NMR Studies of Quantum Choherence and Magnetism in Spin-Ladder Systems

本课题拟利用NMR/NQR研究S=1/2自旋阶梯体系中杂质和载流子掺杂效应下的量子相干性和异常磁性。阐明了SrCu^2O^3中杂质（Zn、Ni和La）诱导的交错极化（IISP）和反铁磁长程有序（AF-LRO）的特性。从准一维（Q1D） IISP沿两条腿估计的相关长度xi^3/ A被发现即使在x=0.001和0.005时也比xi^0/ A长得多。在Zn和Ni掺杂中，与t无关的xi^3/a被缩放到平均距离D^<AV> = 1/(2x)，在La掺杂中，xi^ <AV> = 1/x，例如，当x=0.001时，当D^<AV> = 500时，Zn掺杂中xi^0/a -50。显著拓宽的NQR谱明确地确定了与Zn和Ni掺杂AF- lro相关的AF矩的出现。在1.4 K时，沿a轴方向，杂质之间更多的e Cu位估计为0.04 μ β。AF矩的空间变化表现为指数型，xi^0/a = 8 （x=0.01）和4.5 （x=0.02）与Zn掺杂的LISP几乎相同。当杂质破坏了与间隙的短程共振价键（RVB）态的量子相干性时，尽管Q1D-IISP很弱，但由于阶间相互作用，Q1D-IISP得到了显著增强。另一方面，我们提出T^N由梯间相互作用的强度和S^o的大小决定。我们发现，在T^M=2.2 K以下，由Cu^2O^3阶梯和CuO^2链组成的空穴掺杂两脚阶梯化合物Sr^<2.5>Ca^<11.5>Cu^<24>O^<41>显示出三维（3D）磁序。我们发现小矩- 0.02 μ β自发地排列在阶梯上，而大矩- 0.56 μ β或磁性Cu位点占据了大约四分之一的链。认为这种三维磁序是一种协同现象，是由在阶梯上形成钉住的类cdw态所驱动的，光学电导率测量已经证实了这一点。少

This research project was planed to investigate by means of NMR/NQR the quantum coherence and anoma-lous magnetism in S=1/2 spin-ladder systems through the impurity and carrier doping effects.Characteristics of impurity (Zn, Ni, and La)-induced staggered polarization (IISP) and antiferromagnetic long-range order (AF-LRO) were clarified in gapped spin-1/2 Heisenberg two-leg ladder compound SrCu^2O^3. A correlation length xi^3/a estimated from a quasi-one-dimensional (Q1D) IISP along the two-leg was found to be much longer even in x=0.001 and 0.005 than xi^0/a. The T-independent xi^3/a is scaled to an average distance D^<AV> = 1/(2x) in the Zn and Ni doping and D^<AV> = 1/x in the La doping between the impurities, for example, xi^0/a -50 is estimated in the Zn doping when D^<AV> = 500 for x=0.001. The significantly broadened NQR spectrum has unambiguously identified the appearance of AF moments associated with the AF-LRO in the Zn and Ni doping. Rather uniform AF moments around the middl … More e Cu sites between the impurities are estimated to be in an order of 0.04 mubeta at 1.4 K with their direction along the a axis. The spatial variation of AF moments is shown to be of exponential type with almost the same xi^0/a = 8 (x=0.01) and 4.5 (x=0.02) as in the LISP for the Zn doping. The Q1D-IISP was found to be dramatically enhanced by an interladder interaction even though so weak, once the impurity breaks up the quantum coherence in the short-range resonating valence bond (RVB) state with the gap. On the other hand, we propose that T^N is determined by a strength of the interladder interaction and a size of S^o. We showed that hole-doped two-leg ladder compound Sr^<2.5>Ca^<11.5>Cu^<24>O^<41> comprising Cu^2O^3 ladders and CuO^2 chains reveals three dimensional (3D) magnetic order below T^M=2.2 K.We found that small moments - 0.02 mubeta are spontaneously ordered on the ladders, whereas large moments - 0.56mubeta or the magnetic Cu sites occupying about one-fourth of the chains. It is suggested that this 3D magnetic order is a cooperative phenomenon driven by the formation of pinned CDW-like state on the ladders which the optical conductivity measurement has clarified. Less