433 research outputs found
Superconductivity in SrNi2As2 Single Crystals
The electrical resistivity \rho(T) and heat capacity C(T) on single crystals
of SrNi2As2 and EuNi2As2 are reported. While there is no evidence for a
structural transition in either compound, SrNi2As2 is found to be a bulk
superconductor at T_c=0.62 K with a Sommerfeld coefficient of \gamma= 8.7
mJ/mol K^2 and a small upper critical field H_{c2} \sim 200 Oe. No
superconductivity was found in EuNi2As2 above 0.4 K, but anomalies in \rho and
C reveal that magnetic order associated with the Eu^{2+} magnetic moments
occurs at T_m = 14 K.Comment: 8 pages, 5 figure
Topological change of the Fermi surface in ternary iron-pnictides with reduced c/a ratio: A dHvA study of CaFe2P2
We report a de Haas-van Alphen effect study of the Fermi surface of CaFe2P2
using low temperature torque magnetometry up to 45 T. This system is a close
structural analogue of the collapsed tetragonal non-magnetic phase of CaFe2As2.
We find the Fermi surface of CaFe2P2 to differ from other related ternary
phosphides in that its topology is highly dispersive in the c-axis, being
three-dimensional in character and with identical mass enhancement on both
electron and hole pockets (~1.5). The dramatic change in topology of the Fermi
surface suggests that in a state with reduced (c/a) ratio, when bonding between
pnictogen layers becomes important, the Fermi surface sheets are unlikely to be
nested
Delivering strong 1H nuclear hyperpolarization levels and long magnetic lifetimes through signal amplification by reversible exchange
Hyperpolarization turns typically weak NMR and MRI responses into strong signals so that ordinarily impractical measurements become possible. The potential to revolutionize analytical NMR and clinical diagnosis through this approach reflect this area's most compelling outcomes. Methods to optimize the low cost parahydrogen based approach signal amplification by reversible exchange (SABRE) with studies on a series of biologically relevant nicotinamides and methyl nicotinates are detailed. These procedures involve specific 2H-labelling in both the agent and catalyst and achieve polarization lifetimes of ca. 2 minutes with 50% polarization in the case of 4,6-d2-methylnicotinate. As a 1.5 T hospital scanner has an effective 1H polarization level of just 0.0005% this strategy should result in compressed detection times for chemically discerning measurements that probe disease. To demonstrate this techniques generality, we exemplify further studies on a range of pyridazine, pyrimidine, pyrazine and isonicotinamide analogues that feature as building blocks in biochemistry and many disease treating drugs
Synthesis and hyperpolarisation of eNOS substrates for quantification of NO production by 1H NMR spectroscopy
Hyperpolarization enhances the intensity of the NMR signals of a molecule, whose in vivo metabolic fate can be monitored by MRI with higher sensitivity. SABRE is a hyperpolarization technique that could potentially be used to image nitric oxide (NO) production in vivo. This would be very important, because NO dysregulation is involved in several pathologies, including cardiovascular ones. The nitric oxide synthase (NOS) pathway leads to NO production via conversion of l-arginine into l-citrulline. NO is a free radical gas with a short half-life in vivo (≈5s), therefore direct NO quantification is challenging. An indirect method - based on quantifying conversion of an l-Arg- to l-Cit-derivative by 1H NMR spectroscopy - is herein proposed. A small library of pyridyl containing l-Arg derivatives was designed and synthesised. In vitro tests showed that compounds 4a-j and 11a-c were better or equivalent substrates for the eNOS enzyme (NO2 - production=19-46μM) than native l-Arg (NO2 - production=25μM). Enzymatic conversion of l-Arg to l-Cit derivatives could be monitored by 1H NMR. The maximum hyperpolarization achieved by SABRE reached 870-fold NMR signal enhancement, which opens up exciting future perspectives of using these molecules as hyperpolarized MRI tracers in vivo
Fermi surface of SrFeP determined by de Haas-van Alphen effect
We report measurements of the Fermi surface (FS) of the ternary phosphide
SrFeP using the de Haas-van Alphen effect. The calculated FS of this
compound is very similar to SrFeAs, the parent compound of the high
temperature superconductors. Our data show that the Fermi surface is composed
of two electron and two hole sheets in agreement with bandstructure
calculations. Several of the sheets show strong c-axis warping emphasizing the
importance of three-dimensionality in the non-magnetic state of the ternary
pnictides. We find that the electron and hole pockets have a different
topology, implying that this material does not satisfy a nesting condition.Comment: 5 pages, 4 Figures, 1 Tabl
The Role of Pressure in Inverse Design for Assembly
Isotropic pairwise interactions that promote the self assembly of complex
particle morphologies have been discovered by inverse design strategies derived
from the molecular coarse-graining literature. While such approaches provide an
avenue to reproduce structural correlations, thermodynamic quantities such as
the pressure have typically not been considered in self-assembly applications.
In this work, we demonstrate that relative entropy optimization can be used to
discover potentials that self-assemble into targeted cluster morphologies with
a prescribed pressure when the iterative simulations are performed in the
isothermal-isobaric ensemble. By tuning the pressure in the optimization, we
generate a family of simple pair potentials that all self-assemble the same
structure. Selecting an appropriate simulation ensemble to control the
thermodynamic properties of interest is a general design strategy that could
also be used to discover interaction potentials that self-assemble structures
having, for example, a specified chemical potential.Comment: 29 pages, 8 figure
Structural properties and superconductivity of SrFe2As2-xPx and CaFe2As2-yPy
The SrFe2As2-xPx and CaFe2As2-yPy materials were prepared by a solid state
reaction method. X-ray diffraction measurements indicate the single-phase
samples can be successfully obtained for SrFe2As2-xPx and CaFe2As2-yPy samples.
Clear contraction of the lattice parameters are clearly determined due to the
relatively smaller P ions substation for As. The SDW instability associated
with tetragonal to orthorhombic phase transition is suppressed visibly in both
systems following with the increase of P contents. The highest superconducting
transitions are respectively observed at about 27 K in SrFe2As1.3P0.7 and at
about 13 K in CaFe2As1.7P0.3.Comment: 11 pages, 5 figures, 2 table
On response spectra and Kramers-Kronig relations in superposition rheometry
In a recent paper, we derived expressions for determining the rate-dependent response spectra directly from parallel superposition rheometry data for the case of a certain Lodge-type integral constitutive model. It was shown that, within the confines of linear Yamamoto perturbation theory, the corresponding parallel superposition moduli satisfy the classical Kramers-Kronig relations. Special bases were presented to convert parallel superposition moduli to orthogonal superposition moduli. In the current paper, we obtain similar results for the integral models of Wagner I and, more generally, K-BKZ. These results facilitate the physical interpretation of parallel superposition moduli and direct model-based comparison of parallel and orthogonal superposition moduli in the study of weak nonlinear respons
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