Very High Critical Field and Superior J_c‐Field Performance in NdFeAsO_0.82F_0.18 with T_c of 51 K

Abstract

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 ...

Synthesis and physical properties of FeSe1/2Te1/2 superconductor

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Superconductivity inLaFe1−xCoxAsO

“…Higher doping levels of x = 0.2, 0.5, and 1 give only metallic behavior. The electrical conductivity increases by a factor of eight, from LaFeAsO (3d 6 ) to LaCoAsO (3d 7 ).…”

“…The recent reports of T c ~ 26 K in LaFeAsO 1-x F x (x ~ 0.11) [1][2][3][4][5][6][7][8][9][10] and related materials have attracted a great deal of attention as these materials appear to be unconventional high-Tc superconductors that are based on iron instead of copper. It is interesting to compare and contrast the behavior of the new iron arsenide superconductors with the cuprates.…”

Magnetotransport and thermal properties characterization of 55 K superconductor SmFeAsO0.85F0.15

“…For example it has been estimated that Hc 2 ∼ 63-65T for LaFeAsO 0.9 F 0.1 , Hc 2 ∼ 70T for PrFeAsO 0.85 F 0.15, 230T for high-pressure fabricated NdFeAsO 0.82 F 0.18 and for SmFeAsO 0.85 F 0.15, it is over 200T. [13][14][15][16] Thus Sm substituted compounds can be highly useful in high current/field applications. The tremendous interest in these compounds is not only due to their high critical temperatures, high upper critical fields, and the ability to sustain high current densities at elevated temperatures but also due to the intriguing presence of well-known magnetic iron (Fe) element in the superconducting Fe-As plane.…”