Van Hove singularities and competing instabilities in an altermagnetic metal

Van Hove (VH) singularities in the single-particle band spectrum are important for interaction-driven quantum phases. Whereas VH points are usually spin-degenerate, in newly proposed altermagnets VH singularities can become spin-dependent, due to momentum-dependent spin polarization of the Fermi surfaces arising from combined rotation and time-reversal symmetry. We consider two altermagnetic models (d_x²−y²- and dxy-wave) on a square lattice with spin-polarized VH points, and study their stable fixed-point solutions indicating interaction-induced instabilities using parquet renormalization group. For both models, we find new stable fixed-point solutions of the renormalization group equations which are not connected to the solution in the spin-degenerate limit. This implies that on the square lattice, the system with VH singularities is unstable with respect to altermagnetic perturbations. The leading instability for the d_x²−y² model is real transverse spin density wave. For the d_xy-wave model, it is found to be real transverse spin density wave at large altermagnetic splitting. At small altermagnetic splitting both imaginary charge density wave and real longitudinal spin density waves are dominant.