52 research outputs found
WDX-Analysis of the New Superconductors RO(1-x)F(x)FeAs and Its Consequences on the Electronic Phase Diagram
Polycrystalline samples of RO1-xFxFeAs (0 < x < 0.25) (R = La, Sm, Gd) were
investigated by wavelength-dispersive X-ray spectroscopy (WDX) in the electron
microscope to determine the composition of the samples, in particular the
fluorine content. It was found that the measured fluorine content can deviate
considerably from the initial weight. In the lanthanum compound LaO1-xFxFeAs,
we found good agreement mainly for x > 0.05, but for x < 0.05 the fluorine
hardly goes into the sample. For the samarium compound we measured less than
half the fluorine in the sample as initially weighed at all fluorine
concentrations. These measured values are taken into account when drawing the
electronic phase diagrams of LaO1-xFxFeAs and SmO1-xFxFeAs. This leads to a
more consistent picture of both of the diagrams in comparison to the plot of
the initial weight.Comment: 5 pages, 4 figures, Accepted for publication in Journal of
Superconductivity and Novel Magnetis
Development of a low profile laser Doppler probe for monitoring perfusion at the patient – mattress interface
The clinical importance of pressure ulcers is reviewed confirming the need for continuous monitoring of skin blood perfusion at the patient – mattress interface. The design of a low profile (H≈1mm) laser Doppler probe is then described together with the experimental setup used for evaluation. The results show that the performance of the new sensor does not vary significantly from that of currently available probes over a wide range of operating parameters. The authors conclude that the sensor design provides a low cost perfusion monitoring solution with potential to significantly reduce the risk of bed sores in hospital patients
Imaging Flux Vortices in MgB2 using Transmission Electron Microscopy
We report the successful imaging of flux vortices in single crystal MgB2
using transmission electron microscopy. The specimen was thinned to electron
transparency (350 nm thickness) by focussed ion beam milling. An artefact of
the thinning process was the production of longitudinal thickness undulations
of height 1-2 nm in the sample which acted as pinning sites due to the energy
required for the vortices to cross them. These had a profound effect on the
patterns of vortex order observed which we examine here.
Supplementary information can be downloaded from
http://www-hrem.msm.cam.ac.uk/people/loudon/#publicationsComment: 3 pages, 2 figures to appear in Physica C. Supplementary information
can be downloaded from
http://www-hrem.msm.cam.ac.uk/people/loudon/#publications. The discussion of
the vortex-free region near the sample edge has been revised in response to
referees' comments. Changes have been made to clarify that the specimen
thickness is 250nm parallel to the c-axis but 350nm parallel to the electron
bea
Scanning Hall probe microscopy of unconventional vortex patterns in the two-gap MgB2 superconductor
The low magnetic field vortex patterns nucleation and evolution in a high-quality two-gap superconductor MgB2 single crystal have been investigated by low-temperature scanning Hall probe microscopy. Large areas have been imaged with single-vortex resolution while changing systematically the thermodynamic parameters for field and temperature. The obtained patterns have been studied and compared with those of a reference 2H-NbSe2 single crystal. We found that the observed vortex patterns in MgB2 (e.g., stripes, clusters) appear due to competing vortex-vortex interactions as suggested by the theory of type-1.5 superconductivity
Single crystal growth and properties of MgB2 and Mg(B1-xCx)2
Single crystals of MgB2 and Mg(B1-xCx)2 have been grown using cubic anvil
technique. Tc values vary in a wide range (39-9 K) with carbon content varying
from 0 up to 16%. Using SiC as the precursor leads to C and not to Si
substituted crystals. Micro-hardness measurements performed on MgB2 single
crystals give average value of 1100 kg/mm2.Comment: 6 pages, 3 figures. submitted to Physica C (M2S-Rio proceedings
Inelastic x-ray scattering investigations of lattice dynamics in SmFeAsOF superconductors
We report measurements of the phonon density of states as measured with
inelastic x-ray scattering in SmFeAsOF powders. An unexpected
strong renormalization of phonon branches around 23 meV is observed as fluorine
is substituted for oxygen. Phonon dispersion measurements on
SmFeAsOF single crystals allow us to identify the 21 meV A
in-phase (Sm,As) and the 26 meV B (Fe,O) modes to be responsible for
this renormalization, and may reveal unusual electron-phonon coupling through
the spin channel in iron-based superconductors.Comment: 4 pages, 3 figures, submitted for SNS2010 conference proceeding
MgB2 and Mg1-xAlxB2 single crystals: high-pressure growth and physical properties
Single crystals of MgB2 have been grown with a high-pressure cubic anvil
technique. They grow via the peritectic decomposition of the MgNB9 ternary
nitride. The crystals are of a size up to 2x1x0.1mm3 with a weight up to 230
micrograms. Typically they have transition temperatures between 38 and 38.6 K
with a width of 0.3-0.5 K. Investigations of the P-T phase diagram prove that
the MgB2 phase is stable at least up to 2190C at high hydrostatic pressure in
the presence of Mg vapor under high pressure. Substitution of aluminum for
magnesium in single crystals leads to stepwise decrease of Tc. This indicates a
possible appearance of superstructures or phases with different Tc's. The upper
critical field decreases with Al doping.Comment: 6 pages, 2 figures. submitted to Physica C (M2S-Rio proceedings).
Text and Fig.2 revise
Evidence for two distinct anisotropies in the oxypnictide superconductors SmFeAsO_(0.8)F_(0.2) and NdFeAsO_(0.8)F_(0.2)
Single crystals of the oxypnictide superconductors SmFeAsO_(0.8)F_(0.2) and
NdFeAsO_(0.8)F_(0.2) with T_c in the range of 44 K to 48 K were investigated by
torque magnetometry. An analysis of the data in terms of a recently proposed
model for the anisotropic magnetization in the superconducting state, treating
the penetration depth anisotropy differently than the upper critical field
anisotropy, provides evidence that in the oxypnictide superconductors two
distinct anisotropies are present. As a result the penetration depth anisotropy
differs significantly in magnitude and in temperature dependence from the upper
critical field anisotropy, analogous to MgB_2 but with a reversed sign of
slope. This scenario strongly suggests a new multi-band mechanism in the novel
class of oxypnictide high-temperature superconductors.Comment: published online in J. Supercond. Nov. Mag
A new triclinic modification of the pyrochlore-type KOs2O6 superconductor
A new modification of KOs2O6, the representative of a new structural type
(Pearson symbol aP18, a=5.5668(1)A, b=6.4519(2)A, c=7.2356(2)A, space group
P-1, no.2) was synthesized employing high pressure technique. Its structure was
determined by single-crystal X-ray diffraction. The structure can be described
as two OsO6 octahedral chains relating to each other through inversion and
forming big voids with K atoms inside. Quantum chemical calculations were
performed on the novel compound and structurally related cubic compound.
High-pressure X-ray study showed that cubic KOs2O6 phase was stable up to
32.5(2) GPa at room temperature.Comment: 23 pages, 9 figures,6 tables. Accepted for J. Solid State Che
Chiral singlet superconductivity in the weakly correlated metal LaPt3P
Chiral superconductors are novel topological materials with finite angular momentum Cooper pairs circulating around a unique chiral axis, thereby spontaneously breaking time-reversal symmetry. They are rather scarce and usually feature triplet pairing: a canonical example is the chiral p-wave state realized in the A-phase of superfluid He3. Chiral triplet superconductors are, however, topologically fragile with the corresponding gapless boundary modes only weakly protected against symmetry-preserving perturbations in contrast to their singlet counterparts. Using muon spin relaxation measurements, here we report that the weakly correlated pnictide compound LaPt3P has the two key features of a chiral superconductor: spontaneous magnetic fields inside the superconducting state indicating broken time-reversal symmetry and low temperature linear behaviour in the superfluid density indicating line nodes in the order parameter. Using symmetry analysis, first principles band structure calculation and mean-field theory, we unambiguously establish that the superconducting ground state of LaPt3P is a chiral d-wave singlet
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