Electron spin resonance of defects in the Haldane system ${\mathrm{Y}}_2{\mathrm{BaNiO}}_5$

We calculate the electron paramagnetic resonance (EPR) spectra of the antiferromagnetic spin-1 chain compound ${\mathrm{Y}}_2{\mathrm{BaNi}}_{1-x}{\mathrm{Mg}}_x{\mathrm{O}}_5$ for different values of x and temperature T much lower than the Haldane gap $(\sim 100$ 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 $S=1/2\mathrm{}$ states is clearly observed as the main peak in the spectrum and the remaining structure is completely understood.