120 research outputs found
Superconductivity and magnetism in platinum-substituted SrFe2As2 single crystals
Single crystals of SrFe2-xPtxAs2 (0 < x < 0.36) were grown using the self
flux solution method and characterized using x-ray crystallography, electrical
transport, magnetic susceptibility, and specific heat measurements. The
magnetic/structural transition is suppressed with increasing Pt concentration,
with superconductivity seen over the range 0.08 < x < 0.36 with a maximum
transition temperature Tc of 16 K at x = 0.16. The shape of the phase diagram
and the changes to the lattice parameters are similar to the effects of other
group VIII elements Ni and Pd, however the higher transition temperature and
extended range of superconductivity suggest some complexity beyond the simple
electron counting picture that has been discussed thus far.Comment: 6 pages, 6 figure
Simplified methods of assessing the impact of grid frequency dynamics upon generating plants
The frequency of the national electricity grid is affected by fluctuations in supply and demand, and so continually "judders" in an essentially unpredictable fashion around 50 Hz. At present such perturbations do not seemingly affect Nuclear Electric as most of their plant is run at more or less constant load, but they would like to be able to offer the national grid a mode of operation in which they "followed" the grid frequency: i.e., as the frequency rose above or fell below 50 Hz, the plant's output would be adjusted so as to tend to restore the frequency to 50 Hz. The aim is to maintain grid frequency within 0.2 Hz of its notional value. Such a mode of operation, however, would cause a certain amount of damage to plant components owing to the consequent continual changes in temperature and pressure within them.
Nuclear Electric currently have complex computational models of how plants will behave under these conditions, which allows them to compute plant data (e.g., reactor temperatures) from given grid frequency data. One approach to damage assessment would require several years'-worth of real grid data to be fed into this model and the corresponding damage computed (via "cycle distributions" created by their damage experts). The results of this analysis would demonstrate one of three possibilities: the damage may be acceptable under all reasonable operating conditions; or it may be acceptable except in the case of an exceptional abrupt change in grid frequency (caused by power transmission line failure, or another power station suddenly going off-line, for instance), in which case some kind of backup supply (e.g., gas boilers) would be required; or it may simply be unacceptable.
However, their current model runs in approximately real time, making it inappropriate for such a large amount of data: our problem was to suggest alternative approaches. Specifically, we were asked the following questions:
- Can component damage be reliably estimated directly from cycle distributions of grid frequency? i.e., are there maps from frequency cycle distributions to plant parameter cycle distributions?
- Can a simple model of plant dynamics be used to assess the potential for such maps?
- What methods can be used to select representative samples of grid frequency behaviour?
- What weightings should be applied to the selections?
- Is it possible to construct a "cycle transform" (Fourier transform) which will capture the essential features of grid frequency and which can then be inverted to generate simulated frequency transients?
We did not consider this last question, other than to say "probably not".
We were supplied with data of the actual grid frequency measurements for the evening of 29/7/95, and the corresponding plant responses (obtained using Nuclear Electric's current computational model). A simplified nonlinear mathematical model of the plant was also provided.
Two main approaches were considered: statistical prediction and analytical modelling via a reduction of the simplified plant model
Universal pair-breaking in transition metal-substituted iron-pnictide superconductors
The experimental transport scattering rate was determined for a wide range of
optimally doped transition metal-substituted FeAs-based compounds with the
ThCr2Si2 (122) crystal structure. The maximum transition temperature Tc for
several Ba-, Sr-, and Ca-based 122 systems follows a universal rate of
suppression with increasing scattering rate indicative of a common
pair-breaking mechanism. Extraction of standard pair-breaking parameters puts a
limit of \sim26 K on the maximum Tc for all transition metal-substituted 122
systems, in agreement with experimental observations, and sets a critical
scattering rate of 1.5x10^14 s^-1 for the suppression of the superconducting
phase. The observed critical scattering rate is much weaker than that expected
for a sign-changing order parameter, providing important constraints on the
nature of the superconducting gap in the 122 family of iron-based
superconductors.Comment: 4 pages, 3 figure
Quenched Fe Moment in the Collapsed Tetragonal Phase of CaPrFeAs
We report As NMR studies on single crystals of rare-earth doped iron
pnictides superconductor CaPrFeAs (=0.075 and
0.15). The As spectra show a chemical pressure effect with doping and a
first order structure transition to the collapsed tetragonal phase upon
cooling. A sharp drop of the Knight shift is seen below the structural
transition, whereas is strongly enhanced at low-temperatures. These
evidences indicate quenching of Fe local magnetism and short-range ordering of
Pr moment in the collapsed tetragonal phase. The quenched Fe moment
through structure collapse suggests a strong interplay of structure and
magnetism, which is important for understanding the nature of the collapsed
tetragonal phase.Comment: 5 pages, 5 figure
Superconductivity at 23 K in Pt doped BaFe2As2 single crystals
We report superconductivity in single crystals of the new iron-pnictide
system BaFe1.9Pt0.1As2 grown by a self-flux solution method and characterized
via x-ray, transport, magnetic and thermodynamic measurements. The magnetic
ordering associated with a structural transition at 140 K present in BaFe2As2
is completely suppressed by substitution of 5% Fe with Pt and superconductivity
is induced at a critical temperature Tc=23 K. Full diamagnetic screening in the
magnetic susceptibility and a jump in the specific heat at Tc confirm the bulk
nature of the superconducting phase. All properties of the superconducting
state including transition temperature Tc, the lower critical field Hc1=200 mT,
upper critical field Hc2~65 T, and the slope dHc2/dT are comparable in value to
the those found in other transition-metal-substituted BaFe2As2 series,
indicating the robust nature of superconductivity induced by substitution of
Group VIII elements.Comment: 6 pgs, 4 figs, and 1 tbl, slightly revised, updated reference
Structural collapse and superconductivity in rare earth-doped CaFe2As2
Aliovalent rare earth substitution into the alkaline earth site of CaFe2As2
single-crystals is used to fine-tune structural, magnetic and electronic
properties of this iron-based superconducting system. Neutron and single
crystal x-ray scattering experiments indicate that an isostructural collapse of
the tetragonal unit cell can be controllably induced at ambient pressures by
choice of substituent ion size. This instability is driven by the interlayer
As-As anion separation, resulting in an unprecedented thermal expansion
coefficient of K. Electrical transport and magnetic
susceptibility measurements reveal abrupt changes in the physical properties
through the collapse as a function of temperature, including a reconstruction
of the electronic structure. Superconductivity with onset transition
temperatures as high as 47 K is stabilized by the suppression of
antiferromagnetic order via chemical pressure, electron doping or a combination
of both. Extensive investigations are performed to understand the observations
of partial volume-fraction diamagnetic screening, ruling out extrinsic sources
such as strain mechanisms, surface states or foreign phases as the cause of
this superconducting phase that appears to be stable in both collapsed and
uncollapsed structures.Comment: 15 pages, 18 figure
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