We report measurements of the ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear spin echo decay rate ${}_{}{}^{63}{}_{}{}^{}1$/${\mathit{T}}_{2\mathit{G}}$ in the paramagnetic state of a quasi-two-dimensional antiferromagnet ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ up to 900 K, and from it deduce the temperature dependence of the spin-spin correlation length ξ. ξ shows a crossover from ξ∼exp(J/${\mathit{k}}_{\mathit{B}}$T) in the renormalized classical regime (T≲600 K) to ξ∼J/${\mathit{k}}_{\mathit{B}}$T in the quantum critical regime (T≳600 K), where J is exchange interaction. Our finding that the ratio ${}_{}{}^{63}{}_{}{}^{}$${\mathit{T}}_1$T${/}^{63}$${\mathit{T}}_{2\mathit{G}}$ is temperature independent, where ${}_{}{}^{63}{}_{}{}^{}$${\mathit{T}}_1$ is the ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear spin-lattice relaxation time, gives clear evidence for quantum critical scaling.

• Received 9 June 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.1254