Catalog of cubic, symmetry-protected, non-Fermi liquid, Kondo-type exchange models for doublet impurities
To identify what types of non-Fermi liquid (NFL) behavior are most likely to occur in cubic metals due to doublet impurities, we derive every cubic symmetry-allowed, NFL, Kondo-type exchange coupling that does not need accidental degeneracy for its realization. We find three distinct types of NFL behavior: two-channel Kondo (2CK) behavior for a non-Kramers doublet impurity coupled to local $Γ_8$ conduction electrons; topological Kondo physics for a Kramers doublet impurity and $Γ_4$ or $Γ_5$ conduction electrons; and lastly, spin-half impurity spin-$\frac{3}{2}$ conduction electron Kondo behavior for a Kramers doublet impurity and $Γ_8$ conduction electrons. The first two critical behaviors are not straightforward to realize. In the first case, 2CK physics is not guaranteed, since cubic symmetry does not prevent an effective spatial anisotropy from exceeding the 2CK coupling, which restores a Fermi liquid behavior. In the second case, the topological Kondo interaction is guaranteed to dominate, however, the spin degeneracy of the conduction electrons needs to be lifted e.g. by a magnetic field$-$so that they can be represented by $Γ_4$ or $Γ_5$ triplets$-$which then also lifts the degeneracy of the Kramers doublet. We find that the spin-half impurity spin-$\frac{3}{2}$ conduction electron, NFL, Kondo behavior has the greatest chance of existing in diluted, cubic compounds. We compute the thermodynamics of the topological Kondo model using the numerical renormalization group, and discuss the thermodynamics of the spin-half impurity spin-$\frac{3}{2}$ conduction electron Kondo model. We also identify candidate materials where the corresponding NFL behaviors could be observed.