High magnetic-field ESR in the Haldane spin chains NENP and NINO

Abstract

We present electron spin resonance experiments in the one-dimensional antiferromagnetic $S=1$ spin chains NENP and NINO in pulsed magnetic fields up to 50 T. The measured field dependence of the quantum energy gap for $B||b$ is analyzed using the exact diagonalization method and the density matrix renormalization group method. A staggered anisotropy term $(\ensuremath{-}{1)}^{i}{d(S}_{i}^{x}{S}_{i}^{z}{+S}_{i}^{z}{S}_{i}^{x})$ was considered in addition to a staggered field term $(\ensuremath{-}{1)}^{i}{S}_{i}^{x}{B}_{\mathrm{st}}.$ We show that the spin dynamics in high magnetic fields strongly depends on the orthorhombic anisotropy E.