3D bulk-resolved g-wave altermagnetic order parameter in CrSb.

Electronic phases of matter, such as magnetism and superconductivity, are defined and distinguished by their order parameters quantifying the spontaneous symmetry breaking underlying each phase. Simple cases include the uniform magnetization of ferromagnets1,2 and the isotropic gap function of conventional superconductors3. Unconventional superconductors4 often have a nodal gap function, in which the gap changes sign at nodes on the Fermi surface. This concept of unconventional or nodal order parameter symmetry has recently been extended to numerous magnetic systems5-8, including altermagnets9-13, in which up- and down-spin species have non-degenerate Fermi surfaces. Here we demonstrate that magnetic quantum oscillation14 measurements can provide a high-resolution, bulk-sensitive, three-dimensional (3D) mapping of the order parameter in an unconventional magnet. By rotating a magnetic field through high- and low-symmetry directions of the CrSb Brillouin zone, we show that the altermagnetic band structure of this material leads to a reduction of symmetry for each spin-split Fermi sheet away from nodal orientations. In momentum space, the exchange splitting between up and down spins follows the profile of the Y 4 - 3 = y z ( 3 x 2 - y 2 ) real spherical harmonic-analogous to a g-orbital of the hydrogen atom. Although notoriously difficult to resolve in unconventional superconductors, our work demonstrates that the order parameter symmetry of unconventional magnets can be precisely mapped by quantum oscillatory quasiparticle spectroscopy, establishing CrSb as a prototypical g-wave metallic altermagnet.