SPIN-SPIN CORRELATION IN THE QUANTUM CRITICAL REGIME OF LA2CUO4

We report measurements of the Cu-63 nuclear spin echo decay rate (63)1/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 xi. Xi shows a crossover from xi is similar to exp(J/k(B)T) in the renormalized classical regime (T less than or similar to 600 K) to xi is similar to J/k(B)T in the quantum critical regime (T greater than or similar to 600 K), where J is the exchange interaction. Our finding that the ratio 63T1T/63T2G is temperature independent, where 63T1 is the Cu-63 nuclear spin-lattice relaxation time, gives clear evidence for quantum critical scaling.