Experimental determination of band structures of CeCoSi by an angle-resolved photoelectron spectroscopy

Strongly correlated electron systems exhibit various physical properties closely related to charge, spin, and orbital degrees of freedom; in f-electron systems, multipoles, which are combinations of spin and orbital degrees of freedom, play a major role. The centrosymmetric CeCoSi, which belongs to the non-symmorphic space group (P4/nmm), has attracted much attention from the viewpoint of the odd-parity multipoles below the phase transition temperature T₀. Recently, a Ce 4f-driven antiferroquadrupolar order below T₀ was proposed by a detailed study using single crystal samples, followed by a complete phase diagram and further clarification of the finite T₀ ~12 K even at ambient pressure [1,2]. Subsequently, ⁵⁹Co NMR revealed anomalies suggestive of quadrupolar order [3], and odd-parity multipole order was theoretically proposed [4]. However, a fundamental question has remained unsolved as to why the localized Ce 4f state is so sensitive to external pressure and why the T₀ increases substantially. Here, we report the direct observation of the Fermi surfaces and electronic band dispersions of CeCoSi by angle-resolved photoelectron spectroscopy (ARPES) utilizing synchrotron radiation. The photon energy-dependent ARPES result shows that two- and three-dimensional Fermi surfaces coexist, being consistent with the result of the first-principles calculation. By the Ce 3d-4f resonant photoelectron spectroscopy and the high-resolution ARPES, Ce 4f level is found to be located quite close to the Fermi level (~10 meV), which explains why the pressure-sensitive feature of the phase transition.

[1] H. Tanida, Y. Muro, and T. Matsumura, J. Phys. Soc. Jpn. 87, 023705 (2018).

[2] H. Tanida, K. Mitsumoto, Y. Muro, T. Fukuhara, Y. Kawamura, A. Kondo, K. Kindo, Y. Matsumoto,T. Namiki, T. Kuwai, and T. Matsumura, J. Phys. Soc. Jpn. 88, 054716 (2019).

[3] M. Manago, H. Kotegawa, H. Tou, H. Harima, and H. Tanida, J. Phys. Soc. Jpn. 90, 023702 (2021).

[4] M. Yatsushiro, and S. Hayami, J. Phys. Soc. Jpn. 89, 013703 (2019).