2001-5

5) NMR Study on Even-Legged Quantum Spin Ladder NH₄CuCl₃

The two-legged quantum spin ladder NH₄CuCl₃ has a gapless ground state at zero-field. With increasing field, the gap opens in the two regions of field, causing the two-stepped plateaus in magnetization curve. Cu-NMR study under the field up to 30T has revealed that the spin state consists of discrete two sets: the singlet site with nearly zero internal field and excited triplet site with a large internal field. Only the singlet site is visible in NMR spectra, irrespective of the field being in or out of the plateaus. The local magnetization of the singlet site probed by NMR shift showed a different temperature dependence from that of the macroscopic magnetization. This difference is explained as the localization of the excited triplets, which may be the force to open up the finite gap in the plateau region. In the field out of the plateau, a significant splitting of the resonance peaks is observed at low temperatures below 4.2 K, suggesting the existence of the field induced magnetic order.

Sophia University : Takayuki Goto, Takao Suzuki
Tokyo Institute of Technology : Hidekazu Tanaka
IMR, Tohoku University : Satoshi Awaji, Takahiko Sasaki, Kazuo Watanabe
Reference : S. Hosoya, K. Inokuchi, T. Suzuki, T.Goto, H. Tanaka, S. Awaji, K. Watanabe, T. Sasaki, T. Fukase,T. Shimizu, A. Goto,
"NMR study on the quantum spin ladder NH₄CuCl₃",Physica B, in press.