Electron spin resonance of defects in the Haldane system Y2BaNiO5

We calculate the electron paramagnetic resonance (EPR) spectra of the antif erromagnetic spin-1 chain compound Y2BaNi1-xMgxO5 for different values oft and temperature T much lower than the Haldane gap (similar to 100 K). The l ow-energy spectrum of an anisotropic Heisenberg Hamiltonian, with all param eters determined from experiment, has been solved using the density-matrix renormalization-group method. The observed EPR spectra are quantitatively r eproduced by this model. The presence of end-chain S = 1/2 states is clearl y observed as the main peak in the spectrum and the remaining structure is completely understood. [S0163-1829(99)50742-X].