Search

It has been indicated that the observed Weddell Sea anomaly (WSA) appeared to be an extreme manifestation of the longitudinal variations in the Southern Hemisphere, since the WSA is characterized by greater evening electron density than the daytime density in the region near the Weddell Sea. In the present study, the longitudinal variations of the nighttime F2-layer peak electron density at southern midlatitudes are analyzed using the observations of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites between 2006 and 2008. It is found that significant longitudinal difference (> 150 %) relative to the minimum density at each local time prevails in all seasons, although the WSA phenomenon is only evident in summer under this solar minimum condition. Another interesting feature is that in summer, the maximum longitudinal differences occur around midnight (~23:00- 00:00 LT) rather than in the evening (19:00-21:00 LT) in the evening, when the most prominent electron density enhancement occurs for the WSA phenomenon. Thus the seasonal- local time patterns of the electron density longitudinal variations during nighttime at southern midlatitudes cannot be simply explained in terms of the WSA. Meanwhile, the variations of the geomagnetic configuration and the equivalent magnetic meridional winds/upward plasma drifts are analyzed to explore their contributions to the longitudinal variations of the nighttime electron density. The maximum longitudinal differences are associated with the strongest wind-induced vertical plasma drifts after 21:00 LT in the Western Hemisphere. Besides the magnetic declination-zonal wind effects, the geographic meridional winds and the magnetic inclination also have significant effects on the upward plasma drifts and the resultant electron density. [ABSTRACT FROM AUTHOR]

We studied the effects of sintering temperature on the phase transformation, lattice parameters, full width at half-maximum (FWHM), strain, critical temperature (Tc), critical current density (Jc) and resistivity (?) in MgB2/Fe wires. All samples were fabricated by the in situpowder-in-tube method (PIT) and sintered within a temperature range of 650-900?°C. It was observed that wires sintered at low temperature, 650?°C, resulted in higher Jcup to 12 T and lower Tc. The best transport Jcvalue reached 4200 A cm?2at 4.2 K and 10 T. This is related to the grain boundary pinning due to small grain size. On the other hand, wires sintered at 900?°C had a lower Jcin combination with a higher Tc. [ABSTRACT FROM AUTHOR]

Transition metal (Mn and Ni) doped ZnO nanocrystals with a diameter of 20 nm were fabricated by a low temperature solution processing technique. The incorporation of Ni and Mn into the ZnO lattice is evidenced by the shrinkage or expansion of the lattice parameters of ZnO in response to the transition metal dopants as determined by structure refinements. The E2 (High) phonon lines in Raman spectra of all the samples show large redshift of more than 6 cm[?]1 in comparison with that of the ZnO bulk single crystal. Especially, a further redshift of 3 cm[?]1 and broadening effect in the Raman spectra of the 5% Ni doped ZnO nanocrystals in comparison with the undoped ZnO nanocrystals indicate that the transition metal introduced defects and host disorder into the ZnO. The obtained nanocrystals are ferromagnetic at room temperature. [ABSTRACT FROM AUTHOR]

First, by using of the conventional Rowell analysis, we demonstrated that the addition of nano BN particles can effectively eliminate MgO and pores in MgB2 resulting in a very high density and good connectivity of BN-doped MgB2. Then, another method—low-frequency dielectric impedance analysis—was introduced to characterize the properties of the grain boundaries of MgB2. A comparative impedance study was performed in the frequency range from 100 Hz to 100 MHz on pure and nano BN-doped MgB2. The study revealed some following interesting results: (1) a dielectric resonance around frequency of 108 in both samples was observed, which was argued to be related to an inductance-capacitance and (2) the pure sample has two dielectric relaxations originating from intergrains, while the doped sample has only one intergranular contribution. This convinces that the electric connectivity of the doped sample is really improved by the addition of nano BN particles. Our results indicate that dielectric technique may be a useful tool to characterize the grain boundary properties and grain boundary-related properties of MgB2. [ABSTRACT FROM AUTHOR]

We present the orbital structure of MnTiO3 with polarization dependent x-ray absorption and resonant x-ray emission spectra accompanied with electronic structure calculations. The results clearly indicate a strongly anisotropic O 2p-Mn 3d orbital hybridization whereas the Mn 3d hole state shows a highly delocalized characteristic ascribed to the 3d-4p mixing. The extended Mn 4p orbital could enhance the exchange interaction between Mn (3d)-O (2p)-Mn (3d) leading to an asymmetric charge distribution in Mn-O bonds. The delocalized characteristic of Mn 3d holes is indispensable to the mechanism of spin-dependent-metal-ligand hybridization to explain magnetically induced ferroelectricity. [ABSTRACT FROM AUTHOR]

We report the effects of C4+-irradiation on the superconducting properties of BaFe1.9Ni1.1As2 single crystal. The BaFe1.9Ni1.1As2 single crystal before and after C4+-irradiation was characterized by magnetic, magneto-transport and magneto-optical techniques over a wide range of magnetic fields (0-13 T) and temperatures (2-200 K). We demonstrate that the C4+-irradiation significantly enhances the in-field critical current density (by a factor of up to 1.5 at 5 K) and induces enhanced flux jumping at 2 K, with only a small degradation (by 0.5 K) of the critical temperature, Tc. The vortex phase diagram describing the evolution of the vortex-glass transition temperature with magnetic field and the upper critical field has been resolved for the C4+-irradiated sample. For temperatures below Tc, the resistivity curves and the pinning potential were found to show good scaling, using a modified model for vortex-liquid resistivity. The vortex state is three dimensional at temperatures lower than a characteristic temperature. Good agreement between the thermally activated flux flow model, which is usually employed to account for the resistivity in the vortex-liquid region, and the modified vortex-liquid model, has been observed. [ABSTRACT FROM AUTHOR]

Low grade boron powders were used to fabricate pure and submicron-sized carbon sphere doped MgB2 superconductor. The boron powders used showed low reactivity towards MgB2 formation, as compared to high purity (99%) amorphous boron, which might result from the larger grain size, and the existence of crystalline boron or boron oxide in the former. However, the samples prepared from this boron powder showed comparable Jc values at 20 K and in low field (<1 T) to those from a sample prepared from the high quality boron. Doping submicron-sized carbon spheres had successfully introduced carbon substitution for boron, and so improved the Hc2, Hirr, and in-field Jc properties of MgB2. [ABSTRACT FROM AUTHOR]

The transformation of zero-field-cooled (ZFC) magnetization curves of PLD MgB2 films annealed in situ for different dwell times and at different temperatures is studied within the context of structural analysis. The PLD films were deposited using an off-axis geometry followed by two series of annealing conditions: the dwell time series and the annealing temperature series. ZFC magnetization curves were obtained in an MPMS magnetometer from samples that had been cut into similar shapes. XRD scans on the two series of film samples show enhanced (001) and (002) MgB2 peaks till a dwell time of 9 min at a 650 °C and a 800 °C annealing temperature. TEM results show a granular transformation from an un-annealed film to a 700 °C 9 min in situ annealed film. The possible structural development with annealing time and annealing temperature is then discussed. [ABSTRACT FROM AUTHOR]

The impact of hydrostatic pressure up to 1.2 GPa on the critical current density (Jc) and the nature of the pinning mechanism in MgB2 have been investigated within the framework of the collective theory. We found that the hydrostatic pressure can induce a transition from the regime where pinning is controlled by spatial variation in the critical transition temperature to the regime controlled by spatial variation in the mean free path Furthermore, critical temperature (Tc) and low field Jc are slightly reduced, although the Jc drops more quickly at high fields than at ambient pressure. We found that the pressure raises the anisotropy and reduces the coherence length, resulting in weak interaction of the vortex cores with the pinning centres. Moreover, the hydrostatic pressure can reduce the density of states [Ns(E)], which, in turn, leads to a reduction in the Tc from 39.7 K at P = 0 GPa to 37.7 K at P = 1.2 GPa. [ABSTRACT FROM AUTHOR]

Magnetic and transport properties of 10 wt% SiC doped MgB2 and 5 wt% nano-C doped MgB2 were studied by resistance and critical current density measurements. The results showed improvement of the critical current density for the MgB2 superconductor doped with SiC in comparison with the nano-C doped sample. The flux pinning mechanisms of both doped MgB2 superconductors have been investigated based on the collective theory. It was found that the pinning mechanism in MgB2 was transformed by SiC doping from transition temperature fluctuation induced pinning, δTc pinning, to mean free path fluctuation induced pinning, δℓ pinning, while in the MgB2 doped with nano-C, δTc and δℓ pinning coexist. Their contributions are strongly temperature dependent, however. The δℓ pinning is dominant at low temperature, decreases with increasing temperature, and is suppressed completely at temperatures close to Tc. The δTc pinning mechanism shows the opposite trend. [ABSTRACT FROM AUTHOR]

A correction to the article "Driving Magnetostructural Transitions in Layered Intermetallic Compounds" that was published in the 2013 issue of "Physical Review" is presented.

Transition metals, including Co, Cr, Ni and Mn, were doped into multiferroic Bi0.8La0.2Nb0.01Fe0.99O3 samples that were fabricated by a solid state reaction. X-ray diffraction results show that only the Mn-doped sample did not contain any impurity phases, while the other transition metal dopants destroyed the phase stability of the pure Bi0.8La0.2Nb0.01Fe0.99O3 phase and caused the formation of second phases. All the transition metal doped Bi0.8La0.2Nb0.01Fe0.99O3 samples show significant enhancement in their magnetic moment at room temperature in comparison with the BiFeO3 and Bi0.8La0.2Nb0.01Fe0.99O3 samples without transition metal doping. This is explained by the formation of local ferrimagnetic ordering or ferromagnetic ordering in the transition metal doped Bi0.8La0.2Nb0.01Fe0.99O3, according to the electron configurations of the dopant transition metal ions. [ABSTRACT FROM AUTHOR]

SiC was mixed with Mg and B and reacted by either a one-step in situ or two-step method at 650 or 850 degC. By doing so, it was possible to determine the extent to which scattering via C doping influences the magnitude of field dependent critical current density, Jc(H), compared to pinning via generation of microstructural disorder. The one-step reaction method leads to Mg2Si formation and at the same time to more C doping of MgB2 than the two-step method. Carbon increases both the irreversibility field, Hirr, and upper critical field, Hc2 (T<28 K). However, for the temperatures (6 and 20 K) and fields (up to 7 T) studied, pinning rather than scattering overwhelmingly dominates the magnitude of the field dependent critical current density. [ABSTRACT FROM AUTHOR]

Cold high pressure densification, a method recently introduced at GAP in Geneva, was applied for improving the transport critical current density, Jc, and the irreversibility field, Birr, of monofilamentary in situ MgB2 wires and tapes alloyed with 10 wt% C4H6O5 (malic acid). Tapes densified at 1.48 GPa exhibited after reaction an enhancement of Jc from 2 to 4 x 104 A cm[?]2 at 4.2 K/10 T and from 0.5 to 4 x 104 A cm[?]2 at 20 K/5 T, while the Birr was enhanced from 19.3 to 22 T at 4.2 K and from 7.5 to 10.0 T at 20 K. Cold densification also caused a strong enhancement of B(104), the field at which Jc takes the value 1 x 104 A cm[?]2. For tapes subjected to 1.48 GPa, B(10^{4})^{\parallel } and B(10^{4})^{ \perp } at 4.2 K were found to increase from 11.8 and 10.5 T to 13.2 and 12.2 T, respectively. Almost isotropic conditions were obtained for rectangular wires with aspect ratios a/b<2 subjected to 2.0 GPa, where B(10^{4})^{\parallel }=12.7 and B(10^{4})^{ \perp }=12.5 T were obtained. At 20 K, the wires exhibited an almost isotropic behavior, with B(10^{4})^{\parallel }=5.9 T and B(10^{4})^{ \perp }=5.75 T, Birr(20 K) being [?]10 T. These values are equal to or higher than the highest values reported so far for isotropic in situ wires with SiC or other carbon based additives. Further improvements are expected on optimizing the cold high pressure densification process, which has the potential for fabrication of MgB2 wires of industrial lengths. [ABSTRACT FROM AUTHOR]

We investigated the effect of SiC doping on the critical current density (Jc) in MgB2 thick films using amorphous SiC impurity layers of various thicknesses: 7, 14, 35, and 70 nm. SiC impurity layers were first deposited on the Al2O3(0001) substrates at room temperature by using a pulsed laser deposition system, after which MgB2 films were grown on the SiC deposited precursor substrates by using a hybrid physical-chemical vapor deposition technique at a low growth temperature of 480 degC. All samples showed a high transition temperature of [?]40 K irrespective of the thickness of the impurity layer. The grain sizes of the MgB2 films slightly increased from 400 to 488 nm with increasing thickness of the impurity layer. The MgB2 thick film with a 35 nm thick SiC impurity layer exhibited the highest Jc, while all SiC doped samples showed a higher Jc than a pure MgB2 thick film throughout the whole magnetic field region. These results suggest that the SiC particles of the impurity layer diffused into the MgB2 films during film growth, and the SiC particles, along with the columnar grain boundaries in the MgB2 thick films, act as strong pinning centers. [ABSTRACT FROM AUTHOR]

We report on the fabrication of superconducting MgB2 thin films on textured Cu(100) tape under low pressure and temperature by using a hybrid physical-chemical vapor deposition (HPCVD) technique to explore the possible broad range of deposition. Thermodynamic calculations for the Mg-B system have been carried out and the pressure-temperature phase diagram was obtained. Our results demonstrate that the deposition of superconducting MgB2 films is possible beyond the theoretically calculated growth window, where the sublimation of Mg is taking place. The structural and microstructural investigations reveal that MgB2 films are c-axis-oriented normal to the substrate. The highest Jc of [?]1.34 x 105 A cm[?]2 at 5 K under 3 T is obtained for the film grown at 460 degC. The critical current density (Jc) and flux pinning force density (Fp) of MgB2 films are enhanced with decreasing growth temperature. This could be attributed to the high density of grain boundaries which may act as effective flux pinning centers. These findings suggest an alternative route to fabricate MgB2 tapes at low temperature for large scale applications. [ABSTRACT FROM AUTHOR]

The particle size, purity and form of the starting boron powders play an important role in the superconducting properties of MgB2. In this paper high-purity (99.999%) boron powders with crystalline phase that were processed by ball milling have been used as the precursor powders. It was observed that the critical current density (Jc) value was 9.1 × 103 A cm[?]2 at 5 K and 8 T, which is two times higher than for a sample with ball-milled amorphous boron powders, and a factor of 40 higher than a sample made from as-supplied amorphous boron powders. Significant Jc improvement at high fields is attributed to the Hc2 enhancement caused by the increased disorder. [ABSTRACT FROM AUTHOR]

Direct comparison of the magnetic and transport critical current density (Jc) for the same pieces of copper-sheathed MgB2 wires shows a large discrepancy in magnitude and field dependence of the two. The value of magnetic Jc can differ from the value of transport Jc by a factor of 10 or more. This discrepancy does not occur merely because of the difference in the voltage at which the magnetic and transport Jc are measured, but mainly because of the specific microstructure of MgB2. Such microstructure results in superconducting screening on at least two different length-scales, despite the absence of weak links in MgB2, leading to erroneous magnetic Jc if a simple critical state model is applied to such a system. Consequently, the magnetic Jc cannot be used for analysis of physical processes where the accurate field dependence of Jc needs to be known, such as vortex pinning. The magnetic Jc can still be used for qualitative comparison of MgB2 samples if they are all of the same size and have the same microstructure, or if their size is large enough so that the magnetic Jc does not depend on the sample size. [ABSTRACT FROM AUTHOR]

In this paper we report the results of an extended study of the effect of sugar doping on the structural and electromagnetic properties of MgB2 superconductors. High values of the upper critical field (Bc2) of 36 T and the irreversibility field (Birr) of 27 T have been estimated at the temperature of 5 K in a bulk MgB2 sample with the addition of 10 wt% of sugar. The critical current density (Jc(Ba)) of sugar-doped samples has been significantly improved in the high field region. The value of transport Jc has reached as high as 108 A m[?]2 at 10 T and 5 K for Fe-sheathed sugar-doped MgB2 wire. The analysis of the pinning mechanism in the samples investigated indicated that dominant vortex pinning occurs on the surface type of pinning defects, such as grain boundaries, dislocations, stacking faults etc, for both pure and doped MgB2. In sugar-doped samples, pinning is governed by numerous crystal lattice defects, which appear in MgB2 grains as a result of crystal lattice distortion caused by carbon substitution for boron and nano-inclusions. The drastically improved superconducting properties of sugar-doped samples are also attributed to the highly homogeneous distribution and enhanced reactivity of this dopant with host Mg and B powders. The results of this work suggest that sugar is the optimal source of carbon for doping MgB2 superconductor, especially for application at high magnetic fields. [ABSTRACT FROM AUTHOR]