Two-dimensional antiferromagnetic quantum spin-fluid state inLa2CuO4

Elastic, quasielastic, and inelastic neutron scattering measurements in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ reveal novel two-dimensional antiferromagnetic behavior. At high temperatures the spins are ordered instantaneously over long distances two dimensionally but there is no measurable time-averaged staggered moment. The energy scale of the spin fluctuations is large, corresponding to an effective dispersion of ≳0.4 eV Å. The relevance to the high-temperature superconductivity in this class of materials is noted.