RG Dynamics of Irrelevant Fermion Operators and the Drag Coupling Mechanism
We study the renormalization-group flow of higher-dimensional fermionic interactions $(\psi^\dagger \psi)^{2n}$ in the presence of a Fermi surface. We show that the growth of the BCS four-fermion coupling induces a drag mechanism whereby higher-order fermionic couplings are driven to strong coupling in the infrared. We derive the corresponding beta functions and show that the drag effect applies generically to the whole tower of fermionic operators. Remarkably, although all couplings are driven toward the same strong-coupling scale, the renormalization-group flow preserves a hierarchy in which higher-dimensional operators remain parametrically suppressed relative to the BCS interaction. We then investigate this mechanism in a $2+1$-dimensional non-Fermi liquid coupled to a critical boson. While higher-order fermionic interactions are similarly enhanced along the flow, we show that they do not destabilize the IR stable non-Fermi-liquid fixed point, present for sufficiently large $N$. We briefly discuss possible implications for multicomponent superconductors and other strongly correlated metallic systems.