Switching of the gapped singlet spin-liquid state to an antiferromagnetically ordered state in Sr(Cu1-xZnx)(2)O-3

Magnetic susceptibility and specific heat were measured for a Zn-substituted spin-1/2 Heisenberg two-leg ladder compound Sr(Cu1-xZnx)(2)O-3 (x less than or equal to 0.08) to investigate nonmagnetic impurity effects on the quantum spin system with a large spin gap of about similar to 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 Neel temperature showed a systematic Zn concentration dependence with a broad maximum at around 4% substitution (T-Nmax similar to 8 K) A finite amount of a T-linear term with a coefficient of gamma similar to 3.5 mJ/K-2 was observed in the specific heat above TN for x=0.02 and 0.04, suggesting that the spin excitation in the substituted compound becomes gapless.