The spin ladder materials Srn-1Cun+1O2n, which alternate between a classical and quantum mechanical ground state were presented first. The experimental results supported the theoretical predictions about the ground state structures; the spin-gapped, non-magnetic ground state for the 2-leg ladder system, and a gap-less, ordered ground state for the 3-leg system.
The Haldane material Y2BaNiO5 was confirmed to take the non-magnetic ground state. With the Mg doping to the Cu site, this ground state was found to be stable against the vacancy doping on the spin chain. If positive charge is introduced to the chain, though, the non-magnetic ground state was strongly perturbed; on a macroscopic time scale, the system exhibited a spin-glass like signature, such as history dependent susceptibilities. On the other hand, on a microscopic time scale, however, the spins kept fluctuating down to milli-Kelvin regime in a unconventional way. If one calls a fluctuating spin ground state a `spin liquid', the charge doped Haldane system may be called as a `viscous' spin liquid.
The spin Peierls material CuGeO3 exhibited a non-magnetic ground state. But, with a small amount of non-magnetic perturbation, a classical ground state was induced to appear; well defined Néel order was confirmed with a spontaneous muon spin precession in a Si-doped single crystal.
The experimental results presented in this thesis may be summarized
as in the next table:
|spin Peierls||non-mag.||Néel order||?|
In all of the singlet ground state materials investigated, the non-magnetic ground state was realized in the pure systems, and it induced muon spin relaxation in the fast fluctuation regime. But, when vacancy and/or charge is doped, the response to these perturbations seems to reflect the characters of the individual spin systems. For example, with a small amount of vacancy introduced to the chain, the spin Peierls material (Cu1-xZnx)GeO3 exhibited bulk Néel order, while the Haldane material Y2Ba(Ni1-yMgy)O5 preserved the singlet ground state. The contrasting response to the vacancy doping may reflect the difference in the ground state structures; it seems that the ground state constituent of isolated singlet pairs are unstable against vacancy doping. To test this hypothesis, SR investigations of the Zn-doped 2-leg ladder materials Sr(Cu1-xZnx)2O3 is underway.
The experimental results presented in this thesis may be summarized in
the next figure:
In this plot, the horizontal axis represents characteristic magnitude of perturbations to the system; if a singlet ground state is doped with vacancy and/or charge, the dopant creates unpaired spins which manifest themselves as the low temperature Curie term. The vertical axis reflects how the singlet ground state is perturbed upon doping; if majority of the moments remain singlet and muons detect the dipolar fields from the doping induced local moments, the plot should fall in the region of dashed lines. The pure/vacancy doped Haldane system Y2Ba(Ni1-yMgy)O5 seems to realize this situation. If the system shows bulk magnetic order, the relaxation becomes static (closed symbols) with a saturated relaxation rate, as the Zn doped CuGeO3 exhibited.
One interesting question is why the pure CuGeO3 and the 2-leg ladder system SrCu2O3 doesn't fall in the dashed-line region. There may be two explanations; (1) the native unpaired spins are already correlated and their fluctuation rate () becomes slow, and (2) muon itself works as an impurity to the system, and creates unpaired spins around it; namely, the dipolar field width (a) at the muon site becomes larger than that from the native unpaired spins observed in susceptibility. In the 2-leg ladder system, the enhanced dipolar field widths (a) estimated from the longitudinal field measurements (Table 4 in Chapter 4) seem to suggest a possibility of the latter case, namely, a muon-impurity effect.
Among all the results presented in this thesis, the most interesting one is the unconventional spin dynamics of the charge doped Haldane system. Since this problem involves two degrees of freedom, namely, the `spin' and the `charge', theoretical understanding may be challenging. But this system should contain interesting dynamical features inside.