Fermi Surface Properties and Antisymmetric Spin--Orbit Coupling in Noncentrosymmetric CeCoSi3

We report de Haas--van Alphen (dHvA) effect measurement of the intermediate-valence compound CeCoSi 3 whose crystal structure lacks space-inversion symmetry. Some dHvA frequencies of CeCoSi 3 qualitatively correspond to those of CeRhSi 3 which is a heavy-fermion antiferromagnet exhibiting pressure-induced superconductivity. We observed multiply splitting dHvA frequencies and attributed them to the magnetic breakdown between two separated Fermi surfaces due to the antisymmetric spin--orbit coupling (ASOC). Strikingly, the cyclotron effective masses of the separated Fermi surfaces are largely different, indicating that the mass enhancement depends on the spin direction. The evaluated ASOC energy of CeCoSi 3 is less than that of LaCoGe 3 , which is probably characteristic of a heavy-fermion band with an itinerant f -electron.