1,059 research outputs found
Crystal growth and ambient and high pressure study of the reentrant superconductor Tm_2Fe_3Si_5
We report single crystal growth of the reentrant superconductor Tm_2Fe_3Si_5,
and measurements of the anisotropic static magnetic susceptibility \chi(T) and
isothermal magnetization M(H), ac susceptibility \chi_ac(T), electrical
resistivity \rho(T) and heat capacity C(T) at ambient pressure and \chi_ac(T)
at high pressure. The magnetic susceptibility along the c-axis \chi_c(T) shows
a small maximum around 250 K and does not follow the Curie-Weiss behavior while
the magnetic susceptibility along the a-axis \chi_a(T) follows a Curie-Weiss
behavior between 130 K and 300 K with a Weiss temperature \theta and an
effective magnetic moment \mu_eff which depend on the temperature range of the
fit. The easy axis of magnetization is perpendicular to the c-axis and
\chi_a/\chi_c = 3.2 at 1.8 K. The ambient pressure \chi_ac(T) and C(T)
measurements confirm bulk antiferromagnetic ordering at T_N = 1.1 K. The sharp
drop in \chi_ac below T_N is suggestive of the existence of a spin-gap. We
observe superconductivity only under applied pressures P\geq 2 kbar. The
temperature-pressure phase diagram showing the non-monotonic dependence of the
superconducting transition temperature T_c on pressure P is presented.Comment: 7 pages, 8 figure
Measuring and modeling near-surface reflected and emitted radiation fluxes at the FIFE site
Information is presented pertaining to the measurement and estimation of reflected and emitted components of the radiation balance. Information is included about reflectance and transmittance of solar radiation from and through the leaves of some grass and forb prairie species, bidirectional reflectance from a prairie canopy is discussed and measured and estimated fluxes are described of incoming and outgoing longwave and shortwave radiation. Results of the study showed only very small differences in reflectances and transmittances for the adaxial and abaxial surfaces of grass species in the visible and infrared wavebands, but some differences in the infrared wavebands were noted for the forbs. Reflectance from the prairie canopy changed as a function of solar and view zenith angles in the solar principal plane with definite asymmetry about nadir. The surface temperature of prairie canopies was found to vary by as much as 5 C depending on view zenith and azimuth position and on the solar azimuth. Aerodynamic temperature calculated from measured sensible heat fluxes ranged from 0 to 3 C higher than nadir-viewed temperatures. Models were developed to estimate incoming and reflected shortwave radiation from data collected with a Barnes Modular Multiband Radiometer. Several algorithms for estimating incoming longwave radiation were evaluated and compared to actual measures of that parameter. Net radiation was calculated using the estimated components of the shortwave radiation streams, determined from the algorithms developed, and from the longwave radiation streams provided by the Brunt, modified Deacon, and the Stefan-Boltzmann models. Estimates of net radiation were compared to measured values and found to be within the measurement error of the net radiometers used in the study
Magnetic Ordering and Superconductivity in the REIrGe (RE = Y, La-Tm, Lu) System
We find that the compounds for RE = Y, La-Dy, crystallize in the tetragonal
Ibam (UCoSi type) structure whereas the compounds for RE = Er-Lu,
crystallize in a new orthorhombic structure with a space group Pmmn. Samples of
HoIrGe were always found to be multiphase. The compounds for RE = Y
to Dy which adopt the Ibam type structure show a metallic resistivity whereas
the compounds with RE = Er, Tm and Lu show an anomalous behavior in the
resistivity with a semiconducting increase in as we go down in
temperature from 300K. Interestingly we had earlier found a positive
temperature coefficient of resistivity for the Yb sample in the same
temperature range. We will compare this behavior with similar observations in
the compounds RERuGe and REBiPt. LaIrGe and
YIrGe show bulk superconductivity below 1.8K and 2.5K respectively.
Our results confirm that CeIrGe shows a Kondo lattice behavior and
undergoes antiferromagnetic ordering below 8.5K. Most of the other compounds
containing magnetic rare-earth elements undergo a single antiferromagnetic
transition at low temperatures (T12K) while GdIrGe,
DyIrGe and NdIrGe show multiple transitions. The
T's for most of the compounds roughly scale with the de Gennes factor.
which suggests that the chief mechanism of interaction leading to the magnetic
ordering of the magnetic moments may be the RKKY interaction.Comment: 25 pages, 16 figure
Unusual Ground State Properties of the Kondo-Lattice Compound Yb2Ir3Ge5
We report sample preparation, structure, electrical resistivity, magnetic
susceptibility and heat capacity studies of a new compound YbIrGe.
We find that this compound crystallizes in an orthorhombic structure with a
space group PMMN unlike the compound CeIrGe which crystallizes in
the tetragonal IBAM (UCoSi type) structure. Our resistivity
measurements indicate that the compound YbIrGe behaves like a
typical Kondo lattice system with no ordering down to 0.4 K. However, a
Curie-Weiss fit of the inverse magnetic susceptibility above 100 K gives an
effective moment of only 3.66 which is considerably less than the
theoretical value of 4.54 for magnetic Yb ions. The value of
= -15.19 K is also considerably higher indicating the presence of
strong hybridization. An upturn in the low temperature heat capacity gives an
indication that the system may order magnetically just below the lowest
temperature of our heat capacity measurements (0.4 K). The structure contains
two sites for Yb ions and the present investigation suggests that Yb may be
trivalent in one site while it may be significantly lower (close to divalent)
in the other.Comment: 9 pages, 4 figures. submitted to Phys. Rev.
Estimating carbon stock change in agroforestry and family forestry practices
Paper presented at the 11th North American Agroforesty Conference, which was held May 31-June 3, 2009 in Columbia, Missouri.In Gold, M.A. and M.M. Hall, eds. Agroforestry Comes of Age: Putting Science into Practice. Proceedings, 11th North American Agroforestry Conference, Columbia, Mo., May 31-June 3, 2009.The Carbon Management Evaluation Tool for Voluntary Reporting (COMET-VR) is an online tool that estimates short-term carbon stock (CS) changes under different farm or forest land management systems, including temperate agroforestry practices. It was developed by the USDA Natural Resources Conservation Service in conjunction with Colorado State University. The intended audience includes private farm and forest landowners, NRCS field staff, and technical service providers. Through the online interface, users identify their location, parcel size, surface soil texture, crop rotation history and tillage intensity. The user can choose either of two methods to estimate CS change for their agroforestry practice: 1) for new or future plantings, by using standard prescriptions common to their geographic region, or 2) for a more accurate estimate of an existing planting, by using a summary of live-tree stand inventory data collected from their parcel. Above and below-ground individual tree biomass is calculated using diameter-based allometric equations generalized for tree genera groups. For existing agroforestry plantings, growth estimates are based on empirical models developed from forest inventory data specific to species and region. For new or future plantings, growth estimates were derived for standard agroforestry prescriptions using the Forest Vegetation Simulator. COMET-VR uses the Century soil carbon model to estimate CS change in soil. The output of the tool is a report estimating CS changes over the forthcoming 10 years in the above and below-ground portions of live trees and in the soil. Although specifically designed to meet the requirements of the US Dept. of Energy voluntary greenhouse gas reporting program, COMET-VR may also be applicable to other private and public sector carbon offset programs.Miles L. Merwin (1), Mark Easter (2), Lyn R. Townsend (1), Roel C. Vining (1) and Greg L. Johnson (1) ; 1. USDA Natural Resources Conservation Service, 1201 NE Lloyd Blvd., Portland, OR 97232. 2. Natural Resource Ecology Laboratory, Colorado State University, Ft. Collins, CO.Includes bibliographical references
Efficiency in nanostructured thermionic and thermoelectric devices
Advances in solid-state device design now allow the spectrum of transmitted
electrons in thermionic and thermoelectric devices to be engineered in ways
that were not previously possible. Here we show that the shape of the electron
energy spectrum in these devices has a significant impact on their performance.
We distinguish between traditional thermionic devices where electron momentum
is filtered in the direction of transport only and a second type, in which the
electron filtering occurs according to total electron momentum. Such 'total
momentum filtered' kr thermionic devices could potentially be implemented in,
for example, quantum dot superlattices. It is shown that whilst total momentum
filtered thermionic devices may achieve efficiency equal to the Carnot value,
traditional thermionic devices are limited to efficiency below this. Our second
main result is that the electronic efficiency of a device is not only improved
by reducing the width of the transmission filter as has previously been shown,
but also strongly depends on whether the transmission probability rises sharply
from zero to full transmission. The benefit of increasing efficiency through a
sharply rising transmission probability is that it can be achieved without
sacrificing device power, in contrast to the use of a narrow transmission
filter which can greatly reduce power. We show that devices which have a
sharply-rising transmission probability significantly outperform those which do
not and it is shown such transmission probabilities may be achieved with
practical single and multibarrier devices. Finally, we comment on the
implications of the effect the shape of the electron energy spectrum on the
efficiency of thermoelectric devices.Comment: 11 pages, 15 figure
Antiferromagnetic ordering in the Kondo lattice system YbFeSi
Compounds belonging to the RFeSi series exhibit unusual
superconducting and magnetic properties. Although a number of studies have been
made on the first reentrant antiferromagnet superconductor TmFeSi,
the physical properties of YbFeSi are largely unexplored. In this
work, we attempt to provide a comprehensive study of bulk properties such as,
resistivity, susceptibility and heat-capacity of a well characterized
polycrystalline YbFeSi. Our measurements indicate that Yb
moments order antiferromagnetically below 1.7 K. Moreover, the system behaves
as a Kondo lattice with large Sommerfeld coefficient () of 0.5~J/Yb mol
K at 0.3 K, which is well below T. The absence of superconductivity
in YbFeSi down to 0.3 K at ambient pressure is attributed to the
presence of the Kondo effect.Comment: 10 pages, 3 figures, tex document. A fuller version has appeared in
PRB. Here we have omitted the figures showing the crystal structure and the
fitting of the X-ray pattern. Also the table with the lattice parameters
obtained from fitting has been remove
A round table discussion on forensic science in Australia
This manuscript is an edited transcript of a round table discussion held during the Australian New Zealand Forensic Science Society International Symposium held in Sydney in 2010. The discussants covered a variety of topics, including the management of science, the handling of quality issues, and the report on forensic science from the U.S. National Academies of Science National Research Council. This discussion offers a frank account of the current state of Australian forensic service providers. These views are then considered in the context of recent events unfolding in the United Kingdom and in a broader international context. It poses the question, are there lessons to be learned from the Australian experience that would have relevance to other parts of the world
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