Quantum transport evidence of Weyl fermions in an epitaxial ferromagnetic oxide SrRuO₃ thin film

SrRuO₃, a 4d ferromagnetic perovskite, provides a promising opportunity to seek the existence of Weyl fermions in magnetic materials. However, such a quest has been hampered until our latest report [1] due to difficulties in preparing high-quality specimens [2]. Here, we demonstrate the quantum transport evidence of the Weyl fermions in an epitaxial SrRuO₃ film with the best crystalline quality ever reported. We grew 63-nm thick SrRuO₃ films on SrTiO₃ substrates by our recently developed machine-learning-assisted molecular beam epitaxy [3]. The residual resistivity ratio [ρ(300 K)/ρ(T→0 K)] of the film reached the world’s best value of 84, enabling direct observations of quantum transport evidence of Weyl fermions [1,4]: (i) unsaturated linear positive magnetoresistance (MR), (ii) chiral-anomaly-induced negative MR, (iii) π Berry phase accumulated along cyclotron orbits, (iv) light cyclotron masses, and (v) high quantum mobility of about 10000 cm²/Vs. Therefore, our results establish SrRuO₃ as a magnetic Weyl semimetal.

[1] K. Takiguchi, Y. K. Wakabayashi, et al., Nat. Commun. 11, 4969 (2020).
[2] G. Koster, et al., Rev. Mod. Phys. 84, 253 (2012).
[3] Y. K. Wakabayashi, et al., APL Mater. 7, 101114 (2019).
[4] X. Huang, et al., Phys. Rev. X 5 031023 (2015).