Electron spin resonance of defects in the Haldane system Y₂BaNiO₅

We calculate the electron paramagnetic resonance (EPR) spectra of the antiferromagnetic spin-1 chain compound Y₂BaNi_1-xMg_xO₅ for different values of x and temperature T much lower than the Haldane gap (∼100 K). The low-energy spectrum of an anisotropic Heisenberg Hamiltonian, with all parameters determined from experiment, has been solved using the density-matrix renormalization-group method. The observed EPR spectra are quantitatively reproduced by this model. The presence of end-chain S=1/2 states is clearly observed as the main peak in the spectrum and the remaining structure is completely understood.