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Studies of elemental substitution effect in spin-gap systems by density matrix renormalization group method

密度矩阵重正化群法研究自旋能隙体系中的元素取代效应

基本信息

批准号：
11640363
负责人：
YOKOYAMA Hisatoshi
金额：
$ 1.73万
依托单位：
Tohoku university
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1999
资助国家：
日本
起止时间：
1999 至 2000
项目状态：
已结题

项目摘要

In connection with the pseudogap and impurity effect in high temperature superconductors, we have investigated chiefly elemental substitution effect in various quasi one-dimensional spin systems which show spin-gap behaviors, by using the density matrix renormalization group and exact diagonalization methods. We consider a one-dimensional spin model with two parameters, next-nearest-neighbor coupling α and bond alternation β. This model can be applied to all the materials of our concern, [1] Haldane systems, [2] bond-alternating ones, [3] spin-Peierls ones and [4] spin ladder ones. A series of calculations has been performed for pure states and ones with impurities. Main results are : (1) According to the gap, spin correlation length ζ and the string order parameter, the whole space spanned by α and β for the pure systems (size L=∞ ) belongs to the so-called Haldane phase except for the two limiting cases of 'spin liquid' : β=1 (no bond alternation) and α=∞ (independent two S=1/2 Heise … More nberg chains). (2) When non-magnetic impurities are doped (L is finite) , not only ζ but the behaviors of spin polarization in the states with <S^e_i>≠0 are distinct between the typical Haldane regime and the one near the spin-liquid lines. For the former, ζ and <S^e_i> (namely magnetic excitation) are independent of L and spin polarization is localized near the chain end (impurity site). Meanwhile, for the latter staggered magnetization spreads over the whole chain and its amplitude abruptly increases with decreasing L.Thus, we see that the material groups [1] and [2] belonging to the formar category preserve the gap properties even for highly doping, whereas for [3] and [4] in the latter category a tiny amount of impurity disrupts the gap and leads to an antiferromagnetic long range order. (3) In this model incommensurate spin correlation had been anticipated and partly known. We have elucidated the phase diagram in the whole α-β space for T=0. Moreover, discordance of the incommensurability between the real and momentum spaces has been discussed in the light of quantum fluctuation.Based on the above results for spin systems, we have progressed our calculation to the electronic ones, especially taking account of the pseudogap and stripe structure for high temperaure superconductors. The study in this line will be succeeded in the project of next term. Less

针对高温超导体中的赝能隙和杂质效应，我们利用密度矩阵重整化群和精确对角化方法，主要研究了具有自旋能隙行为的各种准一维自旋系统中的元素替代效应。我们考虑了一个一维自旋模型，其中包含两个参数：次近邻耦合α和键交替β.该模型适用于我们所关心的所有材料，[1] Haldron系统，[2]键交替系统，[3]自旋Peierls系统和[4]自旋阶梯系统。对纯态和含杂质态进行了一系列计算。主要结果如下：（1）根据能隙差距、自旋关联长度和弦序参量，对于纯体系（尺寸L=∞），除了“自旋液体”的两种极限情况：β=1（无键交替）和α=∞（独立的两个S=1/2海瑟）之外，α和β所覆盖的整个空间都属于所谓的Haldom相 ...更多信息 nberg链）。(2)当掺杂非磁性杂质时（L是有限的），在典型的Haldom区和自旋液体线附近的Haldom区中，不仅自旋极化行为不同，而且<S^e_i><0的自旋极化行为也不同。对于前者，ζ和<S^e_i>（即磁激发）与L无关，并且自旋极化局限于链端附近（杂质位点）。同时，对于后一种交错磁化分布在整个链上，并且其幅度随着L的减小而突然增加。因此，我们看到属于前一类的材料组[1]和[2]即使在高掺杂下也保持差距特性，而对于后一类的材料组[3]和[4]，少量的杂质破坏了差距并导致反铁磁长程有序。(3)在这个模型中，无公度自旋关联已经被预料到，并且部分地被知道。我们阐明了T=0时整个α-β空间的相图。从量子涨落的角度讨论了真实的和动量空间不相容性的不一致性，并在自旋系统的上述结果的基础上，将计算推广到电子系统，特别是考虑了高温超导体的赝能隙和条纹结构。这方面的研究将在下学期的项目中取得成功。少