Influence of randomly distributed magnetic nanoparticles on surface superconductivity in Nb films

We report on combined resistance and magnetic measurements in a hybrid structure (HS) of randomly distributed anisotropic CoPt magnetic nanoparticles (MN) embedded in a 160 nm Nb thick film. Our resistance measurements exhibited a sharp increase at the magnetically determined bulk upper-critical fields Hc2(T). Above these points the resistance curves are rounded, attaining the normal state value at much higher fields identified as the surface superconductivity fields Hc3(T). When plotted in reduced temperature units, the characteristic field lines Hc3(T) of the HS and of a pure Nb film, prepared at exactly the same conditions, coincide for H10 kOe they strongly segregate. Interestingly, the characteristic value H=10 kOe is equal to the saturation field of the MN. The behavior mentioned above is observed only for the case where the field is normal to the HS, while is absent when the field is parallel to the film. Our experimental results suggest that the observed enhancement of surface superconductivity field Hc3(T) is possibly due to the not uniform local reduction of the external magnetic field by the dipolar fields of the MN.Comment: to be published in Phys. Rev.

On rationality of the intersection points of a line with a plane quartic

We study the rationality of the intersection points of certain lines and smooth plane quartics C defined over F_q. For q \geq 127, we prove the existence of a line such that the intersection points with C are all rational. Using another approach, we further prove the existence of a tangent line with the same property as soon as the characteristic of F_q is different from 2 and q \geq 66^2+1. Finally, we study the probability of the existence of a rational flex on C and exhibit a curious behavior when the characteristic of F_q is equal to 3.Comment: 17 pages. Theorem 2 now includes the characteristic 2 case; Conjecture 1 from the previous version is proved wron

Stoner gap in the superconducting ferromagnet UGe2

We report the temperature ($T$) dependence of ferromagnetic Bragg peak intensities and dc magnetization of the superconducting ferromagnet UGe2 under pressure ($P$). We have found that the low-$T$ behavior of the uniform magnetization can be explained by a conventional Stoner model. A functional analysis of the data produces the following results: The ferromagnetic state below a critical pressure can be understood as the perfectly polarized state, in which heavy quasiparticles occupy only majority spin bands. A Stoner gap $\Delta(P)$ decreases monotonically with increasing pressure and increases linearly with magnetic field. We show that the present analysis based on the Stoner model is justified by a consistency check, i.e., comparison of density of states at the Fermi energy deduced from the analysis with observed electronic specific heat coeffieients. We also argue the influence of the ferromagnetism on the superconductivity.Comment: 5 pages, 4 figures. to be published in Phys. Rev.

Дополнительные потери энергии при последовательном соединении суперконденсаторов

Проаналізовано особливості послідовного з’єднаня суперконденсаторів. Показано, що несиметрія напруг на них, пов’язана з різною велечиною їх опорів втрат, проявляється в кінці перехідного процесу, який може тривати десятки і сотні годин. Проаналізовано існуючі методи балансування напруг на конденсаторах та їх енергетичні характеристики. Запропоновано метод балансування напруг суперконденсаторів з покращеними характеристиками.The features of series-connected supercapacitors. It is shown that the asymmetry of stress on them, associated with different velechynoyu their resistance loss appears at the end of the transition process , which can take hundreds of hours. Existing methods of balancing voltage on the capacitors and their energy performance. A method for balancing voltages of supercapacitors with improved characteristics.Проанализированы особенности последовательно соединенных суперконденсаторов. Показано, что несимметрия напряжений на них, связанная с разной велечиной их сопротивлений потерь, проявляется в конце переходного процесса, который может длиться десятки и сотни часов. Проанализированы существующие методы балансировки напряжений на конденсаторах и их энергетические характеристики. Предложен метод балансировки напряжений суперконденсаторов с улучшенными характеристиками

Minifúndios e "inquilinato" no estuário amazônico: o caso do projeto agroextrativista Nossa Senhora do Perpétuo Socorro, Ilha Quianduba, município de Abaetetuba, Pará.

As estratégias de sobrevivência dos minifúndios de várzea do estuário do Rio Amazonas, tendo como referência a Ilha Quianduba, situada no Município de Abaetetuba, no Estado do Pará, é a razão deste estudo. O espaço geográfico selecionado foi objeto da ação estatal, quando o Governo Federal, no ano de 2006, interveio na região, por intermédio do Instituto Nacional de Colonização e Reforma Agrária e da Gerência Regional do Patrimônio da União, e viabilizou a transformação de um imóvel rural com área de 2.810,9 hectares, sob jurisdição da União, num Projeto de Assentamento Agroextrativista, considerado uma Unidade de Conservação de Uso Sustentável, na modalidade Reserva Extrativista, por equiparação. As informações sobre as famílias, em número de 600, foram coletadas por intermédio de entrevistas de campo que, depois de tabuladas e confrontadas com a realidade, permitiram fazer inferências sobre as comunidades que integram o projeto, gênero, grau de parentesco, moradia, tempo de ocupação da área, hierarquia da ocupação, tamanho das posses, faixa etária dos componentes das famílias, renda familiar, educação, saúde, saneamento, associativismo, lazer, cultura, extrativismo do açaí, pesca do camarão, olarias, caça, pesca e utilização de crédito rural. As conclusões obtidas estão relacionadas com o reconhecimento de direitos dos moradores que passaram a integrar a base de dados do INCRA, sendo sujeitos de direitos e obrigações, quebrando o paradigma do sistema de inquilinato constituído até então. O emprego de políticas compensatórias pontuais sem considerar uma visão de médio prazo é incapaz de produzir melhoria nas condições de vida dos ribeirinhos, pois a ausência de assistência técnica, crédito rural, infraestrutura e documentação da terra é constatação cabal dessa assertiva. Caso os recursos naturais da Ilha continuem a ser explorados na atual escala, sem que haja atendimento às disposições reguladoras de seu uso, poderão levar a uma situação definida por Hardin (1968) como a tragédia dos comuns. A observação da relação simbiótica do Homem com a natureza representa a visão mais otimista desta proposta

Polarization degrees of freedom in photoinduced two-nucleon knockout from finite nuclei

The polarization degrees of freedom in photoinduced two-nucleon knockout from finite nuclei are studied. It is pointed out that they open good perspectives to study the dynamics of dinucleons in the medium in detail. The ($\gamma,pp$) and ($\gamma,pn$) angular cross sections, photon asymmetries and outgoing nucleon polarizations are calculated for the target nuclei $^{16}$O and $^{12}$C and photonenergies ranging from 100 up to 500 MeV. It is investigated to which degree the two-nucleon emission reaction is dominated by photoabsorption on $^3S_1(T=0)$ proton-neutron and $^1S_0(T=1)$ proton-proton pairs in the nuclear medium. The calculations demonstrate that dominance of $S$ wave photoabsorption in the ($\gamma,pn$) channel does not necessarily imply that the reaction mechanism is similar to what is observed in deuteron photodisintegration.Comment: 27 pages, REVTeX 3.0 with epsf.sty, 11 figures in EPS forma