Quantum Energy Gap in the S=1 Heisenberg Antiferromagnet NENP: Experimental Data and Haldane Conjecture
Since the early work of Ising , a lot of theoretical studies have been devoted to one-dimensional (1d) magnetism [2–6]. Recently, the interest for the 1d Heisenberg antiferromagnets (1d-HAF) with integer spin values has been renewed by Haldane  who predicted for these systems, an energy gap between the singlet ground state and the first excited states.
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