| Budget Amount *help |
¥2,700,000 (Direct Cost: ¥2,700,000)
Fiscal Year 2000: ¥600,000 (Direct Cost: ¥600,000)
Fiscal Year 1999: ¥2,100,000 (Direct Cost: ¥2,100,000)
|
|---|
| Research Abstract |
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
|
