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Formation of gapless triplets in the bond-doped spin-gap antiferromagnet (C$_4$H$_{12}$N$_2$)(Cu$_2$Cl$_6$)

We report results of an electron spin resonance (ESR) study of a spin-gap antiferromagnet (C$_4$H$_{12}$N$_2$)(Cu$_2$Cl$_6$) (nicknamed PHCC) with chlorine ions partially substituted by bromine. We found that up to 10% of nominal doping the contribution of the random defects to the absorption spectra remains at about 0.1% per copper ion, almost the same as in the pure system. Instead, a particular kind of ESR absorption corresponding to gapless S=1 triplets is observed at low temperatures in samples with high nominal bromine content x>5%. Increase of bromine concentration also leads to the systematic broadening of ESR absorption line indicating reduction of the quasi-particles lifetime.