Observation of s-wave superconductivity in CaSb2

CaSb2 has a crystal structure with a special arrangement called “non-symmorphic’’. Due to this structure, it is expected that the bulk electronic energy bands have four-fold degeneracy along a line in the momentum space, which is called Dirac line node. In 2020, we discovered that CaSb2 exhibits superconductivity. Much attention has been paid to the superconducting properties of CaSb2 because non-symmorphic crystal structure and Dirac line node can induce unconventional superconductivity.

 We performed NQR measurement, which can be performed without applying magnetic field, on CaSb2 and measured the temperature dependence of the nuclear spin-lattice relaxation rate. Consequently, we obtained the result supporting conventional s-wave superconductivity. In s-wave superconductors, the nuclear spin-lattice relaxation rate shows a peak structure called coherence peak just below the superconducting transition temperature. Moreover, it shows exponential decay in lower temperature region due to superconducting full gap. Our data have these features and well fitted by the formula based on the BCS theory. Future research plan is going to be revealing how the superconducting properties change by applying pressure.

This research is a collaborative research with a group of Professor Paglione of Department of Physics, University of Maryland. Since the paper is open access, anyone can view it for free from the link below.

Inverse temperature dependence of the nuclear spin-lattice relaxation time T1 of CaSb2. Inset: Temperature dependence of the nuclear spin-lattice relaxation rate of CaSb2. It shows a coherence peak just below the superconducting transition temperature and exponential decay in lower temperature region, well fitted using the BCS theory (gray solid curve). The blue curve is temperature dependence of AC magnetic susceptibility showing a drop at the superconducting transition temperature.

Article information

Takahashi, H; Kitagawa, S; Ishida, K; Kawaguchi, M; Ikeda, A; Yonezawa, S; Maeno, Y

S-Wave Superconductivity in the Dirac Line-Nodal Material CaSb2 Journal Article

In: Journal of the Physical Society of Japan, 90 (073702), pp. 5, 2021.

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