CLOSING OF THE SPIN GAP AND FERROMAGNETISM INDUCED BY MAGNETIC-IMPURITIES

The Hubbard chain with attractive U and a finite concentration of magn etic impurities of spin S is studied using the Bethe Ansatz technique. The impurities weaken the singlet bound states of the Hubbard model r educing the spin gap. The spin gap is closed at a critical concentrati on which depends on the spin and the coupling strength. A ferromagneti c phase is induced for concentrations of impurities larger than the cr itical one, where a fraction of singlet pairs is broken up giving rise to a Fermi sea of spin-polarized itinerant electrons. [S0163-1829(98) 01418-0].