Heavy Fermion Emerging from the Singular Fermi Liquid in Underscreened Kondo Model for Eu Compounds

An underscreened Kondo model for the impurity spin S = 7/2 in a weak crystal field is studied to elucidate the Kondo effect in the Eu compounds. By utilizing the numerical renormalization group method, we first confirmed an appearance of the so-called singular Fermi-liquid (SFL) state in the absence of the crystal field, displaying a type of the non-Fermi-liquid thermodynamics. Then, we found that the Fermi-liquid state with a huge effective mass is induced from the SFL state by introducing the crystal field. The properties of this heavy fermion (HF) were studied in comparison with a large-N bosonization theory. It was shown that the HF state appears even in a region of the strong Kondo coupling and that the Wilson ratio takes the universal value R = 2 in the entire region in spite of the large Eu spin. The relevance of these results to the HF state in the Eu compounds is discussed.