Unconventional Discontinuous Transitions in Isospin Systems (2024)

Z.M. Raines, L.I. Glazman, A.V. Chubukov

in Phys. Rev. Lett.

We show that two-dimensional fermions with dispersion k2k^2 or k4k^4 undergo a first-order Stoner transition to a fully spin-polarized state despite the fact that the spin susceptibility diverges at the critical point. We extend our analysis to systems with dispersion k2αk^{2\alpha} and spin and valley isospin and show that there is a cascade of instabilities into fractional-metal states with some electron bands fully depleted; narrow intermediate ranges of partially depleted bands exist for α<1\alpha<1 or α>2\alpha>2. The susceptibility becomes large near each transition. We discuss applications to biased bi- and trilayer graphene and moiré systems.