Non-Trivial Fixed Points and Truncated SU(4) Kondo Models in a Quasi-Quartet Multipolar Quantum Impurity Problem

The multipolar Kondo problem has seen recent theoretical and experimental interest due to proposals of novel non-Fermi liquid states and the availability of a variety of material platforms. The multipolar nature of local moments, in conjunction with constraining crystal field symmetries, leads to numerous possible interactions and resulting non-Fermi liquid ground states. In this work, inspired by recent experiments on the tetragonal material YbRu2Ge2, which has been shown to exhibit a local moment with a quasi-fourfold degenerate ground state, we consider the Kondo effect for such a quasi-quartet multipolar impurity. In the tetragonal crystal field environment, the local moment supports dipolar, quadrupolar, and octupolar moments. Using renormalization group analysis, we uncover a number of quantum ground states characterized by non-trivial fixed points. It is shown that these fixed points are described by truncated SU(4) Kondo models, where only some of the SU(4) generators (representing the impurity degrees of freedom) are coupled to conduction electrons. Such novel non-trivial fixed points are unique to the quasi-quartet multipolar impurity.