X-band ESR study of the 2D spin-gap system SrCu2(BO3)2

Abstract The X-band ESR spectra of the SrCu 2 (BO 3 ) 2 spin-gap compound have been investigated. We observe extraordinary large linewidths that can be attributed to correlated pairs of triplet excitations. The maximum of the linewidth at 14 K seems to arise from reduced antiferromagnetic fluctuations, relating a reduction of mobile pair excitations with temperature. Attempts to intercalate the copper–borate layers by alkali metals result in compositions with modified spin susceptibility, however, no conclusive route to a new ground state has been uncovered so far.