The temperature dependence of the powdered X-band ESR spectra has been measured in a two-dimensional spin-gap system ${\mathrm{SrCu}}_2{(\mathrm{B}\mathrm{O}}_3{)}_2$ in the temperature range between 5 and 300 K. The spectra are composed of two overlapping components corresponding to isolated triplet excitations and correlated pairs of triplet excitations, respectively. The pairs of triplet excitations can propagate through the crystal resulting in the absence of any fine structure in the broad Lorentzian component contrary to the isolated triplet excitations which are almost completely localized. The temperature dependence of the broad Lorentzian linewidth indicates the importance of the antiferromagnetic correlations between the triplet excitations.

• Received 9 April 2001

DOI:https://doi.org/10.1103/PhysRevB.65.024417