Low Temperature Thermal and Magnetic Properties of Cu-Doped Haldane Gap Antiferromagnet NENP

Abstract

Effects of Cu-impurity are studied by heat capacity and susceptibility measurements down to 30, mK on the Haldane-gap antiferromagnet Ni(C₂H₈N₂)₂NO₂(ClO₄) (NENP). We have observed the characteristic heat capacity and susceptibility which show contributions from free spins of the Cu²⁺-impurities with S=1/2 and the fractional spin S=1/2 states at the ends of the Ni²⁺(S=1)-chain adjacent to the Cu²⁺-impurities. The present results are explained completely by the phenomenological Hamiltonian for a trimmer of S=1/2 proposed by the ESR study on this system.