SPECIFIC-HEAT AND SUSCEPTIBILITY OF THE HEISENBERG CHAIN WITH FINITE CONCENTRATION OF IMPURITIES

The isotropic antiferromagnetic Heisenberg chain of spin 1/2 with a co ncentration x of impurities of spin S is considered. The Bethe ansatz equations for the ground state define two energy bands, which are anal yzed as a function of x and magnetic field. In zero field the ground s tate is a singlet for finite x,, but non-Fermi-liquid-like for x -->, 0. The van Hove singularities of the empty bands define two critical f ields, H-c1(x) and H-c2(x), at which the susceptibility and the specif ic heat coefficient gamma diverge. The non-Fermi-liquid behavior as x --> 0 arises from nonanalyticities as H-c2 vanishes.