We present the following results. ͑1͒ We introduce a doping source for MgB 2 , liquid SiCl 4 , which is free of C, to significantly enhance the irreversibility field ͑H irr ͒, the upper critical field ͑H c2 ͒, and the critical current density ͑J c ͒ with a little reduction in the critical temperature ͑T c ͒. ͑2͒ Although Si can not be incorporated into the crystal lattice, a significant reduction in the a-axis lattice parameter was found, to the same extent as for carbon doping. ͑3͒ Based on the first-principles calculation, it is found that it is reliable to estimate the C concentration just from the reduction in the a-lattice parameter for C-doped MgB 2 polycrystalline samples that are prepared at high sintering temperatures, but not for those prepared at low sintering temperatures. Strain effects and magnesium deficiency might be reasons for the a-lattice reduction in non-C or some of the C-added MgB 2 samples. ͑4͒ The SiCl 4 -doped MgB 2 shows much higher J c with superior field dependence above 20 K compared to undoped MgB 2 and MgB 2 doped with various carbon sources. ͑5͒ We introduce a parameter, RHH ͑H c2 / H irr ͒, which can clearly reflect the degree of flux-pinning enhancement, providing us with guidance for further enhancing J c . ͑6͒ It was found that spatial variation in the charge-carrier mean free path is responsible for the flux-pinning mechanism in the SiCl 4 treated MgB 2 with large in-field J c .

We report that the ͑Ba, K͒Fe 2 As 2 crystal with T c = 32 K shows a pinning potential, U 0 , as high as 10 4 K, with U 0 showing very little field dependence. The ͑Ba, K͒Fe 2 As 2 single crystals become isotropic at low temperatures and high magnetic fields, resulting in a very rigid vortex lattice, even in fields very close to H c2 . The isotropic rigid vortices observed in the two-dimensional ͑2D͒ ͑Ba, K͒Fe 2 As 2 distinguish this compound from 2D high-T c cuprate superconductors with 2D vortices. The vortex avalanches were also observed at low temperatures in the ͑Ba, K͒Fe 2 As 2 crystal. It is proposed that it is the K substitution that induces both almost isotropic superconductivity and the very strong intrinsic pinning in the ͑Ba, K͒Fe 2 As 2 crystal.A high critical current density, J c , upper critical field, B c2 , and irreversibility field, B irr , a high superconducting transition temperature, T c , strong magnetic-flux pinning, good grain connectivity, and isotropic superconductivity are the major physical requirements for superconducting materials used in practical applications operating at low and, in particular, high magnetic fields. The conventional low-T c superconductors, where H c2 is also small, can only carry large J c at very low temperatures. The cuprate high-T c superconductors suffer from poor grain connectivity and easy melting of the vortex lattice, leading to small J c in high magnetic fields at relatively high temperatures. For MgB 2 superconductor with T c of 39 K, B irr is far below H c2 , and J c drops quickly with both field and temperature, preventing its use above 20 K. The newly discovered Fe-based superconductors 1-7 show T c as high as 55 K and B c2 above 200 T, in combination with a small anisotropy for REFeAsO 1−x F x ͑RE-1111 phase, with RE a rare-earth element͒ 8 and an almost isotropic superconductivity for ͑Ba, K͒Fe 2 As 2 ͑122 phase͒. 9 These properties make the Fe-based superconductors extremely promising candidates for high magnetic field applications at relatively high temperatures. The current carrying ability of these superconductors at high fields and temperatures is largely determined by the flux-pinning strength and the behavior of the vortex matter. Therefore, the determination of their intrinsic vortex pinning strength is a central issue from both an applied and a fundamental perspective. Both 1111 and 122 phase compounds have typical two-dimensional ͑2D͒ crystal structures. In RE-1111 phase, where RE is a rare-earth element, the FeAs superconducting layers are separated by insulating LaO layers 10 while in Ba͑K͒-122 phase, the FeAs layer is sandwiched between conductive Ba layers. 5 It is expected that the 122 phase containing two FeAs layers would have small anisotropy and thus higher intrinsic pinning compared to the single layer 1111 phase. Co-doped BaFe 2 As 2 single crystal shows an anisotropy of 1-3 and upper criticalfield values of B c2 ͑B ʈ ab͒ = 20 T and B c2 ͑B ʈ c͒ = 10 T at 20 K, with dB c2 / dT Ϸ 5 T/ K. 11 For single crystals of the optimally do...

An effi cient procedure for the fabrication of highly conductive carbon nanotube/ graphene hybrid yarns has been developed. To start, arrays of vertically aligned multi-walled carbon nanotubes (MWNT) are converted into indefi nitely long MWNT sheets by drawing. Graphene fl akes are then deposited onto the MWNT sheets by electrospinning to form a composite structure that is transformed into yarn fi laments by twisting. The process is scalable for yarn fabrication on an industrial scale. Prepared materials are characterized by electron microscopy, electrical, mechanical, and electrochemical measurements. It is found that the electrical conductivity of the composite MWNT-graphene yarns is over 900 S/cm. This value is 400% and 1250% higher than electrical conductivity of pristine MWNT yarns or graphene paper, respectively. The increase in conductivity is asssociated with the increase of the density of states near the Fermi level by a factor of 100 and a decrease in the hopping distance by an order of magnitude induced by grapene fl akes. It is found also that the MWNT-graphene yarn has a strong electrochemical response with specifi c capacitance in excess of 111 Fg −1 . This value is 425% higher than the capacitance of pristine MWNT yarn. Such substantial improvements of key properties of the hybrid material can be associated with the synergy of MWNT and graphene layers in the yarn structure. Prepared hybrid yarns can benefi t such applications as high-performance supercapacitors, batteries, high current capable cables, and artifi cial muscles.

The discovery of the new family of oxypnictide superconductors, [1,2] including LaFeAsO 0.89 F 0.11 , with critical temperature T c over 26 K, has brought new impetus to the fields of hightemperature superconductivity and strongly correlated electron systems. The new superconductors have the general formula REFeAsO, where RE is a rare earth element, [1][2][3][4][5][6][7][8] and show 2D crystal structures similar to those of high-T c cuprate superconductors. They consist of alternating REO and FeAs layers, providing charge carriers and conducting planes, respectively. T c is strongly dependent on the sizes of the rare earth element ions, [1][2][3][4][5][6][7][8] as well as on F-doping on oxygen sites [1,2] and oxygen deficiency in F-free material.[4] The upper critical field, H c2 , has been estimated to be higher than 55 or 63-65 T in LaFeAsO 0.9 F 0.1, [6,7] 70 T in PrFeAsO 0.85 F 0.15 , and over 100 T in SmFeAsO 0.85 F 0.15 . [7] The two gap superconductivity proposed for LaFeAsO 0.9 F 0.1 suggests that the H c2 could be further increased [6] to a significant extent. This is one of the unique features of the FeAs-based new superconductors. In this work, we show that H c2 (48 K) ¼ 13 T, and that the H c2 (0) values can exceed 80-230 T in a high-pressure (HP) fabricated NdO 0.82 F 0.18 FeAs bulk sample with T c of 51 K. We also demonstrate that the supercurrent density in fields from 1 up to 9 T only drops by a factor of 2-6 for T < 30 K, significantly slower than for MgB 2 and high-T c cuprate superconductors. The very high H c2 of the sample greatly surpasses those of MgB 2 and classic low-temperature superconductors, and the superior J c -field performance is promising for the use of the new NdFeAsO 0.82 F 0.18 superconductors in highfield applications.The X-ray diffraction (XRD) and refinement results shown in Figure 1 indicate that the NdFeAsO 0.82 F 0.18 sample is nearly single phase, with a tetragonal structure of the P4/nmm symmetry and lattice parameters a ¼ 3.953 Å and c ¼ 8.527 Å . The Nd and As are located at the Wyckoff position 2c, with z ¼ 0.143 and 0.659, respectively. The nearest neighbor distances Figure 2. The resistivity is about 9 mV Á cm at 300 K and 3 mV Á cm at 52 K, while the residual resistivity ratio is RRR ¼ r(300 K)/r(52 K) ¼ 3, which means that the scattering becomes large at the onset temperature. The resistance drops to zero at T ¼ 46 K in zero magnetic field. It can be seen that the onset T c drops very slowly with increasing magnetic field. However, the T c (0) decreases quickly to lower temperatures. The upper critical field, H c2 , is defined as the field at which the resistance starts to drop. We use a criterion of 99% of normal resistivity at the onset temperature. The H c2 defined in this way refers to the case of a field parallel to the ab-plane, H C2 ab . The magnetoresistance R(B) was also measured at several temperatures, as shown in Figure 3. The broad R(B) transition is similar to what has been seen in a LaFeAsO 0.82 F 0.18 sample. [6] Using the same analysis used in ...

The vortex liquid-to-glass transition has been studied in Ba0.72K0.28Fe2As2 (BaK-122), Ba(Fe0.91Co0.09)2As2(BaCo-122), and Ba(Fe0.95Ni0.05)2As2(BaNi-122) single crystal with superconducting transition temperature, Tc = 31.7, 17.3, and 18 K, respectively, by magnetoresistance measurements. For temperatures below Tc, the resistivity curves were measured in magnetic fields within the range of 0 ≤ B ≤ 13 T, and the pinning potential was scaled according to a modified model for vortex liquid resistivity. Good scaling of the resistivity ρ(B, T) and the effective pinning energy U0(B,T) were obtained. The vortex state is three-dimensional at temperatures lower than a characteristic temperature T*. The vortex phase diagram was determined based on the evolution of the vortex-glass transition temperature Tg with magnetic field and the upper critical field, Hc2. We found that non-magnetic K doping results in a high glass line close to the Hc2, while magnetic Ni and Co doping causes a low glass line which is far away from the Hc2. Our results suggest that non-magnetic induced disorder is more favourable for enhancement of pinning strength compared to magnetic induced disorder. Our results show that the pinning potential is responsible for the difference in the glass states.

Pressure is well known to significantly raise the superconducting transition temperature, Tc, in both iron pnictides and cuprate based superconductors. Little work has been done, however, on how pressure can affect the flux pinning and critical current density in the Fe-based superconductors. Here, we propose to use hydrostatic pressure to significantly enhance flux pinning and Tc in polycrystalline pnictide bulks. We have chosen Sr4V2O6Fe2As2 polycrystalline samples as a case study. We demonstrate that the hydrostatic pressure up to 1.2 GPa can not only significantly increase Tc from 15 K (underdoped) to 22 K, but also significantly enhance the irreversibility field, Hirr, by a factor of 4 at 7 K, as well as the critical current density, Jc, by up to 30 times at both low and high fields. It was found that pressure can induce more point defects, which are mainly responsible for the Jc enhancement. Our findings provide an effective method to significantly enhance Tc, Jc, Hirr, and the upper critical field, Hc2, for other families of Fe-based superconductors in the forms of wires/tapes, films, and single crystal and polycrystalline bulks.

Flux-pinning mechanism of MgB 2 doped with 10 wt % silicone-oil sintered at low and high temperatures has been investigated by magnetic measurements. The field dependence of the critical current density, j c ͑B͒, was analyzed within the collective pinning model. A crossover field, B sb , from the single vortex to the small vortex bundle-pinning regime was observed. For both types of sintered samples, the temperature dependence of B sb ͑T͒ at low temperature is in good agreement with the ␦l pinning mechanism, i.e., pinning associated with charge-carrier mean free path fluctuation. At temperatures close to the critical temperature, however, there is evidence for ␦T c pinning, which is associated with spatial fluctuations of the transition temperature. These results provide strong evidence that the liquid precursor, silicone oil, produces very small pinning centers and enhances the j c ͑B͒.

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