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 \ensuremath{\gtrsim}0.4 eV A\r{}. The relevance to the high-temperature superconductivity in this class of materials is noted.