Different effects of Ni and Co substitution on the transport properties of BaFe2As2

We report resistivity and Hall effect results on Ba(Fe1-xNix)2As2 and compare them with those in Ba(Fe1-xCox)2As2. The Hall number RH is negative for all x values from 0.01 to 0.14, which indicates that electron carriers dominate the transport both in the magnetic and paramagnetic regime. We analyse the data in the framework of a two-band model. Without any assumption on the number of carriers, we show that the electron resistivity can be estimated with good accuracy in the low temperature paramagnetic range. Although the phase diagrams of the two families are very similar with respect to the extra electrons added in the system, we find that the transport properties differ in several aspects. First, we evidence that the contribution of holes to the transport is more important for Ni doping than for Co doping. Secondly, Ni behaves as a stronger scatterer for the electrons, as the increase of the residual electron resistivity rho/x is about four times larger for Ni than for Co in the most doped samples.Comment: 8 pages, 8 figure

Preliminary evidence that the long-proboscid fly, Philoliche gulosa, pollinates Disa karooica and its proposed Batesian model Pelargonium stipulaceum

AbstractLong-proboscid flies have been documented as the primary pollinators in several long-tubed floral guilds in southern Africa. These specialized pollination systems have become models for studying floral mimicry, adaptation, and coevolution. In this study we document a previously unknown pollinator of Pelargonium stipulaceum (L.f.) Willd. (Geraniaceae) and Disa karooica Johnson & Linder, a species within the Disa draconis complex (Orchidaceae), and propose the occurrence of a new floral model-mimic system in the semiarid interior of South Africa. We find that Philoliche gulosa Wiedemann (Tabanidae) is a pollinator of the putative floral model, Pelargonium stipulaceum and its floral mimic, Disa karooica. We document similarities in floral spur/tube length, spectral reflectance, absence of scent, presence of nectar guides, and overlapping phenology and distribution in these two sympatric plant species. We use colour spectrum analyses to test the idea that divergence in flower colour in the D. draconis complex is driven by adaptation to model Pelargonium species of different colours

Single-valley high-mobility (110) AlAs quantum wells with anisotropic mass

We studied a doping series of (110)-oriented AlAs quantum wells (QWs) and observed transport evidence of single anisotropic-mass valley occupancy for the electrons in a 150 \AA wide QW. Our calculations of strain and quantum confinement for these samples predict single anisotropic-mass valley occupancy for well widths $W$ greater than 53 \AA. Below this, double-valley occupation is predicted such that the longitudinal mass axes are collinear. We observed mobility anisotropy in the electronic transport along the crystallographic directions in the ratio of 2.8, attributed to the mass anisotropy as well as anisotropic scattering of the electrons in the X-valley of AlAs

Multiorbital effects on the transport and the superconducting fluctuations in LiFeAs

The resistivity, Hall effect and transverse magnetoresistance (MR) have been measured in low residual resistivity single crystals of LiFeAs. A comparison with angle resolved photoemission spectroscopy and quantum oscillation data implies that four carrier bands unevenly contribute to the transport. However the scattering rates of the carriers all display the T^2 behavior expected for a Fermi liquid. Near Tc low field deviations of the MR with respect to a H^2 variation permit us to extract the superconducting fluctuation contribution to the conductivity. Though below Tc the anisotropy of superconductivity is rather small, the superconducting fluctuations display a quasi ideal two-dimensional behavior which persists up to 1.4 Tc. These results call for a refined theoretical understanding of the multiband behavior of superconductivity in this pnictide.Comment: 8pages with supplementary material, 6 figure

Hole and Electron Contributions to the Transport Properties of Ba(Fe_(1-x)Ru_x)_2As_2 Single Crystals

We report a systematic study of structural and transport properties in single crystals of Ba(Fe_(1-x)Ru_x)_2As_2 for x ranging from 0 to 0.5. The isovalent substitution of Fe by Ru leads to an increase of the a parameter and a decrease of the c parameter, resulting in a strong increase of the AsFeAs angle and a decrease of the As height above the Fe planes. Upon Ru substitution, the magnetic order is progressively suppressed and superconductivity emerges for x > 0.15, with an optimal Tc ~ 20K at x = 0.35 and coexistence of magnetism and superconductivity between these two Ru contents. Moreover, the Hall coefficient RH which is always negative and decreases with temperature in BaFe2As2, is found to increase here with decreasing T and even change sign for x > 0.15. For x_Ru = 0.35, photo-emission studies have shown that the number of holes and electrons are similar with n_e = n_h ~ 0.11, that is twice larger than found in BaFe2As2 [1]. Using this estimate, we find that the transport properties of Ba(Fe_0.65Ru_0.35)_2As_2 can be accounted for by the conventional multiband description for a compensated semi-metal. In particular, our results show that the mobility of holes is strongly enhanced upon Ru addition and overcomes that of electrons at low temperature when x_Ru > 0.15.Comment: new version with minor correction

Investigation of acceptor levels and hole scattering mechanisms in p-gallium selenide by means of transport measurements under pressure

The effect of pressure on acceptor levels and hole scattering mechanisms in p-GaSe is investigated through Hall effect and resistivity measurements under quasi-hydrostatic conditions up to 4 GPa. The pressure dependence of the hole concentration is interpreted through a carrier statistics equation with a single (nitrogen) or double (tin) acceptor whose ionization energies decrease under pressure due to the dielectric constant increase. The pressure effect on the hole mobility is also accounted for by considering the pressure dependencies of both the phonon frequencies and the hole-phonon coupling constants involved in the scattering rates.Comment: 13 pages, Latex, 4 ps figures. to appear in High Pressure Research 69 (1997

The role of pollinators in the floral diversification and life history of Leucospermum (Proteaceae)

Leucospermum, commonly known as pincushions, are an endemic genus often associated with South Africa and, more specifically, the fynbos ecosystem and Cape Floral Kingdom. The staggering floral polymorphism within the genus suggests a wide variety of pollination modes. Despite this remarkable assortment of floral morphologies within Leucospermum only recently has any research been conducted to explain the role of pollinators in driving diversification. In this study we construct the first molecular phylogeny for the genus and, taking relatedness into account using a phylogenetic generalized least squares approach, determine how pollinators have driven floral divergence. Pollinators were assigned through direct observations and a suite of floral traits that are likely to reflect pollination mode were measured for each taxa. In addition, the ability to autonomously self-pollinate was determined for each taxon and used to test if autogamy is linked to specific classes of pollinators, specialized taxa or recently derived taxa. All fieldwork has been finalized and we are awaiting our phylogenetic analysis before any results can be determined