91°µÍø

Inelastic neutron scattering is used to investigate magnetic
excitations in the quasi-one-dimensional quantum spin-liquid
system Cu$_2$Cl$_{4}\cdot$ D$_8$C$_4$SO$_2$. Contrary to
previously conjectured models that relied on bond-alternating
nearest neighbor interactions in the spin chains, the dominant
interactions are actually next-nearest-neighbor in-chain
antiferromagnetic couplings. The appropriate Heisenberg
Hamiltonian is equivalent to that of a $S=1/2$ 4-leg spin-tube
with almost perfect one dimensionality and no bond alternation. A
partial geometric frustration of rung interactions induces a small
incommensurability of short-range spin correlations.