Electric field control of multiferroic domains in Ni3_3V2_2O8_8 imaged by X-ray polarization enhanced topography

The magnetic structure of multiferroic Ni$_3$V$_2$O$_8$ has been investigated using non-resonant X-ray magnetic scattering. Incident circularly polarized X-rays combined with full polarization analysis of the scattered beam is shown to yield high sensitivity to the components of the cycloidal magnetic order, including their relative phases. New information on the magnetic structure in the ferroelectric phase is obtained, where it is found that the magnetic moments on the "cross-tie" sites are quenched relative to those on the "spine" sites. This implies that the onset of ferroelectricity is associated mainly with spine site magnetic order. We also demonstrate that our technique enables the imaging of multiferroic domains through polarization enhanced topography. This approach is used to image the domains as the sample is cycled by an electric field through its hysteresis loop, revealing the gradual switching of domains without nucleation.Comment: 9 pages, 6 figure

Fermi surface and quasiparticle dynamics of Na(x)CoO2 {x=0.7} investigated by Angle-Resolved Photoemission Spectroscopy

We present an angle-resolved photoemission study of Na0.7CoO2, the host cobaltate of the NaxCoO2.yH2O series. Our results show a large hexagonal-like hole-type Fermi surface, an extremely narrow strongly renormalized quasiparticle band and a small Fermi velocity. Along the Gamma to M high symmetry line, the quasiparticle band crosses the Fermi level from M toward Gamma consistent with a negative sign of effective single-particle hopping (t ): t is estimated to be about 8 meV which is on the order of exchange coupling J in this system. This suggests that t ~ J ~ 10 meV is an important energy scale in the system. Quasiparticles are well defined only in the T-linear resistivity regime. Small single particle hopping and unconventional quasiparticle dynamics may have implications for understanding the unusual behavior of this new class of compounds.Comment: Revised text, Added Figs, Submitted to PR

Field dependence of magnetic ordering in Kagome-staircase compound Ni3V2O8

We present powder and single-crystal neutron diffraction and bulk measurements of the Kagome-staircase compound Ni3V2O8 (NVO) in fields up to 8.5T applied along the c-direction. (The Kagome plane is the a-c plane.) This system contains two types of Ni ions, which we call "spine" and "cross-tie". Our neutron measurements can be described with the paramagnetic space group Cmca for T < 15K and each observed magnetically ordered phase is characterized by the appropriate irreducible representation(s). Our zero-field measurements show that at T_PH=9.1K NVO undergoes a transition to an incommensurate order which is dominated by a longitudinally-modulated structure with the spine spins mainly parallel to the a-axis. Upon further cooling, a transition is induced at T_HL=6.3K to an elliptically polarized incommensurate structure with both spine and cross-tie moments in the a-b plane. At T_LC=4K the system undergoes a first-order phase transition, below which the magnetic structure is a commensurate antiferromagnet with the staggered magnetization primarily along the a-axis and a weak ferromagnetic moment along the c-axis. A specific heat peak at T_CC'=2.3K indicates an additional transition, which we were however not able to relate to a change of the magnetic structure. Neutron, specific heat, and magnetization measurements produce a comprehensive temperature-field phase diagram. The symmetries of the two incommensurate magnetic phases are consistent with the observation that only one phase has a spontaneous ferroelectric polarization. All the observed magnetic structures are explained theoretically using a simplified model Hamiltonian, involving competing nearest- and next-nearest-neighbor exchange interactions, spin anisotropy, Dzyaloshinskii-Moriya and pseudo-dipolar interactions.Comment: 25 pages, 19 figure

Magnetically Driven Ferroelectric Order in Ni\u3csub\u3e3\u3c/sub\u3eV\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e8\u3c/sub\u3e

We show that long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition in Ni3V2O8. The temperature and magnetic-field dependence of the spontaneous polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry using Landau theory for continuous phase transitions, which shows that the spin structure alone can break spatial inversion symmetry leading to ferroelectric order. This phenomenological theory explains our experimental observation that the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter

Strongly linked current flow in polycrystalline forms of the new superconductor MgB2

The discovery of superconductivity at 39 K in MgB2[1] raises many issues. One of the central questions is whether this new superconductor resembles a high-temperature-cuprate superconductor or a low-temperature metallic superconductor in terms of its current carrying characteristics in applied magnetic fields. In spite of the very high transition temperatures of the cuprate superconductors, their performance in magnetic fields has several drawbacks[2]. Their large anisotropy restricts high bulk current densities to much less than the full magnetic field-temperature (H-T) space over which superconductivity is found. Further, weak coupling across grain boundaries makes transport current densities in untextured polycrystalline forms low and strongly magnetic field sensitive[3,4]. These studies of MgB2 address both issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature of our samples, supercurrents flow throughout without the strong sensitivity to weak magnetic fields characteristic of Josephson-coupled grains[3]. Magnetization measurements over nearly all of the superconducting H-T plane show good temperature scaling of the flux pinning force, suggestive of a current density determined by flux pinning. At least two length scales are suggested by the magnetization and magneto optical (MO) analysis but the cause of this seems to be phase inhomogeneity, porosity, and minority insulating phase such as MgO rather than by weakly coupled grain boundaries. Our results suggest that polycrystalline ceramics of this new class of superconductor will not be compromised by the weak link problems of the high temperature superconductors, a conclusion with enormous significance for applications if higher temperature analogs of this compound can be discovered

Giant anharmonicity and non-linear electron-phonon coupling in MgB2_{2}; A combined first-principles calculations and neutron scattering study

We report first-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB$_{2}$. The excellent agreement between theory and our inelastic neutron scattering measurements of the phonon density of states gives confidence that the calculations provide a sound description of the physical properties of the system. The numerical results reveal that the in-plane boron phonons (with E$_{2g}$ symmetry) near the zone-center are very anharmonic, and are strongly coupled to the partially occupied planar B $\sigma$ bands near the Fermi level. This giant anharmonicity and non-linear electron-phonon coupling is key to explaining the observed high T$_{c}$ and boron isotope effect in MgB$_{2}$Comment: In this revised version (to appear in PRL) we also discuss the boron isotope effect. Please visit http://www.ncnr.nist.gov/staff/taner/mgb2 for detail

Investigation of the Spin Density Wave in NaxCoO2

Magnetic susceptibility, transport and heat capacity measurements of single crystal NaxCoO2 (x=0.71) are reported. A transition to a spin density wave (SDW) state at Tmag = 22 K is observable in all measurements, except chi(ac) data in which a cusp is observed at 4 K and attributed to a low temperature glassy phase. M(H) loops are hysteretic below 15 K. Both the SDW transition and low temperature hysteresis are only visible along the c-axis. The system also exhibits a substantial (~40%) positive magnetoresistance below this temperature. Calculations of the electronic heat capacity gamma above and below Tmag and the size of the jump in C indicate that the onset of the SDW brings about the opening of gap and the removal of part of the Fermi surface. Reduced in-plane electron-electron scattering counteracts the loss of carriers below the transition and as a result we see a net reduction in resistivity below Tmag. Sodium ordering transitions at higher temperatures are observable as peaks in the heat capacity with a corresponding increase in resistivity.Comment: 14 pages, 6 figure

Livro Vermelho dos Vertebrados de Portugal

Os Livros Vermelhos têm sido reconhecidos, pelas entidades responsáveis pela conservação da natureza, as organizações não-governamentais, a co- munidade científica e os decisores de projectos com incidência no ordena- mento e gestão do território, como elementos de consulta e instrumento de apoio à tomada de decisão de inegável utilidade. Neles se indica o esta- tuto de ameaça das espécies selvagens, de acordo com critérios quantitati- vos para avaliar níveis de risco de extinção e, ainda, informação sobre as populações, causas de ameaça e medidas de conservação. Estes são documentos em permanente actualização, reflectindo cada edição o melhor conhecimento científico disponível, e a sua elaboração deve ser considerada como uma tarefa de interesse público e mobilizadora de todos os que disponham de informação relevante e actualizada para a avaliação do estatuto das diferentes espécies. Um Livro Vermelho é ainda uma chamada de atenção e uma tomada de consciência perante a diminuição da diversidade biológica à escala global. Desde 1500, registou-se a extinção de 92 espécies de peixes, 5 de anfíbios, 22 de répteis, 131 de aves e 87 de mamíferos (Hylton-Taylor 2000). Regularmente, a União Mundial para a Conservação (IUCN) actualiza e alarga a avaliação do risco de extinção à escala mundial a um ainda maior número de espécies. Assim, de acordo com o exercício realizado em 2004 (IUCN 2004a) verifica-se o seguinte: para os mamíferos, a avaliação revela 23% de espécies ameaçadas, correspondente a 20% do total das espécies co- nhecidas; nas aves, a avaliação perfaz a quase totalidade das espécies conhecidas e atinge os 12% de espécies ameaçadas; os répteis avaliados apresentam uma percentagem muito elevada de espécies ameaçadas (61%), muito embora esta percentagem corresponda apenas a 4% do to- tal das espécies conhecidas; os anfíbios ameaçados são cerca de 31% das espécies conhecidas; nos peixes, grupo cuja proporção de avaliações está muito aquém das espécies conhecidas, os resultados revelam 46% de ameaçadas, correspondentes a 3% do total de espécies conhecidas. Nesta edição são listadas as espécies de peixes dulciaquícolas e migradores, anfíbios, répteis, aves e mamíferos que ocorrem em Portugal. Mas este é sobretudo o livro das espécies ameaçadas que enfrentam risco de extinção e a que, por isso, foram atribuídas as categorias de .Vulnerável., .Em Perigo. ou .Criticamente em Perigo.; é também o livro de algumas espécies às quais foi atribuída a categoria de .Quase Ameaçado. e ainda daquelas com .Informação Insuficiente. sobre a sua distribuição, requisitos de habitat, dimensão ou tendência populacional (entre outros aspectos) o que impediu a avaliação do risco de extinção. Atribuído um estatuto, identificam-se ainda os factores de ameaça, devendo estes ser entendidos como os fenómenos ou processos que sendo contí- nuos, esporádicos ou recorrentes, localizados ou generalizados, afectam as populações ou alteram a estrutura ou o funcionamento dos sistemas na- turais em que elas se integram, de um modo que afecta a sua reprodução ou mesmo sobrevivência. Estes factores são alterações das características físicas, químicas ou biológicas dos habitats das espécies ou acções que causam mortalidade intencional ou acidental ou, ainda, a redução do sucesso reprodutor das espécies. Com esta edição, alcançou-se uma meta no processo de avaliação da fau- na de vertebrados de Portugal, remetendo-se para uma segunda etapa a avaliação das espécies de peixes marinhos e estuarinos. Tendo a clara noção da efemeridade da avaliação do risco de extinção suportada pelas condições e conhecimentos de um momento, espera-se que o Livro Vermelho seja um documento periodicamente actualizado. Finalmente, é elevada a expectativa de todos os que participaram na sua elaboração, de que este venha a contribuir decisivamente para um reforço da conservação das espécies ameaçadas em Portugal

Karyotype and genome size of Iberochondrostoma almacai (Teleostei, Cyprinidae) and comparison with the sister-species I.lusitanicum

This study aimed to define the karyotype of the recently described Iberian endemic Iberochondrostoma almacai, to revisit the previously documented chromosome polymorphisms of its sister species I.lusitanicum using C-, Ag-/CMA3 and RE-banding, and to compare the two species genome sizes. A 2n = 50 karyotype (with the exception of a triploid I.lusitanicum specimen) and a corresponding haploid chromosome formula of 7M:15SM:3A (FN = 94) were found. Multiple NORs were observed in both species (in two submetacentric chromosome pairs, one of them clearly homologous) and a higher intra and interpopulational variability was evidenced in I.lusitanicum. Flow cytometry measurements of nuclear DNA content showed some significant differences in genome size both between and within species: the genome of I. almacai was smaller than that of I.lusitanicum (mean values 2.61 and 2.93 pg, respectively), which presented a clear interpopulational variability (mean values ranging from 2.72 to 3.00 pg). These data allowed the distinction of both taxa and confirmed the existence of two well differentiated groups within I. lusitanicum: one that includes the populations from the right bank of the Tejo and Samarra drainages, and another that reunites the southern populations. The peculiar differences between the two species, presently listed as “Critically Endangered”, reinforced the importance of this study for future conservation plans