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5.2 Haldane material Y2BaNiO5

In Fig.38, we show the crystal structure of the inorganic Haldane material Y2BaNiO5 [97]. The structure is characterized by chains of compressed NiO6 octahedra, which are separated by non-magnetic Y3+ and Ba2+ ions. The shortened Ni-O bonds along the chain (a-axis) lead to a relatively large super-exchange interaction $J\approx 285\sim340$ K, as estimated from the magnetic susceptibility [98,99]. The magnitude of the Haldane gap has been obtained as $E_{\rm g}\sim 100$ K with susceptibility [100,101,99] and inelastic neutron scattering measurements [102,103].

Figure 38: The crystal structure of Y2BaNiO5. $4\times 2\times 1$ unit cells are shown. The chain direction is the a-axis.
\epsfig {file=haldane-struct.eps,width=7cm}

One valuable feature of this material is that both off-chain charge doping (Y3+$\rightarrow$Ca2+) and on-chain vacancy doping (Ni2+$\rightarrow$Zn2+, Mg2+) is possible [98,104,103]. Since charge-doping to the Haldane ground state is a unique feature, some previous measurements have been aimed to clarify the behavior of the doped charge.

DiTusa et al. [103] measured resistivity ($\rho$), X-ray absorption spectroscopy (XAS), and inelastic neutron scattering (NIS) of the Ca/Zn-doped systems, and found the following features:

the doped holes did not induce a clean metallic behavior, but a localized state with hopping ($\rho$).
the doped holes locate mainly at the 2pz orbital of the oxygen ion, which has a lobe in the chain direction (XAS).
charge doping induces a density of states within the Haldane gap (NIS). With vacancy doping (Ni2+$\rightarrow$Zn2+), such `in-gap states' were absent.
The result (3) implies that hole-doping strongly perturbs the non-magnetic ground state, while vacancy-doping preserves the non-magnetic ground state.

Quantitatively, vacancy doped Y2BaNiO5 has shown a mysterious response. Ramirez et al. [104] measured specific heat of the vacancy-doped compounds (Ni2+$\rightarrow$Zn2+) and found that the number of doping-induced spins does not follow the Valence Bond Solid state scenario; the VBS picture predicts the creation of two S=1/2 spins for each vacancy (see Fig.35d), but the Zn-doped Y2BaNiO5 system exhibited one S=1 spin for two Zn ions. To understand this phenomenon, Ramirez et al. suggest a heuristic `singlet-triplet model', which assumes that half of the broken chains form a triplet S=1 and the other half, a singlet S=0. Neither the origin of the couplings between the chain-end spins nor the local structure of the doping-induced spins is clear at the present stage.

To investigate the ground state properties and dynamics of spin systems, $\mu$SR is a powerful technique as introduced in the previous chapter. The following presents $\mu$SR results of the nominally pure/doped Y2BaNiO5, as well as their susceptibility data in low magnetic fields. For our measurements, polycrystalline specimens of nominally pure Y2BaNiO5, Ca doped systems [(Y2-xCax)BaNiO5; x=4.5, 9.5, 14.9 and 30.5%] and Mg doped systems [Y2Ba(Ni1-yMgy)O5; y=1.7 and 4.1%] were prepared at the University of Tokyo, Superconductivity Laboratory, using a standard solid state reaction described below [105]:

Powder X-ray analysis of the samples did not detect any impurity phases. The Ca and Mg concentrations (x and y) were determined with the atomic light-absorption method, which was commercially available (Robertson Microlit Laboratory, NJ, USA).

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Next: 5.2.1 Susceptibility measurements Up: 5 Haldane system Previous: .