Fig. 1: Crystal structure, phase diagram, and reciprocal space of CeCu₂Si₂.

a Schematic phase diagram of CeCu₂Si₂. b Crystal structure of CeCu₂Si₂, with Ce³⁺ ions forming a body-centered tetragonal lattice. c [H, H, L] scattering plane of CeCu₂Si₂, with the shaded area representing a Brillouin zone for the Ce³⁺ ions. The black dots indicate magnetic Bragg peaks in A-type CeCu₂Si₂³⁶, and the dashed ovals are schematic intensity contours of low-energy AF excitations in our S-type CeCu₂Si₂. d Slice of the CeCu₂Si₂ Fermi surface with k_x = k_y, and the arrow represents the intraband nesting vector. The electronic structure used in this work is identical to that in ref. ⁴⁶, and 3D plots of the Fermi surfaces are shown in Supplementary Fig. 7.