Evidence of quantum criticality in the doped Haldane system Y2BaNiO5

Experimental bulk susceptibility chi(T) and magnetization M(H,T) of the S=1 Haldane chain system doped with nonmagnetic impurities, Y2BaNi1-xZnxO5 (x less than or equal to 0.08), are analyzed. A numerical calculation For the low-energy spectrum of noninteracting open segments describes well experime ntal data above 4 K. Below 4 K, we observe power-law behaviors chi(T)simila r to T-alpha and M(H,T)similar to T(1-alpha)f(alpha)(N/T), with alpha (<1) depending on the doping concentration x. This observation suggests the appe arance of a gapless quantum phase due to a broad distribution of effective couplings between the dilution-induced moments.