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Register a new userWater-Quenched Effects of 5 wt.% (Fe, Ti) particle-doped MgB$_2$ Superconductor and Low Limit of Pinning Effect
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begin{abstract} We have studied magnetic properties of water-quenched 5 wt.% (Fe, Ti) particle-doped MgB$_2$ comparing with that of air-cooled one. Generally, grain refinement is achieved by increasing cooling rate, which implies an increase of grainboundaries in the superconductor. Here we show that increased grainboundaries influence what kinds of effects on the field dependence of magnetization and what is the mechanism. As a result, they are served as a pinning center at a high field whereas they are served as a pathway to facilitate the movement of fluxes pinned on volume defects at a low field. As modeling grainboundaries in a superconductor, we explained that they had a flux pinning effect as well as the flux-penetrating promotion effect. As temperature increases, the pinning ability of a grainboundaries decreases, which was caused by increased coherence length. Stacking fault planes and twin boundaries have also been considered by using the model. It explained the reason for that stacking fault planes of MgB$_2$ do not have any pinning effect and the twin boundary of HTSC have the strong pinning or strong flux-penetration effect depending on the direction of the applied field.
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We have studied flux-pinning effects of MgB$_2$ superconductor by doping (Fe, Ti) particles of which radius is 163 nm on average. 5 wt.% (Fe, Ti) doped MgB$_2$ among the specimens showed the best field dependence of magnetization and 25 wt.% one did the worst at 5 K . The difference of field dependence of magnetization of the two increased as temperature increased. Here we show experimental results of (Fe, Ti) particle-doped MgB$_2$ according to dopant level and the causes of the behaviors. Flux-pinning effect of volume defects-doped superconductor was modeled in ideal state. During the study, we had to divide M-H curve of volume defect-dominating superconductor as three discreet regions for analyzing flux pinning effects, which are diamagnetic increase region, $Delta$H=$Delta$B region, and diamagnetic decrease region. As a result, flux-pinning effects of volume defects decreased as dopant level increased over the optimal dopant level, which was caused by decrease of flux-pinning limit of a volume defect. And similar behaviors are obtained as dopant level decreased below the optimal dopant level, which was caused by the decreased number of volume defects. Comparing the theory with experimental results, deviations increased as dopant level increased over the optimal dopant level, whereas the two was well matched on less dopant level than the optimal dopant level. The behavior is considered to be caused by segregation of volume defects. On the other hand, the property of over-doped specimens dramatically decrease as temperature increases, which is caused by double decreases of flux-pinning limit of a volume defect and segregation effect.
The magnetic field dependent critical current density $j_c(B)$ of a MgB$_2$ bulk sample has been obtained by means of magnetization hysteresis measurements. The $j_c(B)$ curves at different temperatures demonstrate a crossover from single vortex pinning to small-bundle vortex pinning, when the field is larger than the crossover field $B_{sb}$. The temperature dependence of the crossover field $B_{sb}(T)$ is in agreement with a model of randomly distributed weak pinning centers via the spatial fluctuations of the transition temperature ($delta T_c$-pinning), while pinning due to the mean free path fluctuations ($delta l$-pinning) is not observed.
Amorphous oxide thin films play a fundamental role in state-of-the art interferometry experiments, such as gravitational wave detectors where these films compose the high reflectance mirrors of end and input masses. The sensitivity of these detectors is affected by thermal noise in the mirrors with its main source being the mechanical loss of the high index layers. These thermally driven fluctuations are a fundamental limit to optical interferometry experiments and there is a pressing need to understand the underlying processes that lead to mechanical dissipation in materials at room temperature. Two strategies are known to lower the mechanical loss: employing a mixture of Ta$_2$O$_5$ with $approx$ 20% of TiO$_2$ and post-deposition annealing, but the reasons behind this are not completely understood. In this work, we present a systematic study of the structural and optical properties of ion beam sputtered TiO$_2$-doped Ta$_2$O$_5$ films as a function of the annealing temperature. We show for the first time that low mechanical loss is associated with a material morphology that consists of nanometer sized Ar-rich bubbles embedded into an atomically homogeneous mixed titanium-tantalum oxide. When the Ti cation ratio is high, however, phase separation occurs in the film which leads to increased mechanical loss. These results indicate that for designing low mechanical loss mixed oxide coatings for interferometry applications it would be beneficial to identify materials with the ability to form ternary compounds while the dopant ratio needs to be kept low to avoid phase separation.
We report high-resolution, bulk Compton scattering measurements unveiling the Fermi surface of an optimally-doped iron-arsenide superconductor, Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. Our measurements are in agreement with first-principles calculations of the electronic structure, revealing both the $X$-centered electron pockets and the $Gamma$-centered hole pockets. Moreover, our data are consistent with the strong three-dimensionality of one of these sheets that has been predicted by electronic structure calculations at the local-density-approximation-minimum As position. Complementary calculations of the noninteracting susceptibility, $chi_0({bf q}, omega)$, suggest that the broad peak that develops due to interband Fermi-surface nesting, and which has motivated several theories of superconductivity in this class of material, survives the measured three dimensionality of the Fermi surface in this family.
m-H loops for virgin and neutron irradiated bulk and powder samples of MgB_{2} were measured in the temperature range 5-30 K in magnetic field B<= 1 T. The irradiation at thermal neutron fluences 9*10^{13} and 4.5*10^{14} cm^{-2} caused very small enhancement of m-H loops at lower temperatures (T<20 K), whereas the effect at high temperatures was unclear due to difficulty in achieving exactly the same measurement temperature prior and after irradiation. However, the irradiation at 4.5*10^{15} cm^{-2} produced clear enhancement of m-H loops (hence J_{c}) at all investigated temperatures, which provides the evidence for the enhancement of flux pinning in MgB_{2} due to ion tracks resulting from n+^{10}B reaction. The potential of this technique for the enhancement of flux pinning in high temperature superconductors is briefly discussed.
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