Elsevier

Physica C: Superconductivity

Volume 468, Issues 15–20, 15 September 2008, Pages 1287-1290
Physica C: Superconductivity

Critical current densities and irreversibility fields of a HgBa2Can−1CunO2n+2+δ sample containing n = 6–15 phases

https://doi.org/10.1016/j.physc.2008.05.049Get rights and content

Abstract

A recent report on multilayered superconductors establishes unique constant Tc although the number of inner CuO2 plane (IP) increases. Many hypotheses proposed that the irreversibility field (Birr) and critical current density (Jc) are depends on the crystal structures. We have measured the Jc and Birr of the HgBa2Can−1CunO2n+2+δ sample containing n = 6–15 phases to investigate the effect of the number of IP’s on the Birr and Jc and its pinning properties. The rate of fall of Jc increases and irreversible lines (IL’s) shift to lower temperatures with increasing the number of IP’s, which suggests that anisotropy increases with n. The IL settled in between those for Bi2Sr2CaCu2Oy with high anisotropy value and YBa2Cu3O7 with less anisotropy value. The double logarithmic plot of irreversibility field versus [1  (T/Tc)] analysis suggests that the flux line melting model is adopted. The flux pinning force density Fp (≈JcB) exhibits scaling behavior when the magnetic field B is normalized by the irreversibility field Birr. Analysis of the normalized pinning force reveals that a surface pinning mechanism is dominant and reduced magnetic field bmax = 0.2 agree with surface pinning mechanism with closely spaced pins.

Introduction

Multilayered (ML) cuprates admit two or more crystallographically inequivalent CuO2 planes in a unit cell: outer planes (OP) with pyramidal (five) oxygen coordination and inner planes with square (four) oxygen coordination. Multilayered cuprates are generally depicted by the formula, MCan−1CunO2n, where M is a charge reservoir layer (CRL) such as HgBa2Oy, Bi2Sr2Oy, Tl2Ba2Oy, Ba(O, F)y etc. The Can−1CunO2n has an infinite layer structure and n means the number of CuO2 planes between the CRL. Multilayered high-Tc cuprates, which have more than three CuO2 planes in a unit cell, demonstrate peculiar properties because of the two types of CuO2 planes. Evidence for the coexistent phase of superconductivity and antiferromagnetism in a unit cell has been obtained in the five-layered high-Tc superconductor HgBa2Ca4Cu5O12+δ [1], [2], [3]. Also our recent investigations have proposed a Tc versus n relationship for multilayered high-Tc superconductors, in which the Tc is almost constant above about n = 5 [4]. We explained the characteristic relationship between Tc versus n using the carrier imbalance model in multilayered cuprates shown by NMR measurements [5]. The OP can have enough carriers for superconductivity for large n even if the number of carriers in IP becomes too small to induce superconductivity.

The Birr and Jc of the samples should be influenced by the n even though that the samples have the same Tc, because a coupling between the OP’s is expected to be weak with increasing the number of IP’s. To know the Jc and Birr for large n, we used the HgBa2Can−1CunO2n+2+δ (Hg-12(n  1)n) sample containing n = 6–15 phases because we have not succeeded in synthesizing single-phase samples for n  7. In this report, the Jc and Birr properties of the sample will be shown and compared with other superconductors.

Section snippets

Experimental

The multilayered (ML) cuprates are normally synthesized under a pressure of several GPa [6]. The details of the preparation for polycrystalline HgBa2Can−1CunO2n+2+δ sample with a nominal composition of n = 10 (Hg-12910) have been reported elsewhere [4]. For grain alignment, the polycrystalline sample was ground to a powder with average grain size 2–3 μm, mixed with an epoxy resin in a sample powder: epoxy resin = 1:3 weight ratio, and kept for 12 h in a high magnetic field of 7 T at room temperature.

Results

Fig. 1 shows the XRD pattern of the aligned Hg-12910 sample. Most peaks could be indexed as only (0 0 l) peaks. The XRD pattern shows the coexistence of the n = 6–15 phases of Hg-12(n  1)n. Fig. 2 shows the temperature dependence of susceptibility of the Hg-12910 sample. The sample showed a high and sharp superconducting transition at temperature 103 K. This means that all the coexistent phases in the sample have the same Tc values, otherwise a lot of transitions would have been observed.

Fig. 3 shows

Conclusions

Critical current densities and irreversiblilty fields have been analyzed for the HgBa2Can−1CunO2n+2+δ sample containing n = 6–15 phases (Hg-12910). The increase in number of inner planes in the unit cell of Hg-12(n  1)n with increase in n, suggests the increase in rate of fall of Jc and shift of the IL’s to lower temperatures, and also indicates the increase in the anisotropy values. The double logarithmic plot of irreversibility field versus [1  (T/Tc)] analysis suggests that the flux line melting

Acknowledgments

P.M. Shirage, D.D. Shivagan and A.Crisan gratefully acknowledge Japan Society for the Promotion of Science (JSPS) for the JSPS Postdoctoral Research Fellowship and JSPS Invited fellowship, respectively.

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