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

We present electron spin resonance experiments in the one-dimensional antiferromagnetic $S=1\mathrm{}$ 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 $\left(-{1)}^i{d(S}_i^x{S}_i^z{+S}_i^z{S}_i^x\right)$ was considered in addition to a staggered field term $(-{1)}^i{S}_i^x{B}_{\mathrm{st}}.$ We show that the spin dynamics in high magnetic fields strongly depends on the orthorhombic anisotropy E.