ABRUPT BUT CONTINUOUS ANTIFERROMAGNETIC TRANSITION IN NEARLY STOICHIOMETRIC LA2CUO4+DELTA

Zero-field La-139 nuclear quadrupole resonance (NQR) has been used to characterize the antiferromagnetic transition in nearly stoichiometric single crystals of La2CuO4+6 (Neel temperatures T(N) between 275 and 318 K). The onset of the NQR Zeeman splitting at T(N) is abrupt but continuous, indicative of a second-order phase transition with either a crossover in critical behavior just below T(N) or a very small critical exponent beta less than or similar to 0.1. The absence of any anomaly at T(N) in the La-139 electric field gradient indicates no accompanying structural distortion.