Nonmagnetic impurity effects on S=1/2 Heisenberg ladder Cu-2(C5H12N2)(2)Cl-4

Magnetic measurements have been performed to investigate nonmagnetic impuri ty effects on a Zn-doped ladder system (Cu1-xZnx)(2) (C5H12N2)(2)Cl-4. The paramagnetic contribution in the susceptibility chi' of the Zn-doped system increases with increasing x below the gap temperature. The susceptibility obeys the Curie law in the temperature range between 0.5 K and 2 K. However , at temperatures below 0.5 K, chi'T drops with decreasing temperature, whi ch suggests the crossover in the Curie constant as predicted by Sigrist and Furusaki [J. Phys. Sec. Jpn. 65 (1996) 2385]. Magnetization curves of the Zn-doped system are compared with that of the pure system. The impurity-ind uced spin behaves like a paramagnetic S=1/2 spin and the spin-gap state sur vives even in the Zn-doped system with the content x up to 0.117. These res ults seem to originate from the weak interladder interaction of the system.