THE HALDANE STATE IN CSNICL3 DETECTED BY MAGNETIZATION MEASUREMENTS -OBSERVATION OF QUANTUM-CLASSICAL CROSSOVER AND THE VALENCE-BOND SOLID-STATE

We have confirmed from susceptibility and magnetization measurements t hat the Haldane state really exists in 1D phase of CsNiCl3 which under goes 3D ordering at T-N = 4.8 K. The observed susceptibility at low fi elds follows the predicted curve for the S = 1 quantum chain with ener gy gap (Delta/k(B) similar to 10 K) down to near T-N. A more definite evidence for the Haldane state is given by studying the effect of the nonmagnetic impurities, We haw revealed a real existence of S = 1/2 ed ge states characteristic of the Haldane state and further revealed a c rossover from the quantum to classical regimes as a function of both t emperature and field, The result is consistent with the model that at low temperatures (T < T-G Delta/k(B)) and low fields (H < H-G Delta/g mu(B)) the Haldane state is well defined to form the Valence-Bond-Soli d while at high temperatures (T > \J\/k(B)) and high fields (H > H-G) the system is not in the Haldane state any more.