139La NQR study in La2CuO4 over temperatures up to 800 K

We have measured 139La NQR spectra and the nuclear spin-lattice relaxation rate for the highest (±7/2<->±5/2) and the middle (±5/2<->±3/2) NQR lines in the parent compound of high-Tc superconductor, La2CuO4, in a wide temperature range up to 800 K. From the abrupt increase of the quadrupole frequency νQ just below the tetra-ortho structural transition temperature TOT which comes from the staggered tilt of the CuO6 octahedra, the critical exponent β is evaluated to be 0.4 which is close to 0.5 expected in the standard mean field approximation for second order phase transition. In higher temperature than TOT, the nuclear spin-lattice relaxation at La site is dominated by the quadrupole relaxation, not reflecting the critical spin dynamics in the CuO2 plane. We successfully discriminate the two types of quadrupole relaxation rate, WQ1 and WQ2. The relaxation rate WQ2 is one order of magnitude larger than WQ1 and increases with the critical exponent α ∝ 1.19 toward TOT. (orig.)