| Research Abstract |
The two-legged quantum spin ladder NH_4CuCl_3 has a gapless ground state at zero-field. With increasing field, the gap opens up in the two regions 5〜12.8 and 17.9〜24.7 T, resulting in the two-stepped plateaus in magnetization curve. Recent specific heat experiments propose the existence of the field-induced magnetic order to which the analogy with the Bose-Einstein condensation of magnons has been pointed out. In order to investigate the microscopic spin state of the plateaus and the magnetic order, Cu-NMR study under the field up to 30T has been performed.
In the low field region around the first plateau, all the NMR peaks are assigned to the singlet site with nearly zero internal field and with a large quadrupole interaction of 39.2MHz. This means that there are the two discrete spin states : the singlet site and excited triplet site with a large hyperfine field around -10T. The latter, which **counts for the finite macroscopic magnetization, is invisible by NMR due to the strong spin fluctuation of on site 3d-spins.
In the high field region between the 1st and 2nd plateaus, the Knight shift of the single site decreases with lowering temperature, and shows a kink at 4K, which corresponds the proposed phase boundary of the field-induced magnetic order (Phase II). Below the kink temperature, the shift stays at steady value rather than showing an upturn as observed in the magnon Bose-Einstein condensation in TlCuCl_3. No splitting or broadening in the singlet peaks arise at the kink temperature. Several satellite peaks, which show faster transverse relaxation, are observed around the singlet peak in this high field region. The splitting separation of a few kOe, which is magnetic origin, is already finite at 7K, well above the kink temperature, slightly increases as lowering temperature. The Knight shift of these satellite peaks also shows a kink at 4K.
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