Abrupt but continuous antiferromagnetic transition in nearly stoichiometric La2CuO4+ delta.

Abstract

Zero-field $^{139}\mathrm{La}$ nuclear quadrupole resonance (NQR) has been used to characterize the antiferromagnetic transition in nearly stoichiometric single crystals of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ (N\'eel temperatures ${\mathit{T}}_{\mathit{N}}$ between 275 and 318 K). The onset of the NQR Zeeman splitting at ${\mathit{T}}_{\mathit{N}}$ is abrupt but continuous, indicative of a second-order phase transition with either a crossover in critical behavior just below ${\mathit{T}}_{\mathit{N}}$ or a very small critical exponent \ensuremath{\beta}\ensuremath{\lesssim}0.1. The absence of any anomaly at ${\mathit{T}}_{\mathit{N}}$ in the $^{139}\mathrm{La}$ electric field gradient indicates no accompanying structural distortion.