Experimental evidence for the lowest excitation mode in thes=1 Haldane-gap system: High-field proton magnetic relaxation in Ni(C2H8N2)2NO2ClO4

Abstract
The nuclear spin-lattice relaxation time T1 of the proton in the s=1 Haldane-gap system Ni(C2 H8 N2 )2 NO2 ClO4 (NENP) has been measured at low temperatures in high magnetic fields (4 THT1 below 4 K are well explained by treating the magnetic excitations as free fermions rather than free bosons.