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In-plane chemical pressure essential for superconductivity in BiCh2-based (Ch: S, Se) layered structure.

Mizuguchi Y, Miura A, Kajitani J, Hiroi T, Miura O, Tadanaga K, Kumada N, Magome E, Moriyoshi C, Kuroiwa Y - Sci Rep (2015)

Bottom Line: BiCh2-based compounds (Ch: S, Se) are a new series of layered superconductors, and the mechanisms for the emergence of superconductivity in these materials have not yet been elucidated.We show that the structure parameter essential for the emergence of bulk superconductivity in both systems is the in-plane chemical pressure, rather than Bi-Ch bond lengths or in-plane Ch-Bi-Ch bond angle.Furthermore, we show that the superconducting transition temperature for all REO0.5F0.5BiCh2 superconductors can be determined from the in-plane chemical pressure.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical and Electronic Engineering, Tokyo Metropolitan University, 1-1, Minami-osawa, Hachioji 192-0397, Japan.

ABSTRACT
BiCh2-based compounds (Ch: S, Se) are a new series of layered superconductors, and the mechanisms for the emergence of superconductivity in these materials have not yet been elucidated. In this study, we investigate the relationship between crystal structure and superconducting properties of the BiCh2-based superconductor family, specifically, optimally doped Ce1-xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1-ySey)2. We use powder synchrotron X-ray diffraction to determine the crystal structures. We show that the structure parameter essential for the emergence of bulk superconductivity in both systems is the in-plane chemical pressure, rather than Bi-Ch bond lengths or in-plane Ch-Bi-Ch bond angle. Furthermore, we show that the superconducting transition temperature for all REO0.5F0.5BiCh2 superconductors can be determined from the in-plane chemical pressure.

No MeSH data available.


Superconductivity phase diagrams of Ce1−xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1−ySey)2.(a) Superconductivity phase diagrams of Ce1−xNdxO0.5F0.5BiS2. For x = 0 and 0.2, superconducting transition is not observed at T > 2 K. For 0.4 ≤ x ≤ 1, superconducting transition with a large shielding signal, with which we could regard the samples as a bulk superconductor (Bulk SC), is observed. Tc increases with increasing x. The inset figure shows a schematic of crystal structure of Ce1−xNdxO0.5F0.5BiS2. (b) Superconductivity phase diagrams of LaO0.5F0.5Bi(S1−ySey)2. For y = 0 and 0.1, superconducting transition is observed but their shielding signals are very small as a bulk superconductor (Filamentary SC). For y ≥ 0.2, superconducting transition with a large shielding signal is observed (Bulk SC). Tc increases with increasing y up to y = 0.5. The inset figure shows a schematic image of crystal structure of LaO0.5F0.5Bi(S1−ySey)2.
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f1: Superconductivity phase diagrams of Ce1−xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1−ySey)2.(a) Superconductivity phase diagrams of Ce1−xNdxO0.5F0.5BiS2. For x = 0 and 0.2, superconducting transition is not observed at T > 2 K. For 0.4 ≤ x ≤ 1, superconducting transition with a large shielding signal, with which we could regard the samples as a bulk superconductor (Bulk SC), is observed. Tc increases with increasing x. The inset figure shows a schematic of crystal structure of Ce1−xNdxO0.5F0.5BiS2. (b) Superconductivity phase diagrams of LaO0.5F0.5Bi(S1−ySey)2. For y = 0 and 0.1, superconducting transition is observed but their shielding signals are very small as a bulk superconductor (Filamentary SC). For y ≥ 0.2, superconducting transition with a large shielding signal is observed (Bulk SC). Tc increases with increasing y up to y = 0.5. The inset figure shows a schematic image of crystal structure of LaO0.5F0.5Bi(S1−ySey)2.

Mentions: Furthermore, our recent studies concerning the effect of isovalent-substitution on superconductivity suggest that optimization of crystal structure is important for the emergence of bulk SC and the ability to attain a high Tc in optimally doped REO0.5F0.5BiCh2. We use the example of Ce1−xNdxO0.5F0.5BiS233 for following discussion. Since in this crystal structure, the valence of Ce and Nd are both 3+, electron carriers in these compounds are essentially the same: the formal valence of Bi is 2.5+. However, bulk SC is induced by the systematic substitution of Ce by Nd, and Tc increases with increasing Nd concentration (x) as shown in Fig. 1a. The emergence of superconductivity in this material was explained by uniaxial lattice shrinkage along the a-axis and optimization of the lattice shrinkage ratio, c/a33. Another example of isovalent-substitution systems is LaO0.5F0.5Bi(S1−ySey)234. In this material, the S2− site within the superconducting layers is systematically substituted by Se2−. Therefore, the formal valence of Bi (2.5+) should not change with Se substitution. Bulk SC is induced by Se substitution, and Tc increases with increasing Se concentration (y) as shown in Fig. 1b. Se substitution enhances the metallic conductivity of this system and induces bulk SC through lattice volume expansion34.


In-plane chemical pressure essential for superconductivity in BiCh2-based (Ch: S, Se) layered structure.

Mizuguchi Y, Miura A, Kajitani J, Hiroi T, Miura O, Tadanaga K, Kumada N, Magome E, Moriyoshi C, Kuroiwa Y - Sci Rep (2015)

Superconductivity phase diagrams of Ce1−xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1−ySey)2.(a) Superconductivity phase diagrams of Ce1−xNdxO0.5F0.5BiS2. For x = 0 and 0.2, superconducting transition is not observed at T > 2 K. For 0.4 ≤ x ≤ 1, superconducting transition with a large shielding signal, with which we could regard the samples as a bulk superconductor (Bulk SC), is observed. Tc increases with increasing x. The inset figure shows a schematic of crystal structure of Ce1−xNdxO0.5F0.5BiS2. (b) Superconductivity phase diagrams of LaO0.5F0.5Bi(S1−ySey)2. For y = 0 and 0.1, superconducting transition is observed but their shielding signals are very small as a bulk superconductor (Filamentary SC). For y ≥ 0.2, superconducting transition with a large shielding signal is observed (Bulk SC). Tc increases with increasing y up to y = 0.5. The inset figure shows a schematic image of crystal structure of LaO0.5F0.5Bi(S1−ySey)2.
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f1: Superconductivity phase diagrams of Ce1−xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1−ySey)2.(a) Superconductivity phase diagrams of Ce1−xNdxO0.5F0.5BiS2. For x = 0 and 0.2, superconducting transition is not observed at T > 2 K. For 0.4 ≤ x ≤ 1, superconducting transition with a large shielding signal, with which we could regard the samples as a bulk superconductor (Bulk SC), is observed. Tc increases with increasing x. The inset figure shows a schematic of crystal structure of Ce1−xNdxO0.5F0.5BiS2. (b) Superconductivity phase diagrams of LaO0.5F0.5Bi(S1−ySey)2. For y = 0 and 0.1, superconducting transition is observed but their shielding signals are very small as a bulk superconductor (Filamentary SC). For y ≥ 0.2, superconducting transition with a large shielding signal is observed (Bulk SC). Tc increases with increasing y up to y = 0.5. The inset figure shows a schematic image of crystal structure of LaO0.5F0.5Bi(S1−ySey)2.
Mentions: Furthermore, our recent studies concerning the effect of isovalent-substitution on superconductivity suggest that optimization of crystal structure is important for the emergence of bulk SC and the ability to attain a high Tc in optimally doped REO0.5F0.5BiCh2. We use the example of Ce1−xNdxO0.5F0.5BiS233 for following discussion. Since in this crystal structure, the valence of Ce and Nd are both 3+, electron carriers in these compounds are essentially the same: the formal valence of Bi is 2.5+. However, bulk SC is induced by the systematic substitution of Ce by Nd, and Tc increases with increasing Nd concentration (x) as shown in Fig. 1a. The emergence of superconductivity in this material was explained by uniaxial lattice shrinkage along the a-axis and optimization of the lattice shrinkage ratio, c/a33. Another example of isovalent-substitution systems is LaO0.5F0.5Bi(S1−ySey)234. In this material, the S2− site within the superconducting layers is systematically substituted by Se2−. Therefore, the formal valence of Bi (2.5+) should not change with Se substitution. Bulk SC is induced by Se substitution, and Tc increases with increasing Se concentration (y) as shown in Fig. 1b. Se substitution enhances the metallic conductivity of this system and induces bulk SC through lattice volume expansion34.

Bottom Line: BiCh2-based compounds (Ch: S, Se) are a new series of layered superconductors, and the mechanisms for the emergence of superconductivity in these materials have not yet been elucidated.We show that the structure parameter essential for the emergence of bulk superconductivity in both systems is the in-plane chemical pressure, rather than Bi-Ch bond lengths or in-plane Ch-Bi-Ch bond angle.Furthermore, we show that the superconducting transition temperature for all REO0.5F0.5BiCh2 superconductors can be determined from the in-plane chemical pressure.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical and Electronic Engineering, Tokyo Metropolitan University, 1-1, Minami-osawa, Hachioji 192-0397, Japan.

ABSTRACT
BiCh2-based compounds (Ch: S, Se) are a new series of layered superconductors, and the mechanisms for the emergence of superconductivity in these materials have not yet been elucidated. In this study, we investigate the relationship between crystal structure and superconducting properties of the BiCh2-based superconductor family, specifically, optimally doped Ce1-xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1-ySey)2. We use powder synchrotron X-ray diffraction to determine the crystal structures. We show that the structure parameter essential for the emergence of bulk superconductivity in both systems is the in-plane chemical pressure, rather than Bi-Ch bond lengths or in-plane Ch-Bi-Ch bond angle. Furthermore, we show that the superconducting transition temperature for all REO0.5F0.5BiCh2 superconductors can be determined from the in-plane chemical pressure.

No MeSH data available.