temperature. The Ce2MIn8 (M = Rh, Ir) compounds belong to the family of the heavy-fermions Ce-based compounds CenMIn3n+2 (M = Co, Rh, Ir; n = 1, 2), where the occurrence of unconventional superconductivity (USC) in many of its members has been motivating intense work in past few years. In particular these compounds present a wide range of interesting properties as Kondo effect, USC, magnetism, non Fermi liquid and Fermi liquid behavior and the occurrence of quantum critical points. Particularly interesting is the tunability of their ground state by pressure and chemical doping that revealed very interesting and rich phase diagrams with properties that are unprecedented for this class of compounds. Indeed the study of the 218 parent compounds is a great opportunity to further study the relationship between magnetism, USC and crystal structure in this family. We report in this work five unpublished phase diagrams of this family: Ce2RhIn8-xCdx, Ce2IrIn8-xCdx, Ce2Rh0.5Ir0.5 In8-xCdx, Ce2RhIn8-xSnx e Ce2RhIn8-xGax. Our results revealed that Cd-doping Ce2MIn8 enhances the antiferromagnetic ordering temperature as a function of the Cd concentration. The results suggest that the Cd-doping in these compounds is inducing an electronic tuning by locally decreasing the Ce 3+ density of states and this is favoring the AFM ordering according com a Doniach-like scenario. However the evolution of the maximum in the resistivity as a function of the Cd concentration and the small variation of the magnetic moment orientation of the doped compounds suggest that Cd-doping is also changing the crystal field (CEF) parameters in these compounds. The magnetic structure studied by X-ray and neutron magnetic scattering showed the Cd-doped samples present just below TN a commensurate antiferromagnetic structure with a propagation vector (¬Ĺ ¬Ĺ 0). The magnetic structure determination indicates that the magnetic moment orientation of the Cd doped samples tend going to the ab-plane. The Sn chemical substitution in Ce2MI n8 (M = Rh, Ir) showed Sn-doping is decreasing the magnetic order by favoring the Kondo effect. This is the opposite of the Cd-doping effect because Sn is now increasing the local density of states of the Ce3+ions. As Sn-doping drives 4f to a more itinerant character the CEF effect are less important for this compounds. On the other hand, the Ga-doped Ce2RhI n8 similarly show a decreasing of the magnetic order due to the enhancement of the Kondo effect by chemical pressure. However the unexpected evolution of the maximum in the resistivity as a function of the Ga suggests in this dopant is changing the crystal field is still an important effect in this compound. Finally we present the 4f magnetism evolution studies on the series of the Ga-based R2CoGa8 for R = Gd ¬Ņ Tm compounds, where one can follow the evolution of the magnetism and the CEF parameters without the presence of the Kondo effectDoutoradoF√≠sica da Mat√©ria CondensadaDoutor em Ci√™ncia

    Diamond Thermal Expansion Measurement Using Transmitted X-ray Back-diffraction.

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    The linear thermal expansion coefficient of diamond has been measured using forward-diffracted profiles in X-ray backscattering. This experimental technique is presented as an alternative way of measuring thermal expansion coefficients of solids in the high-resolution Bragg backscattering geometry without the intrinsic difficulty of detecting the reflected beam. The temperature dependence of the lattice parameter is obtained from the high sensitivity of the transmitted profiles to the Bragg angle variation with temperature. The large angular width of the backscattering profiles allows the application of this technique to mosaic crystals with high resolution. As an application of this technique the thermal expansion coefficient of a synthetic type-Ib diamond (110) single crystal was measured from 10 to 300 K. Extremely low values (of the order of 1 x 10(-7) +/- 5 x 10(-7)) for the linear thermal expansion coefficient in the temperature range from 30 to 90 K are in good agreement with other reported measurements.12349-5

    ASPECTOS E IMPACTOS AMBIENTAIS - UMA VISÃO DA RESPONSABILIDADE SOCIOAMBIENTAL DE UMA EMPRESA DE PEQUENO PORTE

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    Esse artigo vem de forma singela apresentar algumas considera√ß√Ķes no que relaciona empresa e o meio¬† ambiente. Qual seu posionamento diante das novas mudan√ßas e responsabilidades que s√£o exigidas atualmente no cen√°rio sustent√°vel e com uma breve descri√ß√£o dos aspectos e impactos ambientais. Os dados foram coletados atrav√©s de question√°rios na organiza√ß√£o, que continha perguntas objetivas, que por fim averiguava as informa√ß√Ķes e confirmava a base da pesquisa realizada. Desta forma resultou no estudo da resposanbilidade socioambiental de uma empresa de pequeno porte, assim realiza√ß√£o de a√ß√Ķes sociais, praticas de preserva√ß√£o do meio ambiente e conscientizan√ß√Ķes para as futuras gera√ß√Ķes para resguardar os recursos ambientais

    Incoherent electronic band states in Mn substituted BaFe2_{2}As2_{2}

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    Chemical substitution is commonly used to explore new ground states in materials, yet the role of disorder is often overlooked. In Mn-substituted BaFe2_{2}As2_{2} (MnBFA), superconductivity (SC) is absent, despite being observed for nominal hole-doped phases. Instead, a glassy magnetic phase emerges, associated with the S=5/2S=5/2 Mn local spins. In this work, we present a comprehensive investigation of the electronic structure of MnBFA using angle-resolved photoemission spectroscopy (ARPES). We find that Mn causes electron pockets to shrink, disrupting the nesting condition in MnBFA. Notably, we propose that electronic disorder, along with magnetic scattering, primarily contributes to suppressing the itinerant magnetic order in MnBFA. This finding connects the MnBFA electronic band structure properties to the glassy magnetic behavior observed in these materials and suggests that SC is absent because of the collective magnetic impurity behavior that scatters the Fe-derived excitations. Moreover, we suggest that Mn tunes MnBFA to a phase in between the correlated metal in BaFe2_{2}As2_{2} and the Hund insulator phase in BaMn2_{2}As2_{2}.Comment: main 7 pages, 3 figures + supp 5 pages, 5 figure

    Alphaviruses Detected in Mosquitoes in the North-Eastern Regions of South Africa, 2014 to 2018

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    The prevalence and distribution of African alphaviruses such as chikungunya have increased in recent years. Therefore, a better understanding of the local distribution of alphaviruses in vectors across the African continent is important. Here, entomological surveillance was performed from 2014 to 2018 at selected sites in north-eastern parts of South Africa where alphaviruses have been identified during outbreaks in humans and animals in the past. Mosquitoes were collected using a net, CDC-light, and BG-traps. An alphavirus genus-specific nested RT-PCR was used for screening, and positive pools were confirmed by sequencing and phylogenetic analysis. We collected 64,603 mosquitoes from 11 genera, of which 39,035 females were tested. Overall, 1462 mosquito pools were tested, of which 21 were positive for alphaviruses. Sindbis (61.9%, N = 13) and Middelburg (28.6%, N = 6) viruses were the most prevalent. Ndumu virus was detected in two pools (9.5%, N = 2). No chikungunya positive pools were identified. Arboviral activity was concentrated in peri-urban, rural, and conservation areas. A range of Culicidae species, including Culex univittatus, Cx. pipiens s.l., Aedes durbanensis, and the Ae. dentatus group, were identified as potential vectors. These findings confirm the active circulation and distribution of alphaviruses in regions where human or animal infections were identified in South Africa.publishersversionpublishe

    Study of the structural and magnetic properties of epitaxial MnAs films on GaAs(001)

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    Orientadores: Carlos Manuel Giles Antunez de Mayolo, Fernando IikawaDisserta√ß√£o (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Neste trabalho foram estudados filmes finos de MoAs crescidos por epitaxia de feixe molecular em substratos de GaAs (001). As propriedades magn√©ticas destas hetero-estruturas (filme magn√©tico sobre um substrato semicondutor) s√£o fortemente dependentes das propriedades estruturais do filme de MoAs que foram estudadas detalhadamente utilizando t√©cnicas de difra√ß√£o de raios-x no Laborat√≥rio Nacional de Luz S√≠ncrotron. Dentre os principais resultados est√£o a identifica√ß√£o de uma temperatura de ordenamento magn√©tico maior do que os valores para o MnAs massivo, em filmes com acomoda√ß√Ķes cristalinas onde o eixo-c est√° inclinado em rela√ß√£o ao plano (001) do substrato. Tamb√©m foi realizado um estudo detalhado da varia√ß√£o do par√Ęmetro de rede com a temperatura em torno da transi√ß√£o de fase magn√©tica. Este estudo permitiu determinar a evolu√ß√£o das tens√Ķes el√°sticas presentes no filme de MoAs durante a transi√ß√£o onde em um intervalo em torno de 30¬įC acontece uma coexist√™ncia de fases cristalinas pela forma√ß√£o de faixas peri√≥dicas com regi√Ķes alternadas de a -MnAs (hexagonal) e ,b -MnAs (ortorr√īmbico). Tamb√©m se encontrou que os par√Ęmetros de rede contidos no plano do filme se comportam de forma bem diferente. Enquanto que o par√Ęmetro que corresponde ao eixo-c hexagonal n√£o varia muito durante a transi√ß√£o o outro par√Ęmetro de rede contido no filme aumenta fortemente com a temperatura. Estes estudos complementaram medidas magn√©ticas com efeito Kerr magn√©to-√≥ptico simult√Ęneas √†s medidas estruturais, assim como √†s medidas por difra√ß√£o de raios-x com aplica√ß√£o de campo magn√©ticos de at√© 3 kOeAbstract: Thin films ofMnAs grown by molecular beam epitaxy on GaAs (001) were studied by X-ray diffraction. The magnetic properties of a thin film of MnAs are strongly affected by the elastic tensions ofthe film accomodation over the semiconductor substrate and were studied by X-ray diffraction techniques at the Brazilian National Synchrotron Laboratory .It was found that the Curie temperature for the MnAs film is dependent on the type of crystal accomodation over the substrate. In particular for a MnAs accomodation with a tilted hexagonal c-axis with respect to the GaAs (00 1) plane, the transition temperature becomes higher than the value for MnAs bulk. A detailed analysis ofthe evolution ofthe lattice parameters through the transition temperature was performed. These results contribute to the understanding of the phase coexistence in the film through more than 30¬įC where periodic stripes of a -MnAs (hexagonal) and b-MnAs (orthorhombic) are formed to reduce the elastic tension of the film during this first order magnetic transition. It was found that while the hexagonal c-axis lattice parameter almost does not change during the phase transition, the in-plane lattice parameter perpendicular to the c-axis strongly increases with increasing temperature. These structural results complement magnetization measurements using an in situ magneto-optical Kerr effect setup and also structural measurements under applied magnetic fields up to 3 kOeMestradoF√≠sica da Mat√©ria CondensadaMestre em F√≠sic
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