Spin-spin correlation in the quantum critical regime of La2CuO4

We report measurements of the Cu63 nuclear spin echo decay rate 163/T2G in the paramagnetic state of a quasi-two-dimensional antiferromagnet La2CuO4 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/kBT) in the renormalized classical regime (T≲600 K) to ξ∼J/kBT in the quantum critical regime (T≳600 K), where J is exchange interaction. Our finding that the ratio …