Magnetic Fluctuations and Excitations in the S=1 Antiferromagnetic Chains of NENP

Abstract

In this paper we report experimental results concerning the magnetic properties of the S = 1 antiferromagnetic chain, as observed in the new system Ni (en)2N02C104 (alias NENP). Conventional ideas on the ground state properties of antiferromagnetic quantum chains have been recently challenged by HALDANE /1/. According to his theory, the S = 1 Heisenberg antiferromagnetic chain (1D-HAF) would exhibit an energy gap between a non-magnetic singlet ground state and the first excited states, in sharp contrast with the S = 1/2 1D-HAF which exhibits a continuum of excited states /2/. Such a difference between integer and half integer spins has been more clearly verified by numerical calculations /3,4,5/. The best evaluations agree with a rather large value of the energy gap EG ∼ O.4∣J∣ for the case S = 1 /5/. However, the existence of the Haldane gap would affect largely the magnetic properties. The most characteristic features would be an exponential decrease of all the susceptibilities when T ≲ EG/k, and the absence of long-range ordering (LRO) down to very low temperature, due to the finite size of the correlation length /1,3/. unfortunately, the existence of such properties is not a unique signature of the Haldane conjecture. Indeed, similar effects are predicted and observed in the S = ½ alternating chain /6,7/ as in the S = ½ spin-Peierls system /8,9/. This makes more difficult to obtain an unambiguous description of the experimental results.