Unconventional superconducting phenomena in topological Dirac semimetal α-Sn thin films

α-Sn, which has a diamond crystal structure, has recently attracted attention as a rich topological material platform. When grown on an InSb (001) substrate, α-Sn is transformed to a topological Dirac semimetal (TDS) with high-mobility and nontrivial surface and bulk band components [1]. Incorporation of superconductivity into the TDS α-Sn is expected to open a way to topological superconductivity that can host Majorana fermions [2].

We study magnetotransport properties of a 5 nm-thick α-Sn thin film grown on an undoped InSb (001) substrate, which shows a superconducting transition at T_C = 4.2K after being kept in ambient conditions for 20 months. The superconducting α-Sn layer exhibits large in-plane critical magnetic fields (H_C) exceeding the Pauli limit, and H_C changes by 1.5 times when the magnetic field is applied at an angle of 45° and 135° with respect to the current. Furthermore, the critical current changes upon reversing the current direction under an in-plane magnetic field. This superconducting diode effect reaches 2% and changes sign with varying the magnetic field. These results thus highlight rich topological physics in superconducting α-Sn.

[1] L. D. Anh et al, Adv. Mater. 33, 2104645 (2021). [2] S. Kobayashi, & M. Sato, Phys. Rev. Lett. 115, 187001 (2015).