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We report a detailed study of isofield magnetic relaxation and isothermal magnetization measurements with $H$$\parallel$c on an underdoped Ba$_{0.75}$K$_{0.25}$Fe$_2$As$_2$ pnictide single crystal, with superconducting transition temperature $T_c$ = 28 K. The second magnetization peak (SMP) has been observed at temperatures below $T_c$/2 and vanished at higher temperatures. The observed behaviour of the SMP has been studied by measuring the magnetic field dependence of relaxation rate, $R(H)$ and by performing the Maley's analysis. The results suggest that the crossover from collective to plastic pinning observed in the SMP disappears above 12 K with plastic pinning replacing collective pinning. An interesting $H$-$T$ phase diagram is obtained. The critical current density ($J_c$) was estimated using Bean's model and found to be $\sim$ $3.4 \times 10^9$ A/m$^2$ at 10 K in the SMP region, which is comparable to an optimally doped Ba-KFe$_2$As$_2$ superconductor and may be exploited for potential technological applications. The pinning mechanism is found to be unconventional and does not follow the usual $\delta l$ and $\delta T_c$ pinning models, which suggest the intrinsic nature of pinning in the compound., Comment: 8 pages, 9 figures

We describe an experiment to form and characterize a section of a spherically imploding plasma liner by merging six supersonic plasma jets that are launched by newly designed contoured-gap coaxial plasma guns. This experiment is a prelude to forming a fully spherical imploding plasma liner using many dozens of plasma guns, as a standoff driver for plasma-jet-driven magneto-inertial fusion. The objectives of the six-jet experiments are to assess the evolution and scalings of liner Mach number and uniformity, which are important metrics for spherically imploding plasma liners to compress magnetized target plasmas to fusion conditions. This paper describes the design of the coaxial plasma guns, experimental characterization of the plasma jets, six-jet experimental setup and diagnostics, initial diagnostic data from three- and six-jet experiments, and the high-level objectives of associated numerical modeling., Comment: 11 pages, 12 figures, 3 tables, accepted for publication in the April 2018 IEEE Trans. Plasma Sci. special issue on plenary and invited talks from the 2017 International Conference on Plasma Science

As charge carriers traverse a single superconductor ferromagnet interface they experience an additional spin-dependent phase angle which results in spin mixing and the formation of a bound state called the Andreev Bound State. This state is an essential component in the generation of long range spin triplet proximity induced superconductivity and yet the factors controlling the degree of spin mixing and the formation of the bound state remain elusive. Here we demonstrate that point contact Andreev reflection can be used to detect the bound state and extract the resulting spin mixing angle. By examining spectra taken from La1.15Sr1.85Mn2O7 single crystal - Pb junctions, together with a compilation of literature data on highly spin polarised systems, we show that the existence of the Andreev Bound State both resolves a number of long standing controversies in the Andreev literature as well as defining a route to quantify the strength of spin mixing at superconductor-ferromagnet interfaces. Intriguingly we find that for these high transparency junctions, the spin mixing angle appears to take a relatively narrow range of values across all the samples studied. The ferromagnets we have chosen to study share a common property in terms of their spin arrangement, and our observations may point to the importance of this property in determining the spin mixing angle under these circumstances., Comment: 17 pages, 1 table, 3 figures. arXiv admin note: substantial text overlap with arXiv:1503.00533

We report on isofield magnetic relaxation data on a single crystal of $Ba_{1-x}K_xFe_2As_2$ with superconducting transition temperature $T_c$= 32.7 K which exhibit the so called fish-tail effect. A surface map of the superconducting transition temperature shows that the superconducting properties are close to homogeneous across the sample. Magnetic relaxation data, M(t), was used to obtain the activation energy U(M) in order to study different vortex dynamics regimes. Results of this analysis along with time dependent measurements as a function of field and temperature extended to the reversible region of some M(H) curves demonstrate that the irreversibility as well the second magnetization peak position, $H_p(T)$, are time dependent and controlled by plastic motion of the vortex state. In the region delimited by a characteristic field Hon (well below $H_p$), and $H_p$, the vortex dynamics is controlled by collective pinning. For fields below Hon the activation energy, $U_0$, increases with field as expected for collective pinning, but the pinning mechanism is likely to be in the single vortex limit., Comment: 8 pages, 8 figures, one table

Point contact Andreev reflection spectra have been taken as a function of temperature and magnetic field on the polycrystalline form of the newly discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak which disappears at the superconducting transition temperature, dominates all of the spectra. Data taken in high magnetic fields show that this feature survives until 7T at 2K and a flattening of the feature is observed in some contacts. Here we inspect whether these observations can be interpreted within a d-wave, or nodal order parameter framework which would be consistent with the recent theoretical model where the height of the P in the Fe-P-Fe plane is key to the symmetry of the superconductivity. However, in polycrystalline samples care must be taken when examining Andreev spectra to eliminate or take into account artefacts associated with the possible effects of Josephson junctions and random alignment of grains., Comment: Published version

Oxypnictide superconductor NdFeAsO0.85 sample was irradiated with 2 GeV Ta ions at a fluence of 5x10^10 ions/cm2. High resolution transmission electron microscopy study revealed that the irradiation produced columnar-like defects. The effect of these defects on the irreversible magnetisation in polycrystalline randomly oriented fragments was studied as a function of field angle and field sweep rate. We find that the critical current density is enhanced at fields below the matching field (~1 Tesla) but only marginally. The pinning enhancement is anisotropic and maximum along the defect direction at high temperatures but the pinning then becomes more isotropic at low temperatures. The creep rate is suppressed at high temperatures and at fields below the matching field, indicating the columnar defects are efficient pinning sites at these H and T conditions., Comment: 10 pages, 4 figures

Bulk samples of TbFeAsO$_{0.9}$F$_{0.1}$ (T$_{c}$(on) = 50K) were measured by point contact Andreev reflection spectroscopy. The spectra show unambiguous evidence for multiple gap-like features plus the presence of high bias shoulders. By measuring the spectra as a function of temperature with both gold and superconducting niobium tips, we establish that the gap-like features are associated with superconducting order parameter in this material. We discuss whether the well defined zero bias conductance peak that we observe infrequently is associated with a nodal superconducting order parameter., Comment: 11 pages, 5 figures, published version

Here we use global and local magnetometry and Hall probe imaging to investigate the electromagnetic connectivity of the superconducting current path in the oxygen-deficient fluorine-free Nd-based oxypnictides. High resolution transmission electron microscopy and scanning electron microscopy show strongly-layered crystallites, evidence for a ~ 5nm amorphous oxide around individual particles, and second phase neodymium oxide which may be responsible for the large paramagnetic background at high field and at high temperatures. From global magnetometry and electrical transport measurements it is clear that there is a small supercurrent flowing on macroscopic sample dimensions (mm), with a lower bound for the average (over this length scale) critical current density of the order of 103 A/cm2. From magnetometry of powder samples and local Hall probe imaging of a single large conglomerate particle ~120 microns it is clear that on smaller scales, there is better current connectivity with a critical current density of the order of 5 x 104 A/cm2. We find enhanced flux creep around the second peak anomaly in the magnetisation curve and an irreversibility line significantly below Hc2(T) as determined by ac calorimetry., Comment: 11 pages, 4 figures

The newly discovered oxypnictide family of superconductors show very high critical temperatures of up to 55K. Whilst there is growing evidence that suggests a nodal order parameter, point contact Andreev reflection spectroscopy can provide crucial information such as the gap value and possibly the number of energy gaps involved. For the oxygen deficient NdFeAsO0.85 with a Tc of 45.5K, we show that there is clearly a gap value at 4.2K that is of the order of 7meV, consistent with previous studies on oxypnictides with lower Tc. Additionally, taking the spectra as a function of gold tip contact pressure reveals important changes in the spectra which may be indicative of more complex physics underlying this structure., Comment: 11 pages, 3 figures. New references included, extra discussion. This version is accepted in Superconductor Science and Technology

We report on the synthesis of highly oriented a-axis CrO2 films onto (0001) sapphire by atmospheric pressure CVD from CrO3 precursor, at growth temperatures down to 330 degree Celsius, i.e. close to 70 degrees lower than in published data for the same chemical system. The films keep the high quality magnetic behaviour as those deposited at higher temperature, which can be looked as a promising result in view of their use with thermally sensitive materials, e.g. narrow band gap semiconductors., Comment: 13 pages, 4 figures

The influence of the nature of the boron precursor on the superconducting properties of polycrystalline MgB2 was studied. Critical current densities for the MgB2 made from high purity amorphous boron are at least a factor of three higher than typical values measured for standard MgB2 samples made from amorphous precursors. Two possible mechanisms are proposed to account for this difference. Samples made from crystalline boron powders have around an order of magnitude lower Jc compared to those made from amorphous precursors. X-ray, Tc and resistivity studies indicate that this is as a result of reduced current cross section due to the formation of Mg-B-O phases. The samples made from amorphous B contain far fewer Mg-B-O phases than crystalline B despite the fact that the amorphous B contains more B2O3. The different reactivity rates of the precursor powders accounts for this anomaly., Comment: 17 pages, 2 tables, 5 figures, submitted to Superconductor Science and Technology

Composition-spread La1-xSrxMnO3 thin films were prepared by pulsed laser deposition technique from LaMnO3 and SrMnO3 targets. The films were epitaxial with a continuous variation of the out of plane lattice parameter along the direction of composition gradient. Scanning Raman spectroscopy has been employed as a non-destructive tool to characterize the composition-spread films. Raman spectra showed the variation of the structural, Jahn Teller distortions and the presence of coexisting phases at particular compositions that are in agreement with the previous observation on the single crystal samples. Raman spectra on the continuous composition-spread film also reveal the effect of disorder and strain on the compositions., Comment: 14 pages, 4 figures

Two sets of MgB2 samples doped with up to 5 at. % of Al were prepared in different laboratories using different procedures. Decreases in the a and c lattice parameters were observed with Al doping confirming Al substitution onto the Mg site. The critical temperature (Tc) remained largely unchanged with Al doping. For 1 - 2.5 at.% doping, at 20K the in-field critical current densities (Jc's) were enhanced, particularly at lower fields. At 5K, in-field Jc was markedly improved, e.g. at 5T Jc was enhanced by a factor of 20 for a doping level of 1 at.% Al. The improved Jcs correlate with increased sample resistivity indicative of an increase in the upper critical field, Hc2, through alloying., Comment: 17 pages, 4 figures, to be published in Superconductor Science and Technology

The Raman spectrum of the superconductor MgB$_{2}$ has been measured as a function of the Tc of the film. A striking correlation is observed between the $T_{c}$ onset and the frequency of the $E_{2g}$ mode. Analysis of the data with the McMillan formula provides clear experimental evidence for the collapse of the electron phonon coupling at the temperature predicted for the convergence of two superconducting gaps into one observable gap. This gives indirect evidence of the convergence of the two gaps and direct evidence of a transition to an isotropic state at 19 K. The value of the electron phonon coupling constant is found to be 1.22 for films with T$_{c}$ 39K and 0.80 for films with T$_{c}\leq$19K., Comment: 5 pages, 4 figures

Measurements of the temperature dependence of the surface resistance at 3 GHz of 100 micron size grains of MgB_2 separated powder are presented and discussed. The microwave surface resistance data is compared to experimental results of Nb, Bi_2Sr_2CaCu_2O_{8+\delta} (BSCCO) and theoretical predictions of s-wave weak coupling electron-phonon theory (BCS)., Comment: 4 pages, 2 figures; accepted in SUST