Switching of the gapped singlet spin-liquid state to an antiferromagnetically ordered state

Magnetic susceptibility and specific heat were measured for a Zn-substituted spin-1/2 Heisenberg two-leg ladder compound Sr((Formula presented)(Formula presented)(Formula presented)(Formula presented) (x⩽0.08) to investigate nonmagnetic impurity effects on the quantum spin system with a large spin gap of about ∼400 K. A clear anomaly attributed to the onset of an antiferromagnetic ordering was observed both in the magnetic susceptibility and the specific heat at low temperature even at only 1% of Zn. The Néel temperature showed a systematic Zn concentration dependence with a broad maximum at around 4% substitution ((Formula presented)∼8 K). A finite amount of a T-linear term with a coefficient of γ∼3.5 mJ/(Formula presented) was observed in the specific heat above (Formula presented) for x=0.02 and 0.04, suggesting that the spin excitation in the substituted compound becomes gapless.