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Syntheses and NMR study on low dimensional system with small spin-gap

小自旋间隙低维体系的合成及核磁共振研究

基本信息

批准号：
12640360
负责人：
GOTO Takayuki
金额：
$ 1.6万
依托单位：
Sophia University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2000
资助国家：
日本
起止时间：
2000 至 2001
项目状态：
已结题

项目摘要

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.

NH_4CuCl_3的量子自旋梯在零场下具有无隙基态。随着磁场的增加，在5 ~ 12.8和17.9 ~ 24.7 T两个区域内，差距增大，导致磁化曲线出现两个台阶状平台。最近的比热实验提出了存在的磁场诱导的磁秩序的类比与玻色爱因斯坦凝聚的磁振子已被指出。为了研究平台区的微观自旋状态和磁序，我们在30 T的磁场下进行了Cu-NMR研究，在第一平台区附近的低场区，所有的NMR峰都归属于内场接近零的单重位，其四极相互作用为39.2MHz。这意味着有两个离散的自旋态：单重态和激发三重态，在-10T附近有一个大的超精细场。在第一和第二平台之间的高场区，单格位的Knight位移随温度的降低而减小，并在4K处出现扭结，这对应于所提出的场致磁序的相边界（第II相）。在扭折温度以下，位移保持在稳定值，而不是像在TlCuCl_3中的磁振子玻色-爱因斯坦凝聚中观察到的那样显示出上升。在扭结温度下，单重峰没有分裂或加宽。几个卫星峰，这表明更快的横向弛豫，观察到周围的高场区的单重峰。几kOe的分裂分离，这是磁源，在7 K时已经是有限的，远高于扭结温度，随着温度的降低略有增加。这些卫星峰的奈特位移也显示出在4K处的扭结。