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.

两条腿的量子自旋梯NH_4CuCl_3在零磁场下具有无带隙基态，随着磁场的增加，带隙在5〜12.8和17.9〜24.7 T两个区域打开，导致磁化曲线出现两阶平台。最近的比热实验提出了场感磁序的存在，并指出了与磁振子玻色-爱因斯坦凝聚的类比。为了研究高原的微观自旋状态和磁序，在高达30T的场下进行了Cu-NMR研究。在第一个高原周围的低场区域，所有NMR峰都分配到接近零的单重态位点内部场和具有 39.2MHz 的大四极相互作用，这意味着存在两种离散的自旋态：单重态位点和激发三重态位点，后者具有约 -10T 的大超精细场。 ** 计算有限的宏观磁化强度，由于现场 3d 自旋的强烈自旋涨落，通过 NMR 是看不见的。在第一和第二平台之间的高场区域，单位点的奈特位移随着温度的降低而减小，并显示出 4K 处的扭结，这对应于提出的场感磁序相界（第二相），在扭结温度以下，位移保持在稳定值，而不是显示出观察到的上升。在 TlCuCl_3 的磁振子玻色-爱因斯坦凝聚中，在扭结温度下，在单线态峰周围没有出现分裂或展宽，在该高场区域的单线态峰周围观察到了几个卫星峰。几个 kOe（磁起源）在 7K 时已经有限，远高于扭结温度，随着温度的降低，这些卫星峰的奈特位移也显示出扭结。 4K。