MULTICHANNEL KONDO PROBLEM AND SOME APPLICATIONS

We review analytical and numerical results derived from the Bethe ansa tz solution of the n-channel Kondo model of arbitrary spin S as a func tion of temperature, external field, impurity spin S and the number of channels n. Three situations have to be distinguished: (i) If n=2S th e conduction electrons exactly compensate the impurity spin into a sin glet at low temperatures, (ii) if n < 2S the impurity spin is only par tially compensated (undercompensated), and (iii) if n > 2S the impurit y spin is said to be overcompensated giving rise to critical behaviour . The results are discussed in the context of magnetic impurities, e.g . Fe, Cr and Tm in simple metals, the quadrupolar Kondo effect. an imp urity spin embedded in the Takhtajan-Babujian Heisenberg model and ele ctron assisted-tunnelling of an atom in a double-well potential.