142,460 research outputs found

    Mossbauer spectrometer radiation detector

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    A Mossbauer spectrometer with high efficiencies in both transmission and backscattering techniques is described. The device contains a sodium iodide crystal for detecting radiation caused by the Mossbauer effect, and two photomultipliers to collect the radiation detected by the crystal. When used in the transmission technique, the sample or scatterer is placed between the incident radiation source and the detector. When used in a backscattering technique, the detector is placed between the incident radiation source and the sample of scatterer such that the incident radiation will pass through a hole in the crystal and strike the sample. Diagrams of the instrument are provided

    Mossbauer effect and its applications in materials research

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    It is generally recognized that in any well behaved metal fatigue failure, the following sequence of events occurs. The application of stress causes the movement of defects already present in the specimen, besides producing new defects. These defects concentrate in the regions of stress concentration. This defect concentration, which precedes crack nucleation and its eventual growth, actually amounts to environmental changes for Mossbauer atoms located in these regions. Changes in the atomic composition of the Mossbauer absorber atom environment are sensed by examining changes in the sensitive parameters of the Mossbauer spectra. A preliminary study of changes in the isomer shift and line width of commercial nonmagnetic steel specimen is reported

    LaFeAsO1βˆ’x_{1-x}Fx_x: A low carrier density superconductor near itinerant magnetism

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    Density functional studies of 26K superconducting LaFeAs(O,F) are reported. We find a low carrier density, high density of states, N(EF)N(E_F) and modest phonon frequencies relative to TcT_c. The high N(EF)N(E_F) leads to proximity to itinerant magnetism, with competing ferromagnetic and antiferromagnetic fluctuations and the balance between these controlled by doping level. Thus LaFeAs(O,F) is in a unique class of high TcT_c superconductors: high N(EF)N(E_F) ionic metals near magnetism.Comment: Shortened published form. Typos correcte

    Properties of KCo2_2As2_2 and Alloys with Fe and Ru: Density Functional Calculations

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    Electronic structure calculations are presented for KCo2_2As2_2 and alloys with KFe2_2As2_2 and KRu2_2As2_2. These materials show electronic structures characteristic of coherent alloys, with a similar Fermi surface structure to that of the Fe-based superconductors, when the dd electron count is near six per transition metal. However, they are less magnetic than the corresponding Fe compounds. These results are discussed in relation to superconductivity.Comment: 5 page

    Optical properties of cubic and rhombohedral GeTe

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    Calculations of the optical properties of GeTe in the cubic NaCl and rhombohedral ferroelectric structures are reported. The rhombohedral ferroelectric distortion increases the band gap from 0.11 eV to 0.38 eV. Remarkably, substantial changes in optical properties are found even at high energies up to 5 eV. The results are discussed in relation to the bonding of GeTe and to phase change materials based on it

    Prediction of Room Temperature High Thermoelectric Performance in n-type La(Ru,Rh)4Sb12

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    First principles calculations are used to investigate the band structure and the transport related properties of unfilled and filled 4d skutterudite antimonides. The calculations show that, while RhSb3 and p-type La(Rh,Ru)4Sb12 are unfavorable for thermoelectric application, n-type La(Rh,Ru)4Sb12 is very likely a high figure of merit thermoelectric material in the important temperature range 150-300 K.Comment: 3 pages, 3 figures. To appear, Appl. Phys. Let

    Electronic structure of Ba(Fe,Ru)2As2 and Sr(Fe,Ir)2As2 alloys

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    The electronic structures of Ba(Fe,Ru)2_2As2_2 and Sr(Fe,Ir)2_2As2_2 are investigated using density functional calculations. We find that these systems behave as coherent alloys from the electronic structure point of view. In particular, the isoelectronic substitution of Fe by Ru does not provide doping, but rather suppresses the spin density wave characteristic of the pure Fe compound by a reduction in the Stoner enhancement and an increase in the band width due hybridization involving Ru. The electronic structure near the Fermi level otherwise remains quite similar to that of BaFe2_{2}As2_{2}. The behavior of the Ir alloy is similar, except that in this case there is additional electron doping

    Electronic Structure and Bulk Spin Valve Behavior in Ca3_3Ru2_2O7_7

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    We report density functional calculations of the magnetic properties and Fermiology of Ca3_3Ru2_2O7_7. The ground state consists of ferromagnetic bilayers, stacked antiferromagnetically. The bilayers are almost but not exactly half-metallic. In the ferromagnetic state opposite spin polarizations are found for in-plane and out-of-plane transport. Relatively high out of plane conductivity is found for the majority spin, which is relatively weakly conductive in-plane. In the ground state in-plane quantities are essentially the same, but the out of plane transport is strongly reduced.Comment: 5 page
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