MIXED-VALENT REGIME OF THE 2-CHANNEL ANDERSON IMPURITY AS A MODEL FORUBE13

We investigate the mixed-valent regime of a two-configuration Anderson impurity model for uranium ions, with separate quadrupolar and magnet ic doublets. With a new Monte Carlo approach and the noncrossing appro ximation we find (i) a non-Fermi-liquid fixed point with two-channel K ondo model critical behavior, (ii) distinct energy scales for screenin g the low-lying and excited doublets; (iii) a semiquantitative explana tion of magnetic-susceptibility data for U1-xThxBe13 assuming 60% - 70 % quadrupolar doublet ground-state weight, supporting the quadrupolar- Kondo interpretation. [S0031-9007(98)07334-7].