Temperature dependence of the electron-spin-resonance spectrum of the chain-endS=1/2 modes in anS=1 antiferromagnetic chain

Abstract

The S=1/2 degrees of freedom, which have been predicted to occur at the ends of an S=1 Heisenberg-like antiferromagnetic chain, have been observed recently in low-temperature ESR measurements of Ni(${\mathrm{C}}_2$ ${\mathrm{H}}_8$ ${\mathrm{N}}_2$ ${)}_2$ ${\mathrm{NO}}_2$ ${\mathrm{ClO}}_4$ (abbreviated NENP) containing selected impurities. The signal was found to decrease rapidly with temperature, at temperatures well below the Haldane gap. We present here a theoretical analysis of the loss of ESR signal due to the presence of thermal spin excitations on the chain, which can at least qualitatively explain the experiment.