A New Superconductor Family with Various Magnetic Elements
Okamoto Group
There is a complex relationship between superconductivity, where the electrical resistance of a material becomes completely zero at low temperatures, and magnetism, which is the property of a material as a magnet. Generally, superconductivity is destroyed by strong magnetism, so superconductivity does not often appear in materials containing magnetic elements such as iron. In rare cases, however, materials containing magnetic elements exhibit unconventional superconductivity with very high critical temperature or unusual properties that cannot be explained within the framework of existing theories. Uncovering the complex relationship between superconductivity and magnetism may be important for the realization of room-temperature superconductivity. The discovery of unique superconductors is essential for elucidating this relationship.
We discovered that the ternary telluride series Sc6MTe2 is a unique family of d-electron superconductors incorporating various magnetic elements [1]. Sc6MTe2 compounds with M = Mn, Fe, Co, Ni, Ru, Rh, Os, and Ir have been synthesized and reported to crystallize in the hexagonal Zr6CoAl2-type structure, but their physical properties have not been reported thus far [2,3]. A characteristic point of this crystal structure is the fact that the M atoms are trigonal prismatically coordinated by six Sc atoms and form one-dimensional chains along the c axis, as shown in Fig. 1(a). Polycrystalline samples of Sc6MTe2 with various transition metal M were synthesized by the arc-melting method and the bulk superconducting transitions in seven M cases were confirmed based on the electrical resistivity, magnetization, and heat capacity measurements on the obtained polycrystalline samples. A Sc6FeTe2 sample is shown in the lower right panel of Fig. 1(a).
Figure 1(b) shows the electrical resistivity at low temperatures for various M cases. The seven M cases except for M = Mn, the resistivity shows a sharp drop to zero above 2 K. They also show large diamagnetic signal and clear heat capacity jump, indicating the bulk superconducting transition occurs in them. A characteristic feature of the superconductivity in Sc6MTe2 is the M dependence of the superconducting transition temperature Tc. Four compounds with M = 4d and 5d elements displayed almost the same Tc of approximately 2 K, but those with M = 3d elements displayed higher values and increased in the order of Ni, Co, and Fe. Therefore, Sc6FeTe2 showed the highest Tc of 4.7 K.
These results strongly suggest that the 3d electrons of M atoms play an important role in realizing the superconductivity in this system. First principles calculations indicate the presence of significant contribution of Fe 3d orbitals at the Fermi energy, which most likely enhance the Tc of Sc6FeTe2. The heat capacity data of Sc6FeTe2 indicate that the electronic specific heat in Sc6FeTe2 is strongly enhanced by some reason. At present, the origin of this enhancement is still unclear, but it might be interesting if the strong electron correlation of Fe 3d electron plays an important role in this enhancement.
Another important point of this Sc6MTe2 family is that all of Sc, M, and Te sites can be replaced by various elements and physical properties of almost all of them have not been investigated thus far. In fact, following Sc6MTe2, we recently discovered superconductivity in a Zr analogues Zr6MTe2 [4]. The Tc values in Zr6MTe2 is much lower than those for Sc6MTe2, but the highest Tc was realized in Zr6FeTe2 as in the case of Sc6MTe2. Zr6FeSb2 is also found to show superconductivity at 1.3 K [5]. It is expected that more new superconductors will be found in this family and the future research on this family will contribute to a complete understanding of the relationship between superconductivity and magnetic elements.
References
- [1] Y. Shinoda, Y. Okamoto, Y. Yamakawa, H. Matsumoto, D. Hirai, and K. Takenaka, J. Phys. Soc. Jpn. 92, 103701 (2023).
- [2] P. A. Maggard and J. D. Corbett, Inorg. Chem. 39, 4143 (2000).
- [3] L. Chen and J. D. Corbett, Inorg. Chem. 43, 436 (2004).
- [4] H. Matsumoto, Y. Yamakawa, R. Okuma, D. Nishio-Hamane, and Y. Okamoto, J. Phys. Soc. Jpn. 93, 023705 (2024).
- [5] R. Matsumoto, E. Murakami, R. Oishi, S. Ramakrishnan, A. Ikeda, S. Yonezawa, T. Takabatake, T. Onimaru, and M. Nohara, J. Phys. Soc. Jpn. 93, 065001 (2024).